Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction.

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications.

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XHn in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H2), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H2) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Ecosystem warming extends vegetation activity but heightens vulnerability to cold temperatures.

Shifts in vegetation phenology are a key example of the biological effects of climate change1-3. However, there is substantial uncertainty about whether these temperature-driven trends will continue, or whether other factors-for example, photoperiod-will become more important as warming exceeds the bounds of historical variability4,5. Here we use phenological transition dates derived from digital repeat photography6 to show that experimental whole-ecosystem warming treatments7 of up to +9 °C linearly correlate with a delayed autumn green-down and advanced spring green-up of the dominant woody species in a boreal Picea-Sphagnum bog. Results were confirmed by direct observation of both vegetative and reproductive phenology of these and other bog plant species, and by multiple years of observations. There was little evidence that the observed responses were constrained by photoperiod. Our results indicate a likely extension of the period of vegetation activity by 1-2 weeks under a 'CO2 stabilization' climate scenario (+2.6 ± 0.7 °C), and 3-6 weeks under a 'high-CO2 emission' scenario (+5.9 ± 1.1 °C), by the end of the twenty-first century. We also observed severe tissue mortality in the warmest enclosures after a severe spring frost event. Failure to cue to photoperiod resulted in precocious green-up and a premature loss of frost hardiness8, which suggests that vulnerability to spring frost damage will increase in a warmer world9,10. Vegetation strategies that have evolved to balance tradeoffs associated with phenological temperature tracking may be optimal under historical climates, but these strategies may not be optimized for future climate regimes. These in situ experimental results are of particular importance because boreal forests have both a circumpolar distribution and a key role in the global carbon cycle11.

An Investigation of the Influence of Tyrosine Local Interactions on Electron Hopping in a Model Protein.

Multi-step electron transfer reactions are important to the function of many cellular systems. The ways in which such systems have evolved to direct electrons along specific pathways are largely understood, but less so are the ways in which the reduction-oxidation potentials of individual redox sites are controlled. We prepared a series of three new artificial variants of Pseudomonas aeruginosa azurin where a tyrosine (Tyr109) is situated between the native Cu ion and a Ru(II) photosensitizer tethered to a histidine (His107). Arginine, glutamine, or methionine were introduced as position 122, which is near to Tyr109. We investigated the rate of CuI oxidation by a flash-quench generated Ru(III) oxidant over pH values from 5 to 9. While the identity of the residue at position 122 affects some of the physical properties of Tyr109, the rates of CuI oxidation are only weakly dependent on the identity of the residue at 122. The results highlight that more work is still needed to understand how non-covalent interactions of redox active groups are affected in redox proteins.

Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees.

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2 ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller gs , but greater Anet , responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs . The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction.

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

A heme•DNAzyme activated by hydrogen peroxide catalytically oxidizes thioethers by direct oxygen atom transfer rather than by a Compound I-like intermediate.

Hemin [Fe(III)-protoporphyrin IX] is known to bind tightly to single-stranded DNA and RNA molecules that fold into G-quadruplexes (GQ). Such complexes are strongly activated for oxidative catalysis. These heme•DNAzymes and ribozymes have found broad utility in bioanalytical and medicinal chemistry and have also been shown to occur within living cells. However, how a GQ is able to activate hemin is poorly understood. Herein, we report fast kinetic measurements (using stopped-flow UV-vis spectrophotometry) to identify the H2O2-generated activated heme species within a heme•DNAzyme that is active for the oxidation of a thioether substrate, dibenzothiophene (DBT). Singular value decomposition and global fitting analysis was used to analyze the kinetic data, with the results being consistent with the heme•DNAzyme's DBT oxidation being catalyzed by the initial Fe(III)heme-H2O2 complex. Such a complex has been predicted computationally to be a powerful oxidant for thioether substrates. In the heme•DNAzyme, the DNA GQ enhances both the kinetics of formation of the active intermediate as well as the oxidation step of DBT by the active intermediate. We show, using both stopped flow spectrophotometry and EPR measurements, that a classic Compound I is not observable during the catalytic cycle for thioether sulfoxidation.

