▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

The proton inventory technique uses the dependence of enzymic reaction rate on the atom fraction of deuterium present in mixtures of protium oxide and deuterium oxide to deduce for simple cases the number of exchangeable hydrogenic sites that produce isotope effects, and the magnitude of the isotope effect generated at each site. For more complex cases, other information, such as the participation of more than a single step in limiting the rate, may be obtained. The background of the method, the conduct of the experiments and the interpretation of the results are briefly reviewed. The method is then illustrated in its application to various enzyme systems by a series of case histories.

The proton inventory technique.

Using a combination of high resolution and dipolar solid state 15N NMR we have determined H/D isotope effects on the nitrogen-hydron (L = H, D) distances and 15N chemical shielding tensors of strongly hydrogen bonded bisisocyanide salts of the type [(CO)5Cr−C⋮N···L···N⋮C−Cr(CO)5]- X+, where X+ = AsPh4+ (2) and X+ = NPr4+ (3). These compounds have been modeled theoretically by the linear system [C⋮N···L···N⋮C]-Li+ (1). The crystal field acting on the anion was generated by a variety of fixed C···Li distances. For the calculation of dynamical corrections of geometries and NMR chemical shifts, an iterative procedure based on the crude adiabatic approximation was employed, consisting of (i) ab initio calculation of the energy hypersurface at the MP2/6-31+G(d,p) level, (ii) solution of the Schrodinger equation for the anharmonic collinear hydron motion, and (iii) NMR chemical shift calculations using the IGLO-method. The two hydrogen bond distances r1 ≡ N···L and r2 ≡ L···N are found to change in a correlated ...

Solid State15N NMR and Theoretical Studies of Primary and Secondary Geometric H/D Isotope Effects on Low-Barrier NHN−Hydrogen Bonds

7-Azaindole is the chromophoric moiety of 7-azatryptophan, which is an alternative to tryptophan as an optical probe of protein structure and dynamics. The great power of the 7-azaindole chromophore is that it is red shifted both in absorption and emission from tryptophan, that its fluorescence decay is single exponential in water under appropriate conditions, and that its emission is sensitive to solvent. In addition, 7-azatryptophan can be incorporated into synthetic peptides and bacterial protein. In this article, the interactions of 7-azaindole with its environment are discussed. Special attention is directed to the difference in its fluorescence properties in water as opposed to nonaqueous solvents. The sensitivity of 7-azaindole to its environment is demonstrated and then exploited by studying it and its analogs in peptides and in complexes with larger proteins containing many tryptophan residues.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1657&context=chem_pubs

Photophysics and Biological Applications of 7-Azaindole and Its Analogs

Hydrogen bonds and proton transfer in general-catalytic transition-state stabilization in enzyme catalysis.

The hydrolysis of many ortho esters, and some acetals and ketals, is general acid catalyzed, and in some examples these generate linear Bronsted plots over substantial ranges of catalyst acidity. This suggests that the reaction coordinate is primarily a reorganization of heavy atoms since proton transfer from one oxygen to another has been shown to generate strongly curved Bronsted plots. However, the isotopic fractionation factor for the catalytically active proton in these transition states is substantially less than 1.0; in several examples it is less than 0.5. Such values have been thought to require that the reaction coordinate be largely a motion of the hydrogen giving the low fractionation factor. This dilemma has been resolved by the observation and rationalization of fractionation factors as low as 0.28 for stable, hydrogen bridge-bonded complexes, AHA/sup -/. A similar, bounded coordinate is now suggested for the catalytically active protons in question. This permits the reaction coordinate to be pictured. 3 figures, 2 tables.

Robert Eliason

Papers

Kinetic hydrogen isotope effects in the concerted mechanism for the hydrolysis of acetals, ketals, and ortho esters

Broensted .alpha. and isotope effects for vinyl ether hydrolysis

Comparison of general-acid-catalyzed ethyl vinyl ether hydrolysis in 80% dimethyl sulfoxide with that in water

Isotope effects in proton transfer from general acids to carbon bases