α-Helix C-Terminus Acting as a Relay to Mediate Long-Range Hole Migration in Proteins

Abstract

Ab initio calculations suggest that the C-terminus of an α-helix can serve as a novel relay element for long-range charge migration in proteins, as an addition or alternative to the known relay properties of aromatic and S-containing amino acids. The relay ability of an α-helical C-terminus varies with helix length, capping, proximal group competition, and helix twisting/bending. The vertical ionization potential (IPV) is a suitable indicator of hole relay ability and correlates positively with the HOMO energies and inversely with helix length and dipole moment. Different capping groups can yield different effects on the IPV but hardly change the IPV−helix length dependence. A series of recent experimental observations regarding charge migrations by hopping along helices or across different helices and the distance dependence of the charge-transfer rates through helices could be viewed as strong evidence for this prediction that also provides the impetus for further experimental tests and continued theoretical exploration.