Direct evaluation of electronic coupling mediated by hydrogen bonds: implications for biological electron transfer.

Three supramolecular bischromophoric systems featuring zinc(II) and iron(III) porphyrins have been synthesized to evaluate the relative magnitudes of electronic coupling provided by hydrogen, sigma, and pi bonds. Laser flash excitation generates the highly reducing singlet excited state of the (porphinato)zinc chromophore that can subsequently be electron transfer quenched by the (porphinato)iron(III) chloride moiety. Measurement of the photoinduced electron transfer rate constants enables a direct comparison of how well these three types of chemical interactions facilitate electron tunneling. In contrast to generally accepted theory, electronic coupling modulated by a hydrogen-bond interface is greater than that provided by an analogous interface composed entirely of carbon-carbon sigma bonds. These results bear considerably on the analysis of through-protein electron transfer rate data as well as on the power of theory to predict the path traversed by the tunneling electron in a biological matrix; moreover, they underscore the cardinal role played by hydrogen bonds in biological electron transfer processes.