Contemporary Strategies for Immobilizing Metallophthalocyanines for Electrochemical Transformations of Carbon Dioxide.

Metallophthalocyanine (PcM) coordination complexes are well-known mediators of the electrochemical reduction of carbon dioxide (CO2). They have many properties that show promise for practical applications in the energy sector. Such properties include synthetic flexibility, a high stability, and good efficiencies for the reduction of CO2 to useful feedstocks, such as carbon monoxide (CO). One of the ongoing challenges that needs to be met is the incorporation of PcM into the heterogeneous materials that are used in a great many CO2-reduction devices. Much progress has been made in the last decade and there are now several promising approaches to incorporate PcM into a range of materials, from simple carbon-adsorbed preparations to extended polymer networks. These approaches all have important advantages and drawbacks. In addition, investigations have led to new proposals regarding CO2 reduction catalytic cycles and other operational features that are crucial to function. Here, we describe developments in the immobilization of PcM CO2 reduction catalysts in the last decade (2013 to 2023) and propose promising avenues and strategies for future research.

The Impact of Second Coordination Sphere Methionine-Aromatic Interactions in Copper Proteins.

The interplay between the primary and secondary coordination spheres in biological metal sites plays an essential role in controlling their properties. Some of the clearest examples of this are from copper sites in blue and purple copper proteins. Many such proteins contain methionine (Met) in the primary coordination sphere as a weakly bound ligand to Cu. While the effects of replacing the coordinated Met are understood, less so is the importance of its second-sphere interactions. In this combined informatics and experimental study, we first present a bioinformatics investigation of the second-sphere environments in biological Met-Cu motifs. The most common interaction is between the Met-CH3 and the π-face of a phenylalanine (Phe) (81% of surveyed structures), tyrosine (Tyr) (11%), and tryptophan (Trp) (8%). In most cases, the Met-CH3 also forms a contact with a π-face of one of a Cu-ligating histidine-imidazole. Such interactions are widely distributed in different Cu proteins. Second, to explore the impact of the second-sphere interactions of Met, a series of artificial Pseudomonas aeruginosa azurin proteins were produced where the native Phe15 was replaced with Tyr or Trp. The proteins were characterized using optical and magnetic resonance spectroscopies, X-ray diffraction, electrochemistry, and an investigation of the time-resolved electron-transfer kinetics of photosensitizer-modified proteins. The influence of the Cu-Met-Aro interaction on azurin's physical properties is subtle, and the hallmarks of the azurin blue copper site are maintained. In the Phe15Trp variant, the mutation to Phe15 induces changes in Cu properties that are comparable to replacement of the weak Met ligand. The broader impacts of these widely distributed interactions are discussed.

Hydraulically-vulnerable trees survive on deep-water access during droughts in a tropical forest.

Deep-water access is arguably the most effective, but under-studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep-water access may delay plant dehydration. Here, we tested the role of deep-water access in enabling survival within a diverse tropical forest community in Panama using a novel data-model approach. We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990-2015) to vapor pressure deficit, water potentials in the whole-soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water-access depths. Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981-2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress-related mortality risk through deep-water access. The role of deep-water access in mitigating mortality of hydraulically-vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

Warming induces divergent stomatal dynamics in co-occurring boreal trees.

Climate warming will alter photosynthesis and respiration not only via direct temperature effects on leaf biochemistry but also by increasing atmospheric dryness, thereby reducing stomatal conductance and suppressing photosynthesis. Our knowledge on how climate warming affects these processes is mainly derived from seedlings grown under highly controlled conditions. However, little is known regarding temperature responses of trees growing under field settings. We exposed mature tamarack and black spruce trees growing in a peatland ecosystem to whole-ecosystem warming of up to +9°C above ambient air temperatures in an ongoing long-term experiment (SPRUCE: Spruce and Peatland Responses Under Changing Environments). Here, we report the responses of leaf gas exchange after the first two years of warming. We show that the two species exhibit divergent stomatal responses to warming and vapor pressure deficit. Warming of up to 9°C increased leaf N in both spruce and tamarack. However, higher leaf N in the warmer plots translate into higher photosynthesis in tamarack but not in spruce, with photosynthesis being more constrained by stomatal limitations in spruce than in tamarack under warm conditions. Surprisingly, dark respiration did not acclimate to warming in spruce, and thermal acclimation of respiration was only seen in tamarack once changes in leaf N were considered. Our results highlight how warming can lead to differing stomatal responses to warming in co-occurring species, with consequent effects on both vegetation carbon and water dynamics.

