Recruitment Strategies in the Integration of Mobile Health Into Sickle Cell Disease Care to Increase Hydroxyurea Utilization Study (meSH): Multicenter Survey Study.

BACKGROUND: Hydroxyurea is an evidence-based disease-modifying therapy for sickle cell disease (SCD) but is underutilized. The Integration of Mobile Health into Sickle Cell Disease Care to Increase Hydroxyurea Utilization (meSH) multicenter study leveraged mHealth to deliver targeted interventions to patients and providers. SCD studies often underenroll; and recruitment strategies in the SCD population are not widely studied. Unanticipated events can negatively impact enrollment, making it important to study strategies that ensure adequate study accrual. OBJECTIVE: The goal of this study was to evaluate enrollment barriers and the impact of modified recruitment strategies among patients and providers in the meSH study in response to a global emergency. METHODS: Recruitment was anticipated to last 2 months for providers and 6 months for patients. The recruitment strategies used with patients and providers, new recruitment strategies, and recruitment rates were captured and compared. To document recruitment adaptations and their reasons, study staff responsible for recruitment completed an open-ended 9-item questionnaire eliciting challenges to recruitment and strategies used. Themes were extrapolated using thematic content analysis. RESULTS: Total enrollment across the 7 sites included 89 providers and 293 patients. The study acceptance rate was 85.5% (382/447) for both patients and providers. The reasons patients declined participation were most frequently a lack of time and interest in research, while providers mostly declined because of self-perceived high levels of SCD expertise, believing they did not need the intervention. Initially, recruitment involved an in-person invitation to participate during clinic visits (patients), staff meetings (providers), or within the office (providers). We identified several important recruitment challenges, including (1) lack of interest in research, (2) lack of human resources, (3) unavailable physical space for recruitment activities, and (4) lack of documentation to verify eligibility. Adaptive strategies were crucial to alleviate enrollment disruptions due to the COVID-19 pandemic. These included remote approaching and consenting (eg, telehealth, email, and telephone) for patients and providers. Additionally, for patients, recruitment was enriched by simplification of enrollment procedures (eg, directly approaching patients without a referral from the provider) and a multitouch method (ie, warm introductions with flyers, texts, and patient portal messages). We found that patient recruitment rates were similar between in-person and adapted (virtual with multitouch) approaches (167/200, 83.5% and 126/143, 88.1%, respectively; P=.23). However, for providers, recruitment was significantly higher for in-person vs remote recruitment (48/50, 96% and 41/54, 76%, respectively, P<.001). CONCLUSIONS: We found that timely adaptation in recruitment strategies secured high recruitment rates using an assortment of enriched remote recruitment strategies. Flexibility in approach and reducing the burden of enrollment procedures for participants aided enrollment. It is important to continue identifying effective recruitment strategies in studies involving patients with SCD and their providers and the impact and navigation of recruitment challenges. TRIAL REGISTRATION: ClinicalTrials.Gov NCT03380351; https://clinicaltrials.gov/study/NCT03380351. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR2-10.2196/16319.

Social determinants of health and treatment center affiliation: analysis from the sickle cell disease implementation consortium registry.

BACKGROUND: Adults with sickle cell disease (SCD) suffer early mortality and high morbidity. Many are not affiliated with SCD centers, defined as no ambulatory visit with a SCD specialist in 2 years. Negative social determinants of health (SDOH) can impair access to care. HYPOTHESIS: Negative SDOH are more likely to be experienced by unaffiliated adults than adults who regularly receive expert SCD care. METHODS: Cross-sectional analysis of the SCD Implementation Consortium (SCDIC) Registry, a convenience sample at 8 academic SCD centers in 2017-2019. A Distressed Communities Index (DCI) score was assigned to each registry member's zip code. Insurance status and other barriers to care were self-reported. Most patients were enrolled in the clinic or hospital setting. RESULTS: The SCDIC Registry enrolled 288 Unaffiliated and 2110 Affiliated SCD patients, ages 15-45y. The highest DCI quintile accounted for 39% of both Unaffiliated and Affiliated patients. Lack of health insurance was reported by 19% of Unaffiliated versus 7% of Affiliated patients. The most frequently selected barriers to care for both groups were "previous bad experience with the healthcare system" (40%) and "Worry about Cost" (17%). SCD co-morbidities had no straightforward trend of association with Unaffiliated status. The 8 sites' results varied. CONCLUSION: The DCI economic measure of SDOH was not associated with Unaffiliated status of patients recruited in the health care delivery setting. SCDIC Registrants reside in more distressed communities than other Americans. Other SDOH themes of affordability and negative experiences might contribute to Unaffiliated status. Recruiting Unaffiliated SCD patients to care might benefit from systems adopting value-based patient-centered solutions.

