A THERMAL BROADENING ANALYSIS OF ABSORPTION-SPECTRA OF THE D1 D2/CYTOCHROME B-559 COMPLEX IN TERMS OF GAUSSIAN DECOMPOSITION SUBBANDS/

Absorption spectra of the isolated D1/D2/cytochrome b-559 complex have been measured in the temperature range 80-300 K. All spectra were ana lyzed in terms of a linear combination of Gaussian bands and the therm al broadening data interpreted in terms of a model in which the spectr um of each pigment site is broadened by (a) a homogeneous component du e to Linear electron-phonon coupling to a low-frequency protein vibrat ion and (b) an inhomogeneous component associated with stochastic fluc tuations at each pigment site. In order to obtain a numerically adequa te description of the absorption spectra, a minimum number of five sub -bands is required. Further refinement of this sub-band description wa s achieved by taking into account published data from hole burning and absorption difference spectroscopy. In this way, both a six sub-band description and a seven sub-band description were generated. In arrivi ng at the seven sub-band description, the original five sub-band wavel ength positions were essentially unchanged. Thermal broadening analysi s of the seven sub-band description yielded data which displayed the c losest correspondence with the literature observations. The wavelength positions of the sub-bands were near 661, 667, 670, and 675 nm, with two bands near 680 and 684 nm. The two almost isoenergetic sub-bands n ear 680 nm, identified as P680 and pheophytin, have optical reorganiza tion energies around 40 and 16 cm(-1), respectively. All other sub-ban ds, identified as accessory pigments, have optical reorganization ener gies close to 16 cm(-1). The half-bandwidths at room temperature for a ccessory pigments and pheophytin are between 10 and 12 nm (210-250 cm( -1)) with homogeneously broadened line widths in the range 9-11 nm (18 6-232 cm(-1)) and inhomogeneously broadened line widths between 5 and 7 nm (100-140 cm(-1)). For P680, the optical reorganization energy is approximately 40 cm(-1). The half-bandwidth at room temperature is nea r 16 nm (338 cm(-1)) and due to homogeneous and inhomogeneous componen ts of 15 nn (324 cm(-1)) and 5 nm (100 cm(-1)), respectively. Thus, th e room temperature bandwidth, in the case of all pigment sub-bands, is somewhat dominated by the homogeneous broadening.