Divergent species-specific impacts of whole ecosystem warming and elevated CO2 on vegetation water relations in an ombrotrophic peatland.

Boreal peatland forests have relatively low species diversity and thus impacts of climate change on one or more dominant species could shift ecosystem function. Despite abundant soil water availability, shallowly rooted vascular plants within peatlands may not be able to meet foliar demand for water under drought or heat events that increase vapor pressure deficits while reducing near surface water availability, although concurrent increases in atmospheric CO2 could buffer resultant hydraulic stress. We assessed plant water relations of co-occurring shrub (primarily Rhododendron groenlandicum and Chamaedaphne calyculata) and tree (Picea mariana and Larix laricina) species prior to, and in response to whole ecosystem warming (0 to +9°C) and elevated CO2 using 12.8-m diameter open-top enclosures installed within an ombrotrophic bog. Water relations (water potential [Ψ], turgor loss point, foliar and root hydraulic conductivity) were assessed prior to treatment initiation, then Ψ and peak sap flow (trees only) assessed after 1 or 2 years of treatments. Under the higher temperature treatments, L. laricina Ψ exceeded its turgor loss point, increased its peak sap flow, and was not able to recover Ψ overnight. In contrast, P. mariana operated below its turgor loss point and maintained constant Ψ and sap flow across warming treatments. Similarly, C. calyculata Ψ stress increased with temperature while R. groenlandicum Ψ remained at pretreatment levels. The more anisohydric behavior of L. laricina and C. calyculata may provide greater net C uptake with warming, while the more conservative P. mariana and R. groenlandicum maintained greater hydraulic safety. These latter species also responded to elevated CO2 by reduced Ψ stress, which may also help limit hydraulic failure during periods of extreme drought or heat in the future. Along with Sphagnum moss, the species-specific responses of peatland vascular communities to drier or hotter conditions will shape boreal peatland composition and function in the future.

The physiological acclimation and growth response of Populus trichocarpa to warming.

Plant metabolic acclimation to thermal stress remains underrepresented in current global climate models. Gaps exist in our understanding of how metabolic processes (i.e., photosynthesis, respiration) acclimate over time and how aboveground versus belowground acclimation differs. We measured the thermal acclimation of Populus trichocarpa, comparing aboveground versus belowground physiology over time. Ninety genetically identical ramets were propagated in mesocosms that separated root and microbial components. After establishment at 25°C for 6 weeks, 60 clones were warmed +4 or +8°C and monitored for 10 weeks, measuring photosynthesis (A), leaf respiration (R), soil respiration (Rs ), root plus soil respiration (Rs+r ), and root respiration (Rr ). We observed thermal acclimation in both A and R, with rates initially increasing, then declining as the thermal photosynthetic optimum (Topt ) and the temperature-sensitivity (Q10 ) of respiration adjusted to warmer conditions. Photosynthetic acclimation was constructive, based on an increase in both Topt and peak A. Belowground, Rs+r decreased linearly with warming, while Rs rates declined abruptly, then remained constant with additional warming. Plant biomass was greatest at +4°C, with 30% allocated belowground. Rates of mass-based Rr were similar among treatments; however, root nitrogen declined at +8°C leading to less mass nitrogen-based Rr in that treatment. The Q10 -temperature relationship of Rr was affected by warming, leading to differing values among treatments. Aboveground acclimation exceeded belowground acclimation, and plant nitrogen-use mediated the acclimatory response. Results suggest that moderate climate warming (+4°C) may lead to acclimation and increased plant biomass production but increases in production could be limited with severe warming (+8°C).

