3(10)-Helical peptide acting as a dual relay for charge-hopping transfer in proteins

We present a density functional calculational study to clarify that a 3(10)-helix peptide can serve as a novel dual-relay element to mediate long-range charge migrations via its C- and N-termini in proteins. The ionization potential of the 3(10)-helix C-terminus correlates inversely with the helix length, HOMO energy, and dipole moment. In particular, it decreases considerably with the increase in the number of peptide units, even to a smaller value than that of the easily oxidized amino acid residue, which implies the possibility of releasing an electron and forming a hole at the 3(10)-helical C-terminus. On the other hand, the electron affinity of the 3(10)-helical N-terminus correlates positively with the helix length and dipole moment but inversely with the LUMO energy. Clearly, the increasing positive electron-binding energy with the increase in the number of peptide units implies that the 3(10)-helical N-terminus can capture an excess electron and play an electron-relaying role. The relaying ability of the 3(10)-helical C-terminus and N-terminus not only depends on the helix length but also varies subject to the capping effect, the collaboration and competition of proximal groups, and solvent environments. In contrast to the known hole relays such as the side chains of Tyr and Trp and electron relays such as the side chains of protonated Lys and Arg, either the hole relay (the 3(10)-helix C-terminus) or the electron relay (the 3(10)-helix N-terminus) is property-tunable and could apply to different proteins in assisting or mediating long-range charge migrations.