EXCITON LEVEL STRUCTURE AND ENERGY DISORDER OF THE B850 RING AND THE LH2 ANTENNAL COMPLEX

Experimental and theoretical results are presented on the exciton leve l structure of the B850 ring of bacteriochlorophyll a molecules for th e light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila (str ain 10 050) and the effects of energy disorder (due to structural hete rogeneity) on the level structure, The work is an outgrowth of the acc ompanying paper (Wu et al. J. Phys. Chem. B 1997, 1O1, 7641), which re ports on the temperature and pressure dependencies of the LH2 absorpti on spectrum and the zero-phonon hole action spectrum of the lowest ene rgy exciton level of the complex, B870, as well as the structural (non denaturing) change of the complex near 150 K. The effects of energy di sorder are analyzed using the theory of Wu and Small (Chem. Phys. 1997 , 218, 225), which employs symmetry-adapted energy defect patterns. Th e analysis leads to a room temperature value of similar to 100 cm(-1) for the splitting between B870 and the adjacent, strongly allowed E-1 level in the absence of disorder. Using the temperature-dependent data of Wu et al., we arrive at a theoretical estimate for this splitting at temperatures below similar to 150 K of similar to 150 cm(-1) which is 50 cm(-1) smaller than the ''apparent'' value of 200 cm(-1) based o n the 4.2 K B870 action spectrum. The 50 cm(-1) difference is explaine d in terms of a distribution of values for the energy disorder paramet er(s), which leads to a distribution of values for the oscillator stre ngth of B870. Hole-burning data on the temperature dependence of B870' s optical dynamics are presented and analyzed. Below similar to 15 K t he dynamics are dominated by two-level systems of the protein with an effective dephasing frequency that carries a T-alpha dependence with a lpha approximate to 1.3. At temperatures above similar to 20 K the dep hasing is strongly exponentially driven with an activation energy of s imilar to 100-140 cm(-1). A mechanism suggested for this dephasing is that it is due to upward scattering of the B870 level to the adjacent E-1 level by one-phonon absorption. New satellite hole spectra for the LH2 complex (isolated and chromatophores) are presented that lead to the assignment of the weak high-energy tail absorption of the B800 and B850 absorption bands to B850 exciton levels of the B850 ring, which are either symmetry forbidden or predicted to be very weakly absorbing in the absence of energy disorder.