Fluctuating hydrogen-bond networks govern anomalous electron transfer kinetics in a blue copper protein.

We combine experimental and computational methods to address the anomalous kinetics of long-range electron transfer (ET) in mutants of Pseudomonas aeruginosa azurin. ET rates and driving forces for wild type (WT) and three N47X mutants (X = L, S, and D) of Ru(2,2'-bipyridine)2 (imidazole)(His83) azurin are reported. An enhanced ET rate for the N47L mutant suggests either an increase of the donor-acceptor (DA) electronic coupling or a decrease in the reorganization energy for the reaction. The underlying atomistic features are investigated using a recently developed nonadiabatic molecular dynamics method to simulate ET in each of the azurin mutants, revealing unexpected aspects of DA electronic coupling. In particular, WT azurin and all studied mutants exhibit more DA compression during ET (>2 Å) than previously recognized. Moreover, it is found that DA compression involves an extended network of hydrogen bonds, the fluctuations of which gate the ET reaction, such that DA compression is facilitated by transiently rupturing hydrogen bonds. It is found that the N47L mutant intrinsically disrupts this hydrogen-bond network, enabling particularly facile DA compression. This work, which reveals the surprisingly fluctional nature of ET in azurin, suggests that hydrogen-bond networks can modulate the efficiency of long-range biological ET.

Controlling the Oxygen Reduction Selectivity of Asymmetric Cobalt Porphyrins by Using Local Electrostatic Interactions.

The development and improvement of electrocatalysts for the 4H+/4e- reduction of O2 to H2O is an ongoing challenge. The addition of ancillary groups (e.g., hydrogen bonding, Brønsted acid/base) near the active site of metal-containing catalysts is an effective way to improve selectivity and kinetics of the oxygen reduction reaction (ORR). In this regard, iron porphyrins are among the most researched ORR catalysts. Closely related cobalt porphyrin ORR catalysts can function closer to the O2/H2O thermodynamic potential, but they tend to be less selective and follow a different mechanism than for the iron porphyrins. Herein, we explore strategies to extend the ideas about ancillary groups that have been developed for iron porphyrin ORR electrocatalysts to improve the performance of the corresponding cobalt complexes. We describe a series of porphyrin electrocatalysts that are modified versions of Co(5,10,15,20-tetraphenylporphyrin), where the 2-position of one of the phenyl groups contains -NH2, -N(CH3)2, and -N(CH3)3+. Investigations using cyclic voltammetry and hydrodynamic electrochemistry show that the presence of a cationic ancillary group gives rise to a catalyst that is selective for the conversion of O2 to H2O across a wide pH range. In contrast, the other catalysts are selective for reduction of O2 to H2O at pH 0, but produce H2O2 at higher pH. The ORR rate (∼106 M-1 s-1) and selectivity of the -N(CH3)3+-modified catalyst are invariant between pH 0 and 7. Quantum chemical calculations support the hypothesis that the enhancement of selectivity can be attributed to the distinct mechanism of O2 reduction by Co-porphyrins. Specifically, the mechanism relies on anionic, peroxide-bound intermediates. While protic ancillary groups are important in the performance of iron porphyrin ORR catalysts, we suggest that electrostatic stabilizers of O2-bound intermediates are more crucial for cobalt porphyrin ORR catalysts.

Protein Paper from Exfoliated Eri Silk Nanofibers.

