Figure 2.

The K minus BR difference spectra of unlabeled (dotted lines) and [3-¹⁸O]threonine-labeled (solid lines) BR in the 3,700- to 2,000-cm⁻¹ region with the window-tilting angles (φ₀) of 0° (a) and 53.5° (b). The sample was hydrated with D₂O, and spectra were measured at 77 K. These frequencies cover the entire O-H and O-D stretches. One division of the y axis corresponds to 0.001 absorbance unit. Both dotted and solid lines overlap well except for three frequency region shown by arrows, indicating that three threonines change the stretching frequencies of their O-H (O-D) groups on photoisomerization. Diff. Abs., difference absorbance.