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 analyzed in terms of a linear combination of Gaussian bands and the thermal broadening data interpreted in terms of a model in which the spectrum of each pigment site is broadened by (a) a homogeneous component due to Linear electron-phonon coupling to a low-frequency protein vibration and (b) an inhomogeneous component associated with stochastic fluctuations at each pigment site. In order to obtain a numerically adequate description of the absorption spectra, a minimum number of five sub-bands is required. Further refinement of this sub-band description was 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 arriving at the seven sub-band description, the original five sub-band wavelength positions were essentially unchanged. Thermal broadening analysis of the seven sub-band description yielded data which displayed the closest 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 near 680 nm, identified as P680 and pheophytin, have optical reorganization energies around 40 and 16 cm(-1), respectively. All other sub-bands, identified as accessory pigments, have optical reorganization energies close to 16 cm(-1). The half-bandwidths at room temperature for accessory pigments and pheophytin are between 10 and 12 nm (210-250 cm(-1)) with homogeneously broadened line widths in the range 9-11 nm (186-232 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 near 16 nm (338 cm(-1)) and due to homogeneous and inhomogeneous components of 15 nn (324 cm(-1)) and 5 nm (100 cm(-1)), respectively. Thus, the room temperature bandwidth, in the case of all pigment sub-bands, is somewhat dominated by the homogeneous broadening.