Reconstitution of xanthorhodopsin with retinal analogues. (a) Difference spectra accompanying reconstitution with all-trans 9-desmethyl retinal during the first 40 min after addition of retinal. Spectra 1 through 6, absorption changes at 3, 5, 10, 20, 30, 40, min after addition of 3 µM of all-trans 9-desmethyl retinal to xantho-opsin (the spectrum measured 1 min after addition was taken as a baseline). (b) Difference spectra accompanying reconstitution with 13-desmethyl retinal during the first 40 min after addition of retinal (same time intervals as for 9- desmethyl analogue in a). (c) Spectra 1 and 2, two components of spectral changes accompanying reconstitution with 13-desmethyl retinal corresponding to time constants 30 min and 550 min, respectively (from global fit).