Effective Recruitment Strategies for a Sickle Cell Patient Registry Across Sites from the Sickle Cell Disease Implementation Consortium (SCDIC).

Sickle cell disease (SCD) is a genetic disorder predominantly affecting people of African descent and is associated with significant morbidity and mortality. To improve SCD outcomes, the National Heart Lung and Blood Institute funded eight centers to participate in the SCD Implementation Consortium. Sites were required to each recruit 300 individuals with SCD, over 20 months. We aim to describe recruitment strategies and challenges encountered. Participants aged 15-45 years with confirmed diagnosis of SCD were eligible. Descriptive statistics were used to analyze the effectiveness of each recruitment strategy. A total of 2432 participants were recruited. Majority (95.3%) were African American. Successful strategies were recruitment from clinics (68.1%) and affiliated sites (15.6%). Recruitment at community events, emergency departments and pain centers had the lowest yield. Challenges included saturation of strategies and time constraints. Effective recruitment of participants in multi-site studies requires multiple strategies to achieve adequate sample sizes.

Structural, Photophysical, and Photochemical Characterization of Zinc Protoporphyrin IX in a Dimeric Variant of an Iron Storage Protein: Insights into the Mechanism of Photosensitized H2 Generation.

Some of us have previously reported the preparation of a dimeric form of the iron storage protein, bacterioferritin (Bfr), in which the native heme b is substituted with the photosensitizer, Zn(II)-protoporphyrin IX (ZnPP-Bfr dimer). We further showed that the ZnPP-Bfr dimer can serve as a photosensitizer for platinum-catalyzed H2 generation in aqueous solution without the usually added electron relay between photosensitizer and platinum ( Clark , E. R. , Inorg. Chem. 2017 , 56 , 4584 - 4593 ). We proposed reductive or oxidative quenching pathways involving the ZnPP anion radical (ZnPP•-) or the ZnPP cation radical, (ZnPP•+), respectively. The present report describes structural, photophysical, and photochemical properties of the ZnPP in the ZnPP-Bfr dimer. X-ray absorption spectroscopic studies at 10 K showed a mixture of five- and six-coordinated Zn centers with axial coordination by one long Zn-SγMet distance of ∼2.8 Å and ∼40% having an additional shorter Zn-S distance of ∼2.4 Å, in addition to the expected 4 nitrogen atom coordination from the porphyrin. The ZnPP in ZnPP-Bfr dimer was prone to photosensitized oxidation to ZnPP•+. The ZnPP•+ was rapidly reduced by ascorbic acid, which we previously determined was essential for photosensitized H2 production in this system. These results are consistent with an oxidative quenching pathway involving electron transfer from 3ZnPP* to platinum, which may be assisted by a flexible ZnPP axial coordination sphere. However, the low quantum yield for H2 production (∼1%) in this system could make reductive quenching difficult to detect, and can, therefore, not be completely ruled out. The ZnPP-Bfr dimer provides a simple but versatile framework for mechanistic assessment and optimization of porphyrin-photosensitized H2 generation without an electron relay between porphyrin and the platinum catalyst.

Charge-Disproportionation Symmetry Breaking Creates a Heterodimeric Myoglobin Complex with Enhanced Affinity and Rapid Intracomplex Electron Transfer.

