Ultrafast dynamics of biological electron transfer over short distances.

摘要

Photoinduced electron transfer reaction plays a pivotal role in regulating many light-switched enzyme activities such as photosynthesis and circadian rhythm. In the biological system, the donor-acceptor distance may vary from a few angstroms to tens of angstroms. For the past several decades, long distance (>14 angstroms) electron transfer has been well studied. However, understanding the short distance transfer is technically limited due to temporal resolution. In the dissertation, we address this issue with the use of ultrafast spectroscopy and investigate a protein model system of Desulfovibrio vulgaris flavodoxin. Using the flavin cofactor, we initiate the electron-transfer process by ultrashort laser pulse. By monitoring time-dependent laser induced fluorescence and absorption change, we unravel the associated elementary dynamics and elucidate key factors (static reaction energy, environment dynamic relaxation, and distance variant) that affect the rate of electron transfer. With the site-directed mutagenesis, we first block the electron-transfer channel by replacing the tryptophan and tyrosine donor with phenylalanine. The inert mutant thus shows the time constants of active-site solvation ranging from several picoseconds to hundreds of picoseconds. Subsequently, from the native flavodoxin, we isolate the donor of tryptophan and tyrosine adjacent to the acceptor, and determine the individual transfer rate and the complete electron-transfer cycle. At the same time, by altering the critical redox-sensitive residues, different redox potentials for the acceptor to convert from oxidized to one-electron reduced, and from one-electron reduced to two-electron reduced states are generated. From such a great variety of driving forces, the result shows a strong correlation between the rates of electron transfer and the reaction energy. A subpicosecond dynamics is substantially slow down to a few picoseconds with a reduced energy of about 0.5--0.6 eV. Some slower processes in several picoseconds are shown strongly coupled with the active-site solvation, thereby exhibiting a decay behavior in a non-single exponential way. Surprisingly, the rate of back electron transfer is found not directly related to such energy potential change but is otherwise more sensitive to the solvation involvement throughout the cycle. In the final part, we change the donor-acceptor distance by locating the tryptophan donor at different places one at a time. From W60 to the replaced L67W, the edge-to-edge distance to the acceptor is lengthened from 3.7 angstroms to 8--9 angstroms, and the rate of electron transfer is significantly decreased from subpicosecond to nanosecond time scales. In conclusion, our study has resolved several important parameters for understanding the electron transfer occurring in several angstroms that are key to the nonequilibrated reactions in biology.