Theoretical Study Of Charge Transferring Process In Proteins

Charge transfer includes electron transfer and hole transfer. It widely exists in various life activities, such as photosynthesis, metabolism, biological nitrogen fixation, cell signal transduction and so on. Localization dynamics of excess electron in bulk liquid medium has attracted great attention because of its significance in biological chemistry and atmospheric chemistry. These studies involve in basic phenomenon of chemical, biological, physical and medical science and are of great significance to the development of related fields. Backbone of protein widely exists in life system and plays an important role in the process of charge transfer. β-turn is a common secondary structure and can link other secondary structures in protein. Acetamide is a small protein backbone containing only one peptide bond. They are both typical representatives of the protein backbones. In this work, we study relay properties of β-turn structure in gas phase and excess electron dynamics in aqueous acetamide-Ca2+ solutions, using density functional theory (DFT) calculation method and ab initio simulation of molecular dynamics (AIMD), respectively. The primary information is as follows:1. β-turn Peptide Acting as a Dual Relay for Charge Hopping Transfer in ProteinsDensity functional theory calculations (DFT) suggest that β-turn peptide segment can act as a novel dual relay element to facilitate long-range charge hopping transport in proteins, with N-terminus relaying electron hopping transfer and the C-terminus relaying hole hopping migration. Electron or hole binding ability of such a β-turn varies subject to the conformations of oligopeptides and lengths of its linking strands. On one hand, strand extension at the β-turn C-terminus considerably enhances electron binding ability of the β-turn N-terminus owing to its unique electron-positivity in macro-dipole, but does not considerably enhance hole binding ability of the β-turn C-terminus because of competition from other sites within β-strand. On the other hand, strand extension at the β-turn N-terminus greatly enhances hole binding ability of the β-turn C-terminus owing to its distinct electron-negativity in macro-dipole, but does not considerably enhance electron binding ability of the N-terminus because of the sharing responsibility of other sites in β-strand. Thus, in P-hairpin structures, electron or hole binding abilities of both the termini of the β-turn unit degenerate compared with those of the two hook structures, owing to weak polarity of macro-dipole caused by the extension of two anti-parallel strands at the two termini. In general, the high polarity of a macro-dipole always plays a predominant role in determining charge relay properties through modifying the components and energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the β-turn unit, while those local dipoles with low local polarity only play an assisting role cooperatively. Certainly, further exploration is needed to identify the influence of other factors on relay properties in such protein motifs.2. Localization Dynamics of Excess Electron in Aqueous Acetamide-Ca2+ SolutionsIn this work, we chose three configurations to represent three interacting patterns between acetamide and Ca2+ in aqueous solution, in which protein molecule binds metal cation in contact minimum state, solvent-shared state and approximately isolated state. Excess electron exhibits different dynamical moving actions in different configurations. When the two molecules interact with each other in contact minimum state or in solvent-shared state, excess electron localizes in a cavity-shaped structure constructed by water molecules or localizes upon acetamide, forming solvated electron or anionic acetamide, respectively. The lowest unoccupied molecule orbital (LUMO) diffusely distributes in conduction band of water before EE being injected and occupies in acetamide or a cavity in water after EE being injected. Although energy level of LUMO distributing in acetamide is slightly lower than that distributing in water cavity, acetamide does not have an absolutely obvious advantage over water. Meanwhile, existence of weak-oxidizing Ca2+ nearby acetamide weakens absorption of acetamide to excess electron. As a result, water can effectively compete with acetamide to accommodate excess electron by forming in a cavity-shaped structure. When the two molecules are in approximately isolated state, excess electron only localizes on acetamide and forms a stable anion, owing to attraction of acetamide and thrust of Ca2+for excess electron. These phenomena are closely associated with distributions and energy levels of various LUMOs. Different excess electron localization modes also have different effects on interaction between acetamide and Ca2+in turn.