The initial photochemistry of vision

The 11-cis to 11-trans torsional isomerization of retinal chromophore in rhodopsin has long been known to be involved in the early photochemistry of visual process [1]. Recent work on generation of femtosecond pulses in blue-green region of spectrum made it possible to study this process directly. We have done time-resolved absorption experiments on rhodopsin in both protonated and deuterated aