Selective enhancement of resonance Raman spectra of separate bacteriopheophytins in Rb. sphaeroides reaction centers

Abstract

Tunable dye laser excitation of carefully prepared samples of Rb. sphaeroides reaction centers provides richly detailed resonance Raman (RR) spectra of the bacteriopheophytins, H, and the accessory bacteriochlorophylls, B. These spectra demonstrate selective enhancement of the separate bacteriopheophytins on the active (H(L)) and inactive (H(M)) sides of the reaction centers. The spectra are assigned with the aid of normal coordinate analyses using force fields previously developed for porphyrins and reduced porphyrins. Comparison of the H(L) and H(M) vibrational mode frequencies reveals evidence for greater polarization of the acetyl substituent in H(L) than H(M). This polarization is expected to make H(L) easier to reduce, thereby contributing to the directionality of electron transfer from the special pair, P. In addition, the acetyl polarization of H(L) is increased at low temperature (100 K), helping to account for the increase in electron transfer rate. The polarizing field is suggested to arise from the Mg(2+) of the neighboring accessory bacteriochlorophyll, which is 4.9 A from the acetyl O atom. The 100 K spectra show sharpening and intensification of a number of RR bands, suggesting a narrowing of the conformational distribution of chromophores, which is consistent with the reported narrowing of the distribution in electron transfer rates. Excitation at 800 nm produces high-quality RR spectra of the accessory bacteriochlorophylls, and the spectral pattern is unaltered on tuning the excitation to 810 nm in resonance with the upper exciton transition of P. Either the resonance enhancement of P is weak, or the bacteriochlorophyll RR spectra are indistinguishable for P and B.

Strongly coupled bacteriochlorin dyad studied using phase-modulated fluorescence-detected two-dimensional electronic spectroscopy

“…The strongest mode seen in the data is ~155 cm -1 . A FC active vibrational mode at this frequency is well established for bacteriochlorin-derived molecules, and arises on the bacteriochlorin macrocycle skeleton [33,36,50]. Oscillations of 2D peak at this frequency have been consistently reported[1, 2, 14, 51] across a variety of photosynthetic proteins with bacteriochlorin-derived chromophores.…”

“…Thus, the electronic coupling as well as the Huang-Rhys factors are within the parameter range known for chromophores in various photosynthetic proteins [47]. Resonance Raman experiments and normal mode analyses on bacteriochlorin derived molecules [33][34][35][36] suggest that a prominent 580 cm -1 FC active mode arises due to pyrrole and pyrroline ring deformations. Assuming that a similar mode is also present on the bacteriochlorin skeleton, non-adiabatically enhanced vibrational wavepackets due to a vibrational-electronic resonance might be expected in this system [10].…”

“…We adopt the phase-modulated approach to F-2DES developed by Marcus and co-workers [29]. The electronic couplings and FC displacements [24,[33][34][35][36] deduced from the absorption spectrum of the dyad, closely mimic those of a typical photosynthetic protein. Here we use this model system to demonstrate the feasibility of F-2DES spectroscopy for coherence and kinetic analysis [11,12,25] of coherent wavepackets performed routinely in 2DES experiments.…”

Low-frequency resonance Raman studies of the H(M202)G cavity mutant of bacterial photosynthetic reaction centers

“…Certain bands in this region are sensitive to the structure of BChl; however, modes in the very low-frequency region (<500 cm )1 ), which include out-of-plane deformations of the macrocycle, substituent deformations, and metal-ligand stretching modes are generally more sensitive to structural changes than the mid-to high-frequency in-plane stretching and bending vibrations of the macrocycle (Palaniappan et al 1995;Laporte et al 1996;Czarnecki et al 1997aCzarnecki et al , b, c, 1999Chen et al 2004Chen et al , 2005. In this connection, we and others have previously examined the low-frequency resonance Raman (RR) spectra of the BChl and BPh cofactors in a variety of wild-type and genetically modified RCs (Cherepy et al 1994(Cherepy et al , 1995(Cherepy et al , 1997aPalaniappan et al 1995;Laporte et al 1996;Czarnecki et al 1997aCzarnecki et al , b, c, 1999Eads et al 2000;Frolov et al 2002;Chen et al 2004Chen et al , 2005. In one series studies, we examined RCs wherein the BChls (P and the accessory pigments) were 15 N or 26 Mg labeled and the His residues were 15 N labeled (Czarnecki et al 1997b, c).…”

Effects of ethanol, formaldehyde, and gentle heat fixation in confocal resonance Raman microscopy of purple nonsulfur bacteria

“…The carotenoids typically found in Rhodobacter sphaeroides are spheroidene with its strongest line at 1,519 cm −1 or neurosporene with its strongest line at 1,528 cm −1 , whereas Rhodospirillum rubrum holds a spirilloxanthin with the strongest line at 1,509 cm −1 (Koyama, 1991, 1995). Additional Raman chromophores resonant to 532 nm excitation in Rhodobacter are cytochromes c and bacteriopheophytins (Eads et al, 2000; Imhoff et al, 1984; Varotsis et al, 1995).…”