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-amin onaphthalene-5-sulphonate group in an excited singlet state (donor) an d a nitroxide radical (acceptor) was incorporated into a hydrophobic c avity of bovine serum albumin (BSA). The kinetics of reversible and ir reversible intramolecular electron transfer from the donor to the acce ptor were monitored by luminescence and ESR techniques in the temperat ure range from 77 K to 300 K. The temperature dependence of the microp olarity and the intramolecular dynamics in the vicinity of the donor a nd the acceptor groups were monitored by fluorescence and ESR techniqu es, respectively. The Arrhenius dependence of the reversible ET consta nt (K-e) was non-linear and the apparent activation energy (E(app)) ch anges from 0 eV (at T = 77-100 K) to 0.25 eV (at T = 298 K). The tempe rature region of the E(app) increase (100-240 K) was close to the temp erature of the increase of relaxation shift of the donor luminescent s pectra (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 th e Frank-Condon factor for reversible electron transfer were estimated. The role of molecular dynamics in the vicinity of the donor and the a cceptor groups is also discussed.