We report rapid photoinitiated intracomplex electron transfer (ET) within a "charge-disproportionated" myoglobin (Mb) dimer with greatly enhanced affinity. Two mutually supportive Brownian Dynamics (BD) interface redesign strategies, one a new "heme-filtering" approach, were employed to "break the symmetry" of a Mb homodimer by pairing Mb constructs with complementary highly positive and highly negative net surface charges, introduced through D/E → K and K → E mutations, respectively. BD simulations using a previously developed positive mutant, Mb(+6) = Mb(D44K/D60K/E85K), led to construction of the complementary negative mutant Mb(-6) = Mb(K45E, K63E, K95E). Simulations predict the pair will form a well-defined complex comprising a tight ensemble of conformations with nearly parallel hemes, at a metal-metal distance ∼18-19 Å. Upon expression and X-ray characterization of the partners, BD predictions were verified through ET photocycle measurements enabled by Zn-deuteroporphyrin substitution, forming the [ZnMb(-6), Fe(3+)Mb(+6)] complex. Triplet ET quenching shows charge disproportionation increases the binding constant by no less than ∼5 orders of magnitude relative to wild-type Mb values. All progress curves for charge separation (CS) and charge recombination (CR) are reproduced by a generalized kinetic model for the interprotein ET photocycle. The intracomplex ET rate constants for both CS and CR are increased by over 5 orders of magnitude, and their viscosity independence is indicative of true interprotein ET, rather than dynamic gating as seen in previous studies. The complex displays an unprecedented timecourse for CR of the CS intermediate I. After a laser flash, I forms through photoinduced CS, accumulates to a maximum concentration, then dies away through CR. However, before completely disappearing, I reappears without another flash and reaches a second maximum before disappearing completely.

Distance-independent charge recombination kinetics in cytochrome c-cytochrome c peroxidase complexes: compensating changes in the electronic coupling and reorganization energies.

Charge recombination rate constants vary no more than 3-fold for interprotein ET in the Zn-substituted wild type (WT) cytochrome c peroxidase (CcP):cytochrome c (Cc) complex and in complexes with four mutants of the Cc protein (i.e., F82S, F82W, F82Y, and F82I), despite large differences in the ET distance. Theoretical analysis indicates that charge recombination for all complexes involves a combination of tunneling and hopping via Trp191. For three of the five structures (WT and F82S(W)), the protein favors hopping more than that in the other two structures that have longer heme → ZnP distances (F82Y(I)). Experimentally observed biexponential ET kinetics is explained by the complex locking in alternative coupling pathways, where the acceptor hole state is either primarily localized on ZnP (slow phase) or on Trp191 (fast phase). The large conformational differences between the CcP:Cc interface for the F82Y(I) mutants compared to that the WT and F82S(W) complexes are predicted to change the reorganization energies for the CcP:Cc ET reactions because of changes in solvent exposure and interprotein ET distances. Since the recombination reaction is likely to occur in the inverted Marcus regime, an increased reorganization energy compensates the decreased role for hopping recombination (and the longer transfer distance) in the F82Y(I) mutants. Taken together, coupling pathway and reorganization energy effects for the five protein complexes explain the observed insensitivity of recombination kinetics to donor-acceptor distance and docking pose and also reveals how hopping through aromatic residues can accelerate long-range ET.

Evolving the [myoglobin, cytochrome b(5)] complex from dynamic toward simple docking: charging the electron transfer reactive patch.

We describe photoinitiated electron transfer (ET) from a suite of Zn-substituted myoglobin (Mb) variants to cytochrome b(5) (b(5)). An electrostatic interface redesign strategy has led to the introduction of positive charges into the vicinity of the heme edge through D/E → K charge-reversal mutation combinations at "hot spot" residues (D44, D60, and E85), augmented by the elimination of negative charges from Mb or b(5) by neutralization of heme propionates. These variations create an unprecedentedly large range in the product of the ET partners' total charges (-5 < -q(Mb)q(b(5)) < 40). The binding affinity (K(a)) increases 1000-fold as -q(Mb)q(b(5)) increases through this range and exhibits a surprisingly simple, exponential dependence on -q(Mb)q(b(5)). This is explained in terms of electrostatic interactions between a "charged reactive patch" (crp) on each partner's surface, defined as a compact region around the heme edge that (i) contains the total protein charge of each variant and (ii) encompasses a major fraction of the "reactive region" (Rr) comprising surface atoms with large matrix elements for electron tunneling to the heme. As -q(Mb)q(b(5)) increases, the complex undergoes a transition from fast to slow-exchange dynamics on the triplet ET time scale, with a correlated progression in the rate constants for intracomplex (k(et)) and bimolecular (k(2)) ET. This progression is analyzed by integrating the crp and Rr descriptions of ET into the textbook steady-state treatment of reversible binding between partners that undergo intracomplex ET and found to encompass the full range of behaviors predicted by the model. The generality of this approach is demonstrated by its application to the extensive body of data for the ET complex between the photosynthetic reaction center and cytochrome c(2). Deviations from this model also are discussed.

