Analysis of absorption spectra of purple bacterial reaction centers in the near infrared region by higher order derivative spectroscopy

Reaction centers (RCs) of purple bacteria are uniquely suited objects to study the mechanisms of the photosynthetic conversion of light energy into chemical energy. A recently introduced method of higher order derivative spectroscopy [I.K. Mikhailyuk, H. Lokstein, A.P. Razjivin, A method of spectral subband decomposition by simultaneous fitting the initial spectrum and a set of its derivatives, J. Biochem. Biophys. Methods 63 (2005) 10-23] was used to analyze the NIR absorption spectra of RC preparations from Rhodobacter (R.) sphaeroides strain 2R and Blastochloris (B.) viridis strain KH, containing bacteriochlorophyll (BChl) a and b, respectively. Qy bands of individual RC porphyrin components (BChls and bacteriopheophytins, BPheo) were identified.The results indicate that the upper exciton level Py+ of the photo-active BChl dimer in RCs of R. sphaeroides has an absorption maximum of 810 nm. The blue shift of a complex integral band at approximately 800 nm upon oxidation of the RC is caused primarily by bleaching of Py+, rather than by an electrochromic shift of the absorption band(s) of the monomeric BChls. Likewise, the disappearance of a band peaking at 842 nm upon oxidation of RCs from B. viridis indicates that this band has to be assigned to Py+. A blue shift of an absorption band at approximately 830 nm upon oxidation of RCs of B. viridis is also essentially caused by the disappearance of Py+, rather than by an electrochromic shift of the absorption bands of monomeric BChls. Absorption maxima of the monomeric BChls, BB and BA are at 802 and 797 nm, respectively, in RCs of R. sphaeroides at room temperature. BPheo co-factors HB and HA peak at 748 and 758 nm, respectively, at room temperature. For B. viridis RCs the spectral positions of HB and HA were found to be 796 and 816 nm, respectively, at room temperature.