Computational Insights into the Charge Relaying Properties of β-Turn Peptides in Protein Charge Transfers

Density functional theory calculations suggest that -turn peptide segments can act as a novel dual-relay elements to facilitate long-range charge hopping transport in proteins, with the N terminus relaying electron hopping transfer and the C terminus relaying hole hopping migration. The electron- or hole-binding ability of such a -turn is subject to the conformations of oligopeptides and lengths of its linking strands. On the one hand, strand extension at the C-terminal end of a -turn considerably enhances the electron-binding of the -turn N terminus, due to its unique electropositivity in the macro-dipole, but does not enhance hole-forming of the -turn C terminus because of competition from other sites within the -strand. On the other hand, strand extension at the N terminal end of the -turn greatly enhances hole-binding of the -turn C terminus, due to its distinct electronegativity in the macro-dipole, but does not considerably enhance electron-binding ability of the N terminus because of the shared responsibility of other sites in the -strand. Thus, in the -hairpin structures, electron- or hole-binding abilities of both termini of the -turn motif degenerate compared with those of the two hook structures, due to the decreased macro-dipole polarity caused by the extending the two terminal strands. In general, the high polarity of a macro-dipole always plays a principal role in determining charge-relay properties through modifying the components and energies of the highest occupied and lowest unoccupied molecular orbitals of the -turn motif, whereas local dipoles with low polarity only play a cooperative assisting role. Further exploration is needed to identify other factors that influence relay properties in these protein motifs.