Interfacial hydration, dynamics and electron transfer: multi-scale ET modeling of the transient [myoglobin, cytochrome b5] complex.

Formation of a transient [myoglobin (Mb), cytochrome b(5) (cyt b(5))] complex is required for the reductive repair of inactive ferri-Mb to its functional ferro-Mb state. The [Mb, cyt b(5)] complex exhibits dynamic docking (DD), with its cyt b(5) partner in rapid exchange at multiple sites on the Mb surface. A triple mutant (Mb(3M)) was designed as part of efforts to shift the electron-transfer process to the simple docking (SD) regime, in which reactive binding occurs at a restricted, reactive region on the Mb surface that dominates the docked ensemble. An electrostatically-guided brownian dynamics (BD) docking protocol was used to generate an initial ensemble of reactive configurations of the complex between unrelaxed partners. This ensemble samples a broad and diverse array of heme-heme distances and orientations. These configurations seeded all-atom constrained molecular dynamics simulations (MD) to generate relaxed complexes for the calculation of electron tunneling matrix elements (T(DA)) through tunneling-pathway analysis. This procedure for generating an ensemble of relaxed complexes combines the ability of BD calculations to sample the large variety of available conformations and interprotein distances, with the ability of MD to generate the atomic level information, especially regarding the structure of water molecules at the protein-protein interface, that defines electron-tunneling pathways. We used the calculated T(DA) values to compute ET rates for the [Mb(wt), cyt b(5)] complex and for the complex with a mutant that has a binding free energy strengthened by three D/E → K charge-reversal mutations, [Mb(3M), cyt b(5)]. The calculated rate constants are in agreement with the measured values, and the mutant complex ensemble has many more geometries with higher T(DA) values than does the wild-type Mb complex. Interestingly, water plays a double role in this electron-transfer system, lowering the tunneling barrier as well as inducing protein interface remodeling that screens the repulsion between the negatively-charged propionates of the two hemes.

Faster interprotein electron transfer in a [myoglobin, b5] complex with a redesigned interface.

Direct measurements of electron transfer (ET) within a protein-protein complex with a redesigned interface formed by physiological partner proteins myoglobin (Mb) and cytochrome b(5) (b(5)) reveal interprotein ET rates comparable to those observed within the photosynthetic reaction center. Brownian dynamics simulations show that Mb in which three surface acid residues are mutated to lysine binds b(5) in an ensemble of configurations distributed around a reactive most-probable structure. Correspondingly, charge-separation ET from a photoexcited singlet zinc porphyrin incorporated within Mb to the heme of b(5) and the follow-up charge-recombination exhibit distributed kinetics, with median rate constants, k(f)(s) = 2.1 × 10(9) second(-1) and k(b)(s) = 4.3 × 10(10) second(-1), respectively. The latter approaches that for the initial step in photosynthetic charge separation, k = 3.3 × 10(11) second(-1).

Photoinitiated singlet and triplet electron transfer across a redesigned [myoglobin, cytochrome b5] interface.

