The Theoretical Study Of Relay Stations For Charge Transfer In Proteins

The charge transfer is an basic and fundamental question of the life science, including electron transfer, proton transfer and hole transfer. Electron and proton transfer problems are widely present in many life processes, such as respiration, metabolism, photosynthesis, nitrogen fixation, gene replication, enzymatic reactions and signal transduction of biology and so on. The investigation and study of charge transfer in vivo body is currently one of the hot issues, and its solution is of great significance for promoting the development of chemistry, life science, medicine, and other related fields. In long range charge transfer process, many groups or residues act as a springboard for electron transfer in the form of a relay station, prompting the successful completion of the charge-transfer process. Some amino acids and protein structures widely exist in life, and they can act as relay stations to take part in charge transfers in protein. In this chapter, phenylalanine is selected as a representative of the aromatic amino acids, and the310-helix is selected as a representative of the protein helical structures. We carried out a series of meaningful work, and made some valuable research to study their ability of releasing or binding an excess electron. The primary information is as follows:1. Coexistence of Solvated Electron and Benzene-Centered Valence Anion in the Negatively Charged Benzene-Water ClustersWe present a combined M06functional calculation and ab initio molecular dynamics simulation study of an excess electron (EE) in a microhydrated aromatic complex (modeled by benzene-water binary clusters, Bz(H20)n). Calculated results illustrate that Bz ring and water clusters are indeed linked through the π…HO interactions in the neutral Bz(H20)n (n=1-8) clusters, and the size of the water cluster does not influence the nature of its interaction with the π system for the oligo-hydrated complexes. The states and the dynamics of an EE trapped in such Bz-water clusters were also determined. All of possible localized states for the EE can be roughly classified into two types:(i) single, ring-localized states (the Bz-centered valence anions) in which an EE occupies the LUMO of the complexes originating from the LUMO (π*) of the Bz ring, and the π…HO interactions are enhanced for increase of electron density of the Bz ring. In this mode, the carbon skeleton of the Bz part is significantly deformed due to increase of electron density and nonsymmetric distribution of electron density induced by the interacting H-O bonds;(ii) solvated states, in which an EE is trapped directly as a surface state by the dangling hydrogen atoms of water molecules or as a solvated state in a mixed cavity formed by Bz and water cluster. In the latter case, Bz may also participate in capturing an EE using its C-H bonds in the side edge of the aromatic ring as a part of the cavity. In general, a small water cluster is favorable to the Bz-centered valence anion state, while a large one prefers a solvated electron state. Fluctuations and rearrangement of water molecules can sufficiently modify the relative energies of the EE states to permit facile conversion from the Bz-centered to the water cluster-centered state. This indicates that aromatic Bz can be identified as a stepping stone in electron transfer and the weak π…HO interaction plays an important role as the driving force in conversion of the two states.2. A310-Helical Peptide Acting as a Dual Relay for Charge Hopping Transfer in ProteinsWe present a density functional theory calculational study for clarifying that a310-helix peptide can serve as a novel dual relay element to mediate long-range charge migrations via its C-and N-terminus in proteins. The ionization potential of the310-helix C-terminus correlates inversely with the helix length, HOMO energy, and dipole moment. In particular, it decreases considerably with the increase 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 the310-helical C-terminus. On the other hand, the electron affinity of the310-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 of peptide units implies that the310-helical N-terminus can capture an excess electron and play an electron-relaying role. The relaying ability of the310-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, etc. In contrast with the known hole relays such as the side-chains of Tyr, Trp and electron relays such as the side-chains of protonated Lys and Arg, etc., either the hole relay (the310-helix C-terminus) or the electron relay (the310-helix N-terminus) is property-tunable and could apply to different proteins for assisting or mediating charge long-rang migrations.