FIGURE 3.

Typical dark state absorbance spectra (red curves) of a purified type-2 (A) and type-1 (B) pigment. Both spectra exhibit a major peak (α-band) and a small satellite (β-band), both originating in the chromophore, and a γ-band near 280 nm, mainly originating in protein residues. The α-band derives from the whole conjugated polyene system (S0-S1) (cf. Figure 1), while the β-band derives from a smaller segment, and its intensity also depends on the torsion in the polyene chain. Upon short illumination of the monostable type-2 pigment (A) in the presence of hydroxylamine, the liberated retinal is converted into retinaloxime (blue curve). The Meta state of bistable type-2 pigments, also reacts with hydroxylamine generating retinaloxime, but usually quite slowly. Short illumination of type-1 pigments (B) in the presence of hydroxylamine hardly affects the photocycle and the return to the ground state. However, upon prolonged illumination hydroxylamine will slowly attack photo-intermediates, mainly M and N, releasing retinaloxime.