We describe a strategy by which reactive binding of a weakly bound, 'dynamically docked (DD)' complex without a known structure can be strengthened electrostatically through optimized placement of surface charges, and discuss its use in modulating complex formation between myoglobin (Mb) and cytochrome b(5) (b(5)). The strategy employs paired Brownian dynamics (BD) simulations, one which monitors overall binding, the other reactive binding, to examine [X --> K] mutations on the surface of the partners, with a focus on single and multiple [D/E --> K] charge reversal mutations. This procedure has been applied to the [Mb, b(5)] complex, indicating mutations of Mb residues D44, D60, and E85 to be the most promising, with combinations of these showing a nonlinear enhancement of reactive binding. A novel method of displaying BD profiles shows that the 'hits' of b(5) on the surfaces of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) progressively coalesce into two 'clusters': a 'diffuse' cluster of hits that are distributed over the Mb surface and have negligible electrostatic binding energy and a 'reactive' cluster of hits with considerable stability that are localized near its heme edge, with short Fe-Fe distances favorable to electron transfer (ET). Thus, binding and reactivity progressively become correlated by the mutations. This finding relates to recent proposals that complex formation is a two-step process, proceeding through the formation of a weakly bound encounter complex to a well-defined bound complex. The design procedure has been tested through measurements of photoinitiated ET between the Zn-substituted forms of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) and Fe(3+)b(5). Both mutants convert the complex from the DD regime exhibited by Mb(WT), in which the transient complex is in fast kinetic exchange with its partners, k(off) >> k(et), to the slow-exchange regime, k(et) >> k(off), and both mutants exhibit rapid intracomplex ET from the triplet excited state to Fe(3+)b(5) (rate constant, k(et) approximately 10(6) s(-1)). The affinity constants of the mutant Mbs cannot be derived through conventional analysis procedures because intracomplex singlet ET quenching causes the triplet-ground absorbance difference to progressively decrease during a titration, but this effect has been incorporated into a new procedure for computing binding constants. Most importantly, these measurements reveal the presence of fast photoinduced singlet ET across the protein-protein interface, (1)k(et) approximately 2 x 10(8) s(-1).

Electrostatic redesign of the [myoglobin, cytochrome b5] interface to create a well-defined docked complex with rapid interprotein electron transfer.

Cyt b(5) is the electron-carrier "repair" protein that reduces met-Mb and met-Hb to their O(2)-carrying ferroheme forms. Studies of electron transfer (ET) between Mb and cyt b(5) revealed that they react on a "Dynamic Docking" (DD) energy landscape on which binding and reactivity are uncoupled: binding is weak and involves an ensemble of nearly isoenergetic configurations, only a few of which are reactive; those few contribute negligibly to binding. We set the task of redesigning the surface of Mb so that its reaction with cyt b(5) instead would occur on a conventional "simple docking" (SD) energy landscape, on which a complex exhibits a well-defined (set of) reactive binding configuration(s), with binding and reactivity thus no longer being decoupled. We prepared a myoglobin (Mb) triple mutant (D44K/D60K/E85K; Mb(+6)) substituted with Zn-deuteroporphyrin and monitored cytochrome b(5) (cyt b(5)) binding and electron transfer (ET) quenching of the (3)ZnMb(+6) triplet state. In contrast, to Mb(WT), the three charge reversals around the "front-face" heme edge of Mb(+6) have directed cyt b(5) to a surface area of Mb adjacent to its heme, created a well-defined, most-stable structure that supports good ET pathways, and apparently coupled binding and ET: both K(a) and k(et) are increased by the same factor of approximately 2 x 10(2), creating a complex that exhibits a large ET rate constant, k(et) = 10(6 1) s(-1), and is in slow exchange (k(off) << k(et)). In short, these mutations indeed appear to have created the sought-for conversion from DD to simple docking (SD) energy landscapes.

Kinetic-dynamic model for conformational control of an electron transfer photocycle: mixed-metal hemoglobin hybrids.

