Near-Infrared-Excitation Resonance Raman Studies of Bacterial Reaction Centers

Abstract

Resonance Raman (RR) spectroscopy has proven to be an effective probe of the structure of the bacteriochlorophyll (BChl) and bacteriopheophytin (BPh) pigments in bacterial photosynthetic reaction centers (RCs) [1–3]. Most RR studies have been performed with excitation resonant with either the B-states or Q_x-states of the pigments. Interpretation of the RR scattering observed with B-state excitation is complicated by the fact that the absorption bands of all six pigments overlap. This is not the case for the Q_x region; however, these absorptions are relatively weak which leads to relatively poor quality RR spectra. In contrast, the Q_y absorption bands of the special pair (P) BChls, the accessory BChls, and the BPhs are energetically better separated and relatively intense. The combination of these spectral characteristics makes the Q_y region an attractive target for RR studies of RCs. Early Q_y-excitation RR studies of RCs were hampered by the lack of suitable detectors or laser excitation sources [4]. However, with the advent of the charge couple device (CCD) and Ti:sapphire laser, this is no longer a problem. Regardless, the most serious impediment to Q_y-excitation RR studies of RCs is the presence of strong fluorescence intrinsic to the RCs or from small amounts of exogenous pigments present in the preparations. Despite this limitation, high quality Q_y-excitation RR spectra of RCs can be obtained [5–7].