Conformational changes following reduction of the bacteriopheophytin electron acceptor in reaction centers of Rhodopseudomonas viridis

Abstract

An electron acceptor, designated I, functions between the bacteriochlorophyll dimer (B₂) and the quinone acceptor (Q) in bacterial photosynthetic reaction centers. The acceptor is thought to be a complex composed of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPheo). We have generated the trapped B₂I⁻Q⁻ redox state at 100 K in reaction centers of Rhodopseudomonas viridis, and have examined the optical and EPR properties of this redox state as a function of temperature. Changes in the optical absorption spectra show that the B₂I⁻Q⁻ state converts from a low temperature form, which is frozen in at temperatures below 150 K, to a ‘relaxed’ form at higher temperatures. The conversion is not reversible. Optical dichroism and EPR spectra show that the conversion of the B₂I⁻Q⁻ state from the low temperature to relaxed form is not the result of a change in the electron distribution between BChl and BPheo. Instead, this conversion appears to affect the optical absorption of the accessory BChl. We propose that nuclear relaxations occur within the accessory BChl and/or surrounding protein following the reduction of I at temperatures above 150 K. Comparison of these optical spectra to those of the transient B₂⁺I⁻ state suggests that these processes may be functional during the ps electron transfer from I⁻ to Q.