It is becoming increasingly clear that the transfer of an electron across a protein-protein interface is coupled to the dynamics of conformational conversion between and within ensembles of interface conformations. Electron transfer (ET) reactions in conformationally mobile systems provide a "clock" against which the rapidity of a dynamic process may be measured, and we here report a simple kinetic (master equation) model that self-consistently incorporates conformational dynamics into an ET photocycle comprised of a photoinitiated "forward" step and thermal return to ground. This kinetic/dynamic (KD) model assumes an ET complex exists as multiple interconverting conformations which partition into an ET-optimized (reactive; R) population and a less-reactive population ( S). We take the members of each population to be equivalent by constraining them to have the same conformational energy, the same average rate constant for conversion to members of the other population, and the same rate constants for forward and back ET. The result is a mapping of a complicated energy surface onto the simple "gating", two-well surface, but with rate constants that are defined microscopically. This model successfully describes the changes in the ET photocycle within the "predocked" mixed-metal hemoglobin (Hb) hybrid, [alpha(Zn), beta(Fe3+N 3 (-))], as conformational kinetics are modulated by variations in viscosity (eta = 1-15 cP; 20 degrees C). The description reveals how the conformational "routes" by which a hybrid progresses through a photocycle differ in different dynamic regimes. Even at eta = 1 cP, the populations are not in fast exchange, and ET involves a complex interplay between conformational and ET processes; at intermediate viscosities the hybrid exhibits "differential dynamics" in which the forward and back ET processes involve different initial ensembles of configurational substates; by eta = 15 cP, the slow-exchange limit is approached. Even at low viscosity, the ET-coupled motions are fairly slow, with rate constants of <10 (3) s (-1). Current ideas about Hb function lead to the testable hypothesis that ET in the hybrid may be coupled to allosteric fluctuations of the two [alpha 1, beta 2] dimers of Hb.

Dynamic docking of cytochrome b5 with myoglobin and alpha-hemoglobin: heme-neutralization "squares" and the binding of electron-transfer-reactive configurations.

Intracomplex electron transfer (ET) occurs most often in intrinsically transient, low affinity complexes. As a result, the means by which adequate specificity and reactivity are obtained to support effective ET is still poorly understood. We report here on two such ET complexes: cytochrome b5 (cyt b5) in reaction with its physiological partners, myoglobin (Mb) and hemoglobin (Hb). These complexes obey the Dynamic Docking (DD) paradigm: a large ensemble of weakly bound protein-protein configurations contribute to binding in the rapid-exchange limit, but only a few are ET-active. We report the ionic-strength dependence of the second-order rate constant, k2, for photoinitiated ET from within all four combinations of heme-neutralized Zn deuteroporphyrin-substituted Mb/alphaHb undergoing ET with cyt b5, the four "corners" of a "heme-neutralization square". These experiments provide insights into the relative importance of both global and local electrostatic contributions to the binding of reactive configurations, which are too few to be observed directly. To interpret the variations of k2 arising from heme neutralization, we have developed a procedure by which comparisons of the ET rate constants for a heme-neutralization square permit us to decompose the free energy of reactive binding into individual local electrostatic contributions associated with interactions between (i) the propionates of the two hemes and (ii) the heme of each protein with the polypeptide of its partner. Most notably, we find the contribution from the repulsion between propionates of partner hemes to the reactive binding free energy to be surprisingly small, DeltaG(Hb) approximately +1 kcal/mol at ambient temperature, 18 mM ionic strength, and we speculate about possible causes of this observation. To confirm the fundamental assumption of these studies, that the structure of a heme-neutralized protein is unaltered either by substitution of Zn or by heme neutralization, we have obtained the X-ray structure of ZnMb prepared with the porphyrin dimethyl ester and find it to be nearly isostructural with the native protein.

NMR spectroscopy can characterize proteins encapsulated in a sol-gel matrix.

Proteins encapsulated within sol-gel matrices (SG) have the potential to fill many scientific and technological roles, but these applications are hindered by the limited means of probing possible structural consequences of encapsulation. We here present the first demonstration that it is possible to obtain high-resolution, solution NMR measurements of proteins encapsulated within a SG matrix. With the aim of determining the breadth of this approach, we have encapsulated three paramagnetic proteins with different overall charges: the highly acidic human Fe3+ cytochrome b5 (cyt b5); the highly basic horse heart cytochrome c (cyt c); and the nearly neutral, sperm whale cyanomet-myoglobin. The encapsulated anionic and neutral proteins (cyt b5; myoglobin) undergo essentially free rotation, but show minor conformational perturbations as revealed by shifts of contact-shifted peaks associated with the heme and nearby amino acids.

