Chirality Dependent Charge Transfer Rate in Oligopeptides.

It is shown that "spontaneous magnetization" occurs when chiral oligopeptides are attached to ferrocene and are self-assembled on a gold substrate. As a result, the electron transfer, measured by electrochemistry, shows asymmetry in the reduction and oxidation rate constants; this asymmetry is reversed between the two enantiomers. The results can be explained by the chiral induced spin selectivity of the electron transfer. The measured magnetization shows high anisotropy and the "easy axis" of magnetization is along the molecular axis.

The use of HeRo catheter in catheter-dependent dialysis patients with superior vena cava occlusion.

OBJECTIVES: Hemodialysis (HD) patients with superior vena cava (SVC) occlusion have limited access options. Femoral access is commonly employed but is associated with high complication rates. Hemodialysis Reliable Outflow (HeRO) catheters can be used in tunneled catheter-dependent (TCD) patients who have exhausted other access options. The HeRO graft bypasses occlusion and traverses stenosis with outflow directly into the central venous circulation. At our institution we have used the inside-out central venous access technique (IOCVA) to traverse an occluded vena cava for HeRO graft placement. We review our experience with this technique. METHODS: A retrospective chart review was conducted of patients with HeRO graft placement at our institution. All were dependent on a tunneled femoral dialysis catheter due to central venous occlusion (CVO). The IOCVA technique was used in each case. This technique was used as last resort for patients who had no other dialysis access option. Demographics, patency rates, complications, and mortality were recorded. RESULTS: A total of 11 HeRO grafts were placed in 11 patients from January 2012 to June 2013, with 100% technical success rate. Three grafts were ligated due to steal syndrome. Two grafts were lost due to thrombosis. Five of 11 patients experienced a 30-day complication. Three patients died within the follow-up period; however, none were directly related to the graft placement. Follow up range was 65-573 days; 5 of 11 grafts were used for dialysis at the end of the follow-up period. The 12-month patency rate was 30%. CONCLUSIONS: HeRO grafts are one option for dialysis patients with CVO. There is, however, a high incidence of steal syndrome and other complications. These grafts should be offered as a final potential alternative to catheter dependence.

A three-step kinetic model for electrochemical charge transfer in the hopping regime.

Single-step nonadiabatic electron tunneling models are widely used to analyze electrochemical rates through self-assembled monolayer films (SAMs). For some systems, such as nucleic acids, long-range charge transfer can occur in a "hopping" regime that involves multiple charge transfer events and intermediate states. This report describes a three-step kinetic scheme to model charge transfer in this regime. Some of the features of the three-step model are probed experimentally by changing the chemical composition of the SAM. This work uses the three-step model and a temperature dependence of the charge transfer rate to extract the charge injection barrier for a SAM composed of a 10-mer peptide nucleic acid that operates in the hopping regime.

The single-molecule conductance and electrochemical electron-transfer rate are related by a power law.

This study examines quantitative correlations between molecular conductances and standard electrochemical rate constants for alkanes and peptide nucleic acid (PNA) oligomers as a function of the length, structure, and charge transport mechanism. The experimental data show a power-law relationship between conductances and charge transfer rates within a given class of molecules with the same bridge chemistry, and a lack of correlation when a more diverse group of molecules is compared, in contrast with some theoretical predictions. Surprisingly, the PNA duplexes exhibit the lowest charge-transfer rates and the highest molecular conductances. The nonlinear rate-conductance relationships for structures with the same bridging chemistries are attributed to differences in the charge-mediation characteristics of the molecular bridge, energy barrier shifts and electronic dephasing, in the two different experimental settings.

Use of MRI in preoperative planning for women with newly diagnosed DCIS: risk or benefit?

BACKGROUND: The role of magnetic resonance imaging (MRI) in preoperative planning for women diagnosed with breast cancer remains controversial. The risks and benefits in women with newly diagnosed ductal carcinoma in situ (DCIS) are largely unknown. PATIENTS AND METHODS: Retrospective chart review comparing women treated for DCIS who did and did not undergo MRI for preoperative planning. End points included number of additional biopsies prompted by MRI, surgical reexcision rates, weight of excisions, mastectomy rates, and conversion to mastectomy after attempted breast conservation. RESULTS: 218 patients met study criteria. Sixty-four patients did not undergo preoperative MRI, and 154 patients did. There was no statistically significant difference (P = not significant, NS) in reexcision rates between the 34.1 % (42/123) of women who did and 20/51 (39.2 %) women who did not undergo MRI. Despite use of preoperative MRI, 11/123 women (8.9 %) were converted to mastectomy due to positive margins compared with 4/51 (7.8 %) in the women who did not undergo MRI (P = NS). In women undergoing MRI, average weight of excision at definitive surgery was 49.5 g, while in women who did not undergo MRI, average weight of excision at definitive surgery was 48.7 g. CONCLUSIONS: Our data show that MRI does not significantly decrease reexcision rates or conversion to mastectomy after attempted breast-conservation surgery. Based on our findings, we do not believe preoperative MRI adds benefit to the care of this patient population. Prospective trials are necessary to further investigate the risks and benefits of preoperative MRI in women with DCIS.

Effect of backbone flexibility on charge transfer rates in peptide nucleic acid duplexes.

