FACTORS AFFECTING PHOTOINDUCED ELECTRON-TRANSFER IN A DONOR-ACCEPTOR PAIR (D-A) INCORPORATED INTO BOVINE SERUM-ALBUMIN

A donor-acceptor hybrid molecule (D-A) consisting of a 1-dimethyl-aminonaphthalene-5-sulphonate group in an excited singlet state (donor) and a nitroxide radical (acceptor) was incorporated into a hydrophobic cavity of bovine serum albumin (BSA). The kinetics of reversible and irreversible intramolecular electron transfer from the donor to the acceptor were monitored by luminescence and ESR techniques in the temperature range from 77 K to 300 K. The temperature dependence of the micropolarity and the intramolecular dynamics in the vicinity of the donor and the acceptor groups were monitored by fluorescence and ESR techniques, respectively. The Arrhenius dependence of the reversible ET constant (K-e) was non-linear and the apparent activation energy (E(app)) changes from 0 eV (at T = 77-100 K) to 0.25 eV (at T = 298 K). The temperature region of the E(app) increase (100-240 K) was close to the temperature of the increase of relaxation shift of the donor luminescent spectra (T = 100-240 K). On the basis of the obtained kinetic data and of the data of the micropolarity in the vicinity of the donor and the acceptor groups, values of standard Gibbs energy (Delta G(0)) change, the reorganization energy (E(r)), the resonance integral (V-DA) and the Frank-Condon factor for reversible electron transfer were estimated. The role of molecular dynamics in the vicinity of the donor and the acceptor groups is also discussed.