Equilibrium/nonequilibrium initial configurations in forward/reverse electron transfer within mixed-metal hemoglobin hybrids.

In a protein-protein electron transfer (ET) photocycle, the "forward" ET reaction is initiated with the excited complex, [3DA], in an equilibrium ensemble of configurations, the majority of which exhibit less than the maximal ET matrix element. In contrast, the charge-separated intermediate complex is formed in a nonequilibrium set of configurations with maximal ET matrix elements and would be expected to return to the ground state with the largest rate constant possible unless conformational interconversion first "breaks the connection" and the complex converts to less-reactive substates. According to this analysis, the forward and back ET reactions should show a differential response to viscosity, and the latter could even show an increased rate constant under conditions which suppress departure from the reactive configuration(s). We now report that the viscosity dependences of forward and back ET rate constants for the photocycle within the [alpha2(Zn),beta2(Fe3+N3-)] mixed-metal hemoglobin hybrid at pH 7 show the anticipated behavior: kf decreases as viscosity increases, but, in sharp contrast, kb increases strongly.

Hopping in the electron-transfer photocycle of the 1:1 complex of Zn-cytochrome c peroxidase with cytochrome c.

The physiological electron-transfer (ET) partners, cytochrome c peroxidase (CcP) and cytochrome c (Cc)1, can be modified to exhibit photoinitiated ET through substitution of Zn (or Mg) for Fe in either partner. Laser excitation of the Zn-porphyrin (ZnP) to its triplet excited state (3ZnP) initiates direct heme-heme ET to the ferriheme center of its partner across the protein-protein interface. This photoinitiated ET produces the charge-separated intermediate, I = [ZnP+CcP, Fe2+Cc], with a metalloporphyrin pi-cation radical (ZnP+) in the donor protein and a ferroheme acceptor protein. I, in general, is thought to return to the ground state by a thermal ET process that involves direct heme-heme back-ET to complete a simple photocycle. We here contrast intracomplex ET between yeast iso-1 Cc and ZnCcP(WT) (wild-type) with that for two ZnCcP(X) variants: X = W191F, with redox-active W191 replaced by Phe; WYM4, a W191F mutant with further replacement of four other potentially redox-active sites (W51F, Y187F, Y229F, and Y236F). The results show that W191 acts as an ET mediator, which "short-circuits" the direct heme-heme back-ET through a two-step, hopping process in which the ZnP+ cation radical formed by photoinitiated ET rapidly oxidizes W191, and the resultant W191+, in turn, rapidly oxidizes Fe2+Cc.

Differential influence of dynamic processes on forward and reverse electron transfer across a protein-protein interface.

We propose that the forward and reverse halves of a flash-induced protein-protein electron transfer (ET) photocycle should exhibit differential responses to dynamic interconversion of configurations when the most stable configuration is not the most reactive, because the reactants exist in different initial configurations: the flash-photoinitiated forward ET process begins with the protein partners in an equilibrium ensemble of configurations, many of which have little or no reactivity, whereas the reactant of the thermal back ET (the charge-separated intermediate) is formed in a nonequilibrium, "activated" protein configuration. We report evidence for this proposal in measurements on (i) mixed-metal hemoglobin hybrids, (ii) the complex between cytochrome c peroxidase and cytochrome c, and (iii and iv) the complexes of myoglobin and isolated hemoglobin alpha-chains with cytochrome b(5). For all three systems, forward and reverse ET does respond differently to modulation of dynamic processes; further, the response to changes in viscosity is different for each system.

Electrostatic influence on rotational mobilities of sol-gel-encapsulated solutes by NMR and EPR spectroscopies.

