The K minus BR difference spectra in the 3,530- to 3,350-cm−1 region for the wild type (dotted lines) and the mutants (solid lines) T17V (a), T24V (b), T46V (c), T47V (d), T55V (e), T89S (f), T90V (g), T121V (h), T142N (i), T178N (j), and T205V (k). The sample was hydrated with D2O, and the window-tilting angle (φ0) is 53.5°. One division of the y axis corresponds to 0.001 absorbance unit.