Thermal Broadening Analysis of the Light Harvesting Complex II Absorption Spectrum

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Abstract
Absorption spectra in the Qy region of the light harvesting complex II (LHCII) have been measured in the temperature range 70-300 K. The spectra were analyzed by evaluating the temperature dependence (a) of the total bandwidth and (b) of the sub-bands obtained by numerical decomposition in terms of double Gaussians. The thermal broadening of the bands are interpreted, in both cases, as a homogeneous component, due to the presence of linear electron-phonon coupling, plus an inhomogeneous component, due to both statistical energy fluctuations at each pigment site and heterogeneity of the sample itself. Sub-bands analysis, in which eight major components are identified, yields a reorganization energy 9 cm(-1) < or = Svm < or = 14 cm(-1) and an inhomogeneous contribution in the range 120-170 cm(-1). In all cases the bands are substantially symmetrical in the 70-300 K temperature range. This observation gains theoretical support from an analysis of the band moments, when the influence of a low-frequency vibrational mode is considered. Analysis of the total absorption band yields Svm approximately 70 cm(-1); however, this high value is reduced to Svm approximately 11-20 cm(-1) when the red-most sub-band, with maximum at 684 nm, is eliminated at all temperatures. These data are discussed in terms of the underlying transitions, giving strong support to the presence of extreme red absorption bands in LHCII. The presence of another low-frequency mode with vm > 20-30 cm(-1) is also proposed.