Figure 5.

The M minus BR difference spectra of unlabeled (solid lines) and 3-¹⁸O-threonine-labeled (dotted lines) BR in the 1,780–1,730 cm⁻¹ region, typical frequencies of the C=O stretching vibration of protonated carboxylic acids. The sample was hydrated with H₂O (Left) and D₂O (Right), and the window was tilted at 0°, 17.8°, 35.7°, and 53.5° (from top to bottom). One division of y axis corresponds to 0.002 absorbance unit.