Kq. Lao et al., EFFECTS OF APPLIED ELECTRIC-FIELDS ON THE QUANTUM YIELDS OF THE INITIAL ELECTRON-TRANSFER STEPS IN BACTERIAL PHOTOSYNTHESIS .1. QUANTUM YIELD FAILURE, Journal of physical chemistry, 97(50), 1993, pp. 13165-13171
The effect of an applied electric field has been measured for the quan
tum yield of formation of the P+H-, P+Q(A)-, and 3P states of nonorien
ted Rb. sphaeroides reaction centers at 80 K in poly(vinyl alcohol) (P
VA) films and in glycerol-buffer glasses. Both the shape and magnitude
of the field dependence are very similar for all three observables in
both media where the data overlap. This result suggests that the redu
ction of the quantum yield occurs at a very early step in the charge s
eparation process. More than 50% reduction in the quantum yield is obs
erved at fields of 2.5 x 10(6) V/cm in PVA films. Possible mechanisms
include field-dependent nonradiative decay of 1P, a field-dependent ra
te of formation or decay of P+B- if P+B- is important, and field-depen
dent formation or decay of an unrelaxed form of P+H-. Small electric f
ield effects are observed on the recombination rate of the radical pai
r P+H- and the magnetic field dependence of the 3P quantum yield. This
is likely because those orientational subpopulations of reaction cent
ers which survive to become P+H- and 3p in an electric field are those
which did not experience quantum yield failure at an early step. The
field-induced effects provide a basis for a possible mechanism to regu
late photosynthetic charge separation by transmembrane potentials.