The influence of geometrical fluctuations on electron tunneling barriers in proteins

Abstract

We have recently developed a new classification of the amino acid side chains based on doublet acceptor energy levels (DALs). These energy levels, taken as the eigenvalue of the highest occupied molecular orbital in the radical anion, provide insight into the shape of the tunneling barrier in protein electron transfer. We have used this classification to characterize several transfer pathways in myoglobin. We present here a study of the effect of changes in geometry on the doublet acceptor levels. We find that, in general, this influence is rather small for reasonable changes in bond lengths and angles (e.g., those resulting from thermal fluctuations in the protein structure). However, we have found that for serine, threonine and water, changes in geometry that might take place as a result of fluctuations in hydrogen bond geometries lead to rather large changes in DAL. We investigate these correlations in some detail. The relation between protein fluctuations and changes in the shape of the electron tunneling barrier is discussed. Additionally, based on the energetics of changes in the OH bond distance, we discuss aspects of the relation between electron transfer and proton transfer.