Figure 6.

Dependence of the frequency of the C=O stretch of D85 in the M state on external pH. The midpoint of the frequency shift is at 5.2 (±0.3). Frequencies of band maxima have been calculated by fitting a Gaussian to the bands at the respective pH (cf. Fig. 5). The full width at half maximum of the band (12 ± 0.6 cm⁻¹) is invariant to the external pH.