Figure 6.

Increasing the bulk of the substituent at the C9 position of retinal does not accelerate the decay of the O intermediate in the photocycle of the Leu-93 → Ala mutant. Transient absorbance traces were measured at 625 nm in the Leu-93 → Ala mutant with 9-ethylretinal (trace A) or 9-isopropylretinal (trace B). The kinetics were measured at 22°C in sodium phosphate buffer (10 mM) at pH 7 with 0.15 M KCl and 3 M NaCl, because formation of a chromophore with a single visible absorbance peak was observed only at high ionic strengths.