Selective enhancement of resonance Raman spectra of separate bacteriopheophytins inRb. 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 (HL) and inactive (HM) 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 HL and HM vibrational mode frequencies reveals evidence for greater polarization of the acetyl substituent in HL than HM. This polarization is expected to make HL easier to reduce, thereby contributing to the directionality of electron transfer from the special pair, P. In addition, the acetyl polarization of HL 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 Mg2+ of the neighboring accessory bacteriochlorophyll, which is 4.9 Å 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. © 2000 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 57: 64–76, 2000

Structural Asymmetry of Bacterial Reaction Centers: A Qy Resonant Raman Study of the Monomer Bacteriochlorophylls

“…Selective RR probing has so been achieved for each of the two H L and H M BPheo cofactors, using resonant excitation at each of their Q x transitions, which are distinct at low temperature, 26,27 as well as, more recently, using resonant excitations in the Q y band. [28][29][30] In the bacterial RC, the Q y transitions of the monomer BChls are well separated both from those of the primary donor and BPheo cofactors (Figure 1). Moreover, there is good evidence 31,32 that the Q y transitions of the BChl L and BChl M cofactors are not exactly overlapping in Rb sphaeroides RCs, B M absorbing some 10 nm further to the red than B L .…”

“…9,11,17 These experiments however are very difficult because the Q y bands correspond to the lowest singlet, S 1 transitions of the bacteriochlorins, which are very radiative. Different attempts have been made to overcome this technical problem, 33 either recording the RR spectra in a classical, straightforward way 30,34 or using the so-called SERDS difference method. 35,36 Although some of the results obtained by these groups were not entirely consistent (see discussions in refs 30, 33, and 35), the data for the accessory BChls excited at or near to 800 nm appeared quite encouraging, with excellent agreement between the more recent data obtained by the two methods, particularly in the lower frequency regions.…”

“…These differences can in principle be used for selectively exciting RR spectra of each of the two cofactors. Selective RR probing has so been achieved for each of the two H L and H M BPheo cofactors, using resonant excitation at each of their Q x transitions, which are distinct at low temperature, , as well as, more recently, using resonant excitations in the Q y band. − …”

“…These characteristics of the Q y transitions constitute an incentive for attempting selective resonant Raman excitations of either BChl L or BChl M around 800 nm which would allow detailed comparisons of the structures and local environments of the two cofactors, in line with their possible role in the functional asymmetry of the RC. ,, These experiments however are very difficult because the Q y bands correspond to the lowest singlet, S 1 transitions of the bacteriochlorins, which are very radiative. Different attempts have been made to overcome this technical problem, either recording the RR spectra in a classical, straightforward way , or using the so-called SERDS difference method. , Although some of the results obtained by these groups were not entirely consistent (see discussions in refs , , and 35), the data for the accessory BChls excited at or near to 800 nm appeared quite encouraging, with excellent agreement between the more recent data obtained by the two methods, particularly in the lower frequency regions. We thus attempted careful recording of low-temperature RR spectra at full resonance with their Q y transition, with the double objective of (i), accurately recording the higher frequency regions of the spectra, which involve most of the currently identified reporter bands for the structural and environmental characteristics of BChl, and (ii) of selectively recording RR spectra of either B L or B M , to compare their structures and environments.…”

Vibrational analysis of Ni(II)‐ and Cu(II)‐octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopy

“…Spiro and co-workers performed vibrational analyses of free-base Ni(II)-1,5-dihydroxy-1,5-dimethyloctaethylbacteriochlorin, 20 tetraphenylbacteriochlorin 21 and recently also of bacteriopheophytin. 22 For all these molecules, the results indicate some similarity with corresponding porphyrin modes so that the normal mode classification of the latter remains applicable. Bocian's group employed their concept to calculate the normal modes of Cu(II)-tetraphenylchlorin/Cu(II)-tetraphenylbacteriochlorin, 23 chlorophyll a 24 and bacteriochlorophyll.…”