Charge transfer (CT) properties are compared between peptide nucleic acid structures with an aminoethylglycine backbone (aeg-PNA) and those with a γ-methylated backbone (γ-PNA). The common aeg-PNA is an achiral molecule with a flexible structure, whereas γ-PNA is a chiral molecule with a significantly more rigid structure than aeg-PNA. Electrochemical measurements show that the CT rate constant through an aeg-PNA bridging unit is twice the CT rate constant through a γ-PNA bridging unit. Theoretical calculations of PNA electronic properties, which are based on a molecular dynamics structural ensemble, reveal that the difference in the CT rate constant results from the difference in the extent of backbone fluctuations of aeg- and γ-PNA. In particular, fluctuations of the backbone affect the local electric field that broadens the energy levels of the PNA nucleobases. The greater flexibility of the aeg-PNA gives rise to more broadening, and a more frequent appearance of high-CT rate conformations than in γ-PNA.

Charge transfer through modified peptide nucleic acids.

We studied the charge transfer properties of bipyridine-modified peptide nucleic acid (PNA) in the absence and presence of Zn(II). Characterization of the PNA in solution showed that Zn(II) interacts with the bipyridine ligands, but the stability of the duplexes was not affected significantly by the binding of Zn(II). The charge transfer properties of these molecules were examined by electrochemistry for self-assembled monolayers of ferrocene-terminated PNAs and by conductive probe atomic force microscopy for cysteine-terminated PNAs. Both electrochemical and single molecular studies showed that the bipyridine modification and Zn(II) binding do not affect significantly the charge transfer of the PNA duplexes.

Evidence for a near-resonant charge transfer mechanism for double-stranded peptide nucleic acid.

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions.

The blue copper protein from Pseudomonas aeruginosa, azurin, immobilized at gold electrodes through hydrophobic interaction with alkanethiol self-assembled monolayers (SAMs) of the general type [-S-(CH(2))(n)-CH(3)] (n = 4, 10, and 15) was employed to gain detailed insight into the physical mechanisms of short- and long-range biomolecular electron transfer (ET). Fast scan cyclic voltammetry and a Marcus equation analysis were used to determine unimolecular standard rate constants and reorganization free energies for variable n, temperature (2-55 degrees C), and pressure (5-150 MPa) conditions. A novel global fitting procedure was found to account for the reduced ET rate constant over almost five orders of magnitude (covering different n, temperature, and pressure) and revealed that electron exchange is a direct ET process and not conformationally gated. All the ET data, addressing SAMs with thickness variable over ca. 12 A, could be described by using a single reorganization energy (0.3 eV), however, the values for the enthalpies and volumes of activation were found to vary with n. These data and their comparison with theory show how to discriminate between the fundamental signatures of short- and long-range biomolecular ET that are theoretically anticipated for the adiabatic and nonadiabatic ET mechanisms, respectively.

Distance dependence of the charge transfer rate for peptide nucleic acid monolayers.

Charge transfer studies have been performed for self-assembled monolayers of single-stranded and double-stranded peptide nucleic acids (PNAs) having a C-terminus cysteine and an N-terminus ferrocene group as a redox reporter. The decay of the charge transfer rate with distance was strong for short single-stranded PNA molecules and weak for long single-stranded and double-stranded PNAs. Possible mechanisms for this "softening" of the distance dependence are discussed. The nature of the mechanism change can be explained by a transition of the charge transport mechanism from superexchange-mediated tunneling for short PNAs to a "hopping" mechanism for long PNAs.

Effect of deuterium substitution on electron transfer at cytochrome c/SAM interfaces.

The involvement of protons in the heterogeneous electron transfer between cytochrome c and a gold electrode to which it is attached was studied by comparing the electron transfer rate constants for H2O and D2O solutions. Rate constants were measured as a function of the electrochemical cell solution and the protein incubant solution, i.e., k (0)(incubant, cell). Two separate isotope effects exist: a cell "isotope effect", KIE cell = k (0)(H2O, H2O): k (0)(H2O, D2O), which is manifest at short time scales (<30 s) and arises from the viscosity difference between H2O and D2O, and an incubant isotope effect, KIE inc= k (0)(H2O, H2O): k (0)(D2O, H2O), which is manifest at longer times (>2 h) and results from H/D exchange. The two isotope effects are approximately equal ( approximately 1.2) and a total isotope effect KIE total = k (0)(H2O, H2O): k (0)(D2O, D2O) can be constructed that is the product of KIE cell and KIE inc. The nature of the electron transfer process, possible coupling to a proton transfer process, and the involvement of specific hydrogens in the transfer mechanism are discussed.

Validity and reliability of the medicine ball throw for kindergarten children.

The purpose of this study was to establish validity and reliability evidence for the medicine ball throw test for kindergarten students, an underrepresented group in the literature. The subjects were 105 students, 5-7 years old, BMI 17.44 +/- 3.17 kg x m(-2), 43% female and 57% male. Intraclass correlation coefficients (ICCs) were used to examine reliability, and Pearson correlation coefficients and a paired t-test were used to examine validity. To accomplish this, the kindergarten students completed the medicine ball throw test on two different days and the modified pull-up test, the "criterion" measure, on another day. For the medicine ball throw, each student sat on the floor before throwing the medicine ball forward like a chest pass three times. The medicine ball throw was highly reliable both within 1 day (ICCs = 0.93 and 0.94 for day 1 and day 2, respectively) and across 2 days (ICC = 0.88), with all reliability estimates over the acceptable level of 0.80. The medicine ball throw scores were positively related with height (r = 0.34) and weight (r = 0.34), and there was a significant difference between the 5-year-old group (mean +/- SD; 111.78 +/- 34.93) and the 6-year-old group (135.60 +/- 39.77), t = -3.23, p = 0.002, which supports correlational and known-difference evidence of validity for the medicine ball throw test. Even though no correlation was found between the medicine ball throw test and the modified pull-up test, r = -0.04, other forms of validity evidence (i.e., known-difference and correlational) were apparent. In conclusion, the medicine ball throw test seems to be a valid and reliable measure of upper-body strength for kindergarten children.