FIGURE 4.

Spectral changes during the reductive half-reaction with 2-PEA. UV-visible absorption spectra were recorded after mixing AGAO (100 μm monomer) with 2 mm 2-PEA (A), 2 mm 2-PEA plus 300 mm Na₂SO₄ (B), 2 mm 2-PEA plus 100 mm NaCl (C), or 2 mm 2-PEA plus 300 mm NaBr (D) in 50 mm HEPES buffer, pH 6.8, at 4 °C under anaerobic conditions. The spectra obtained at 0 (red), 2.30, 3.84, 6.4, 8.96, 14.1, 24.3, 44.8, 117, 209, 332, 600, and 1023 ms are shown using darker colors to represent later times. The arrows indicate the direction of the spectral changes. Deduced absorption spectra are shown for TPQ_ox (red), TPQ_ssb (yellow), TPQ_psb (green), TPQ_amr (black), and TPQ_sq (blue) in E, F, G, and H, calculated from the measurements of A (no salt), B (300 mm Na₂SO₄), C (100 mm NaCl), and D (300 mm NaBr), respectively. The absorption spectra for TPQ_amr·X⁻ (cyan) generated during the experiments using 100 mm NaCl (C), and 300 mm NaBr (D) are shown in G and H, although they are defined to be identical to that of TPQ_amr (black). The inset of A shows the time course of the absorbance at 468 nm in the spectral changes of A (no salt, red), B (300 mm Na₂SO₄, black), C (100 mm NaCl, blue), and D (300 mm NaBr, green).