VOLUME CONTRACTION ON PHOTOEXCITATION OF THE REACTION-CENTER FROM RHODOBACTER-SPHAEROIDES R-26 - INTERNAL PROBE OF DIELECTRICS

Reaction centers of Rhodobacter sphaeroides undergo a similar to 20 A(3)/mole volume contraction in <50 ns after excitation. The rapid volume change is tentatively assigned to electrostriction. From its magnitude, we infer that the effective dielectric coefficient is 10-15 if the compressibility of the reaction center is similar to that of globular proteins. The volume contraction is not sensitive to replacement of the natural ubiquinone at the Q(A) site by other quinones or to the occupancy of the Q(A) site. The quenching caused by pressure on the reaction centers most likely occurs on a faster time scale than that of electron transfer.