The exfoliation of silk fiber is an attractive method to produce silk micro- and nanofibers that retain the secondary structure of native silk. However, most fibrillation methods used to date require the use of toxic and/or expensive solvents and the use of high energy. This study describes a low cost, scalable method to produce microfibrillated silk nanofibers without the use of toxic chemicals by controlling the application of shear using commercially scalable milling and homogenization equipment. Manipulation of the degumming conditions (alkaline concentration and degumming temperature) and the shear in milling and/or homogenization enabled control over the degree of fibrillation. The microfibrillated silk was then characterized to determine structural change during processing and the stability of the resulting suspensions at different pH. Silk nanofibers obtained from milling degummed silk were characterized using atomic force microscopy. Nanofibers obtained both with and without high-pressure homogenization were then used to produce silk "protein paper" through casting. Silk degumming conditions played a critical role in determining the degree of microfibrillation and the properties of the cast silk papers. The silk papers produced from homogenized nanofibers showed excellent mechanical properties, high water absorption, and wicking properties. The silk papers were excellent for supporting the attachment and growth of human skin keratinocytes, demonstrating application possibilities in healthcare such as wound healing.

Performance of Anti-Topoisomerase I Antibody Testing by Multiple-Bead, Enzyme-Linked Immunosorbent Assay and Immunodiffusion in a University Setting.

BACKGROUND/OBJECTIVE: The criterion standard for anti-topoisomerase I antibody (anti-topo I antibody) testing in systemic sclerosis (SSc) uses immunodiffusion (ID) techniques, but enzyme-linked immunosorbent assay (ELISA) and multiple-bead technology are often used in current settings to save time and cost. Our aim was to assess the performance of the multiple-bead assay, ELISA, and ID testing methods. METHODS: We conducted a retrospective study of patients at the University of Michigan whose extractable nuclear antigen 10 autoantibody panel tested positive for the anti-topo I antibody by multiple-bead technology during a 1-year period. All samples positive by multiple-bead assay were sent to the RDL Laboratories and reflexed for ELISA, and all anti-topo I antibodies positive by ELISA were further tested by ID. Clinical data were reviewed by a rheumatologist and assessed for presence of SSc. Data were analyzed via frequency tables. RESULTS: Approximately 9500 extractable nuclear antigen 10 panels were ordered by physicians at the University of Michigan. Of these, 129 patients were positive for the anti-topo I antibody by multiple-bead assay, 51 were positive by multiple-bead assay and ELISA, and 21 were positive by multiple-bead assay, ELISA, and ID. We found that 26.4% of patients positive by multiple-bead assay, 47.1% positive by multiple-bead assay and ELISA, and 95.2% positive by multiple-bead assay, ELISA, and ID had SSc. CONCLUSIONS: Multiple-bead assays have a high rate of false-positive results for the anti-topo I antibody in patients without clinical evidence of SSc. A stepwise approach of confirmation of positive multiple-bead assay results using both ELISA and ID improves the predictive value of antibody testing for the diagnosis of SSc.

Acclimation and adaptation components of the temperature dependence of plant photosynthesis at the global scale.

The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common-garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

The interaction between methionine and two aromatic amino acids is an abundant and multifunctional motif in proteins.

Many types of non-covalent interactions give rise to a protein's natural structure and function. One such interaction involves an aromatic amino acid (phenylalanine (Phe), tryptophan (Trp), or tyrosine (Tyr)) and the sulfur of methionine (Met), the so-called methionine-aromatic interaction. The Met-aromatic interaction is well-established, and it is defined as involving one aromatic and one Met residue. However, in a small-scale survey, we recently noted that more than one aromatic residue can interact with one Met in a "bridging" motif of the general form Aro-Met-Aro. In the present work, a systematic survey of all protein structures available in the Protein Data Bank was carried out. About 70% of those structures contain any Met-aromatic interaction and over 40% contain a Met-aromatic bridge. Analysis of a smaller subset of protein structures, which omits entries with low resolution or high sequence homology, shows the same distribution. The relationship of bridging interactions and longer aromatic amino acid chains also was explored using network theory approaches. Met-Aro bridges were found in 8.4% of extended aromatic chains. Analysis of a different subset of proteins that contain embedded metal ions as reference points revealed that many Met-Aro bridges are at/near protein surfaces. These analyses, and some specific examples, lead to the proposal that Met-aromatic bridges play biological roles as stabilizers and protectors of protein structures, motifs for molecular recognition, and electron transfer mediators.