OXIMINE FORM OF THE PEPTIDE-BOND AS A TRANSIENT MODIFICATION IN ENZYME REDOX REACTIONS

In a study devoted to proton-assisted intramolecular electron transfer (IET) in multiheme cytochromes (an alternative to through-space IET), the possibility that the peptide bond takes up an oximine form at least for a very short time has appeared to be critical. Therefore, a detailed quantum mechanical study of the time-dependent aspects of the tautomerism of the peptide bond has been carried out, using the formamide dimer as a test model. The surprising conclusion is that, due to the rapid decrease of the lifetime of the oximine form when the H bridge length decreases, it is the N-O bridge vibration that determines the return to the stable amine form. Thus, the in situ lifetime of the oximine form is ca. 0.2 ps. An interesting generalization of this result is that proton relay processes (whether assisting IET or not) are time controlled by the slow H bridge vibration.