The rotational mobilities of small solute molecules encapsulated in tetramethyl orthosilicate (TMOS) sol-gels have been investigated by EPR spectroscopy of encapsulated nitroxide probes and by high-resolution NMR spectroscopic measurements of transferred NOE's (trNOE's), of T(1)'s, and of T(1)'s in the rotating frame (T(1)rho). The two spectroscopic methods are sensitive to motions on different time scales and hence, are nicely complementary. Suites of neutral, positively, and negatively charged nitroxide probes (EPR) and of simple diamagnetic small molecules (NMR) were selected to disclose influences of electrostatic interactions with the sol-gel walls and to probe the presence of multiple populations of molecules in distinct regions of the sol-gel pores. For neutral and negatively charged solute probes, both techniques disclose a single population with a significantly increased average rotational correlation time, which we interpret at least in part as resulting from exchange between free-volume and transiently immobilized surface populations. The electrostatic attraction between cationic probes and the negatively charged sol-gel walls causes the positively charged probes to be more effectively immobilized and/or causes a greater percentage of probes to undergo this transient immobilization. The EPR spectra directly disclose a population of cationic probes which are immobilized on the X-band EPR time scale: tau(c) greater than or approximately equal 10(-7) s. However, NMR measurements of trNOE's and of T(1)rho demonstrate that this population does exchange with the free-volume probes on the slower time scale of NMR. This approach is equally applicable to the study of solutes within other types of confined spaces, as well.

Conformational substates of the oxyheme centers in alpha and beta subunits of hemoglobin as disclosed by EPR and ENDOR studies of cryoreduced protein.

Exposure of frozen solutions of oxyhemoglobin to gamma-irradiation at 77 K yields EPR- and ENDOR-active, one-electron-reduced oxyheme centers which retain the conformation of the diamagnetic precursor. EPR spectra have been collected for the centers produced in human HbO(2) and isolated alphaO(2) and betaO(2) chains, as well as alphaO(2)beta(Zn), alpha(Zn)betaO(2), and alphaO(2)beta(Fe(3+)) hybrids, each in frozen buffer and in frozen glasses that form in the presence of glycols and sugars and also in the presence of IHP. These reveal two spectroscopically distinct classes of such ferriheme centers (g(1)

Dynamic docking and electron-transfer between cytochrome b5 and a suite of myoglobin surface-charge mutants. Introduction of a functional-docking algorithm for protein-protein complexes.

Horse myoglobin (Mb) provides a convenient "workbench" for probing the effects of electrostatics on binding and reactivity in the dynamic [Mb, cytochrome b(5)] electron-transfer (ET) complex. We have combined mutagenesis and heme neutralization to prepare a suite of six Mb surface-charge variants: the [S92D]Mb and [V67R]Mb mutants introduce additional charges on the "front" face, and incorporation of the heme di-ester into each of these neutralizes the charge on the heme propionates which further increases the positive charge on the "front" face. For this set of mutants, the nominal charge of Mb changes by -1 to +3 units relative to that for native Mb. For each member of this set, we have measured the bimolecular quenching rate constant (k(2)) for the photoinitiated (3)ZnDMb --> Fe(3+)b(5) ET reaction as a function of ionic strength. We find: (i) a dramatic decoupling of binding and reactivity, in which k(2) varies approximately 10(3)-fold within the suite of Mbs without a significant change in binding affinity; (ii) the ET reaction occurs within the "thermodynamic" or "rapid exchange" limit of the "Dynamic Docking" model, in which a large ensemble of weakly bound protein-protein configurations contribute to binding, but only a few are reactive, as shown by the fact that the zero-ionic-strength bimolecular rate constant varies exponentially with the net charge on Mb; (iii) Brownian dynamic docking profiles allow us to visualize the microscopic basis of dynamic docking. To describe these results we present a new theoretical approach which mathematically combines PATHWAY donor/acceptor coupling calculations with Poisson-Boltzmann-based electrostatics estimates of the docking energetics in a Monte Carlo (MC) sampling framework that is thus specially tailored to the intermolecular ET problem. This procedure is extremely efficient because it targets only the functionally active complex geometries by introducing a "reactivity filter" into the computations themselves, rather than as a subsequent step. This efficiency allows us to employ more computationally expensive and accurate methods to describe the relevant intermolecular interaction energies and the protein-mediated donor/acceptor coupling interactions. It is employed here to compute the changes in the bimolecular rate constant for ET between Mb and cyt b(5) upon variations in the myoglobin surface charge, pH, and ionic strength.