Experimental and theoretical results are presented on the exciton level structure of the B850 ring of bacteriochlorophyll a molecules for the light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila (strain 10 050) and the effects of energy disorder (due to structural heterogeneity) on the level structure. The work is an outgrowth of the accompanying paper (Wu et al. J. Phys. Chem. B 1997, 101, 7641), which reports on the temperature and pressure dependencies of the LH2 absorption spectrum and the zero-phonon hole action spectrum of the lowest energy exciton level of the complex, B870, as well as the structural (nondenaturing) change of the complex near 150 K. The effects of energy disorder are analyzed using the theory of Wu and Small (Chem. Phys. 1997, 218, 225), which employs symmetry-adapted energy defect patterns. The analysis leads to a room temperature value of ∼100 cm-1 for the splitting between B870 and the adjacent, strongly allowed E1 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 ∼150 K of ∼150 cm-1, which is 50 cm-1 smaller than the “apparent” value of 200 cm-1 based on the 4.2 K B870 action spectrum. The 50 cm-1 difference is explained in terms of a distribution of values for the energy disorder parameter(s), which leads to a distribution of values for the oscillator strength of B870. Hole-burning data on the temperature dependence of B870's optical dynamics are presented and analyzed. Below ∼15 K the dynamics are dominated by two-level systems of the protein with an effective dephasing frequency that carries a T α dependence with α ≈ 1.3. At temperatures above ∼20 K the dephasing is strongly exponentially driven with an activation energy of ∼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 E1 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.

One-and two-color pump/probe femtosecond and hole-burning data are reported for the isolated B800-850 (LH2) antenna complex of Rhodopseudomonas acidophila (strain 10050). The two-color profiles are interpretable in terms of essentially monophasic B800fB850 energy transfer with kinetics ranging from 1.6 to 1.1 ps between 19 and 130 K for excitation at or to the red of the B800 absorption maximum. The B800 zero-phonon hole profiles obtained at 4.2 K with burn frequencies located near or to the red of this maximum yielded a transfer time of 1.8 ps. B800 hole-burning data (4.2 K) are also reported for chromatophores at ambient pressure and pressures of 270 and 375 MPa. At ambient pressure the B800-B850 energy gap is 950 cm -1 , while at 270 and 375 MPa it is close to 1000 and 1050 cm -1 , respectively. However, no dependence of the B800fB850 transfer time on pressure was observed, consistent with data for the B800-850 complex of Rhodobacter sphaeroides. The resilience of the transfer rate to pressure-induced changes in the energy gap and the weak temperature dependence of the rate are consistent with the model that has the spectral overlap (of Förster theory) provided by the B800 fluorescence origin band and weak vibronic absorption bands of B850. However, both the time domain and hole-burning data establish that there is an additional relaxation channel for B800, which is observed when excitation is located to the blue of the B800 absorption maximum. Several explanations for this faster channel are considered, including that it is due to intra-B800 energy transfer or a manifestation of coupling of B800 with quasi-degenerate upper exciton levels of the B850 molecules. The data indicate that it is not due to vibrational relaxation.