Energy transfer and spectral dynamics of the three lowest energy Qy-states of the Fenna-Matthews-Olson antenna complex

Temperature-dependent (1.8-10 K) spectral hole burning results are presented for the three lowest Q(y)-states of the Fenna-Matthews-Olson (FMO) bacteriochlorophyll a antenna complex of Chlorobium tepidum that contribute to the 825 nm absorption band. Both holewidth and hole growth kinetic data are presented. The data indicate that the three states he at 823, 825, and 827 nm and, furthermore, that they are associated with the lowest energy state of the subunit of the FMO trimer; Structural heterogeneity results in the three states being energetically inequivalent. The time constants for downward energy transfer from the 823 and 825 nm states are 37 and 117 ps, respectively. It is argued that the time constants for the 823 nm --> 825 and 823 nm --> 827 nm pathways are 117 and 54 ps, respectively. The dispersive hole growth kinetic data for the three states obtained at 1.8 and 8.0 K are shown to be consistent with the excited-state lifetimes and homogeneous widths of the zero-phonon absorption lines. Analysis of the data leads to a lifetime of 2 ns for. the 827 nm state. The temperature dependencies of the zero-phonon lines associated with this and the other two states are identical and characteristic of coupling to the two-level systems of the protein.