FIGURE 3.

Double difference FT-IR spectra showing perturbations of azide antisymmetric stretching bands. To identify any structural changes at the OEC azide site, manganese-depleted PSII data were subtracted from chloride-depleted PSII data. These double difference spectra were created by subtraction of data shown in Fig. 2, C and D. In A, subtracted spectra were acquired with one laser flash (Fig. 2, C and D, red). In B, subtracted spectra were acquired with three laser flashes (Fig. 2, C and D, blue). In C, a control double difference spectrum, in which no vibrational bands are expected, was generated by subtraction of one half of the data in (Fig. 2 D, red) from the other half of the data set and dividing by the square root of two. Delta absorbance on the y axis indicates that the spectra show changes in absorbance. All samples contained 1.5 mM recrystallized DCBQ. The tick marks on the y axis represent 5 × 10⁻⁴ AU.