Figure 6. USI spectra fitted to model photointermediate components.

The absorbance of USI 1, shown in Figure 4, has both UV and visible bands which suggests that USI 1 is an equilibrated mixture of two photointermediate species whose equilibration is fast. The top left panel above shows how the UV portion of USI 1's absorbance (▲) can be modeled in terms of a deprotonated SB form (solid line, see text for model details) analogous to bovine rhodopsin's BSI intermediate (dashed line). The dotted line in the same panel, which does not fit the UV portion of the USI 1 data, shows the corresponding PSB form of the model BSI for Siberian hamster UV cone pigment. The top right panel shows how the visible absorbing portion of USI 1's absorbance models well as a PSB form (solid line) analogous to bovine rhodopsin's Batho intermediate (dashed line), but not as a deprotonated SB form of Siberian hamster Batho (dotted line). Similarly, the middle panels show how absorbance of both USI 2 (■) and USI 3 (♦) are well fit by deprotonated SB forms of the model Siberian hamster Lumi I and Lumi II spectra (solid lines), derived from the bovine Lumi I and Lumi II (dashed lines) but not by model PSB forms of those photointermediates (dotted lines). The lower panels show how absorbance of USIs 4 (+) and 5 (○) can be decomposed into two bands, presumably corresponding to mixtures of protonated and deprotonated SB species, but the correspondence of these components to bovine rhodopsin photointermediates is not clear.