Fig. 2.

Reaction-induced FTIR detection of the protonated water cluster, W₅⁺, produced by the S₁-to-S₂ transition. (A) Positive 2,880-cm⁻¹ band from the cationic water cluster in calcium PSII (black), compared with calcium-depleted (gray). (B) Calcium PSII after treatment with 100 mM ammonia (black), compared with calcium-depleted (gray; repeated from A, gray). (C) Calcium PSII after exchange into ²H₂O buffer (black), compared with calcium-depleted in ¹H₂O buffer (gray; repeated from A, gray). The O–D stretching band in C, black has been omitted for clarity. The calcium-depleted difference spectrum was acquired with a single flash to a dark-adapted sample and corresponds to S₂′-minus-S₁. (D) Baseline (S₁-minus-S₁) acquired from dark-adapted, calcium-depleted PSII. Spectra are averages of 31 (A, black), 16 (B, black), 12 (C, black), and 15 (A–D, gray) samples. y axis ticks represent 1 × 10⁻⁴ absorbance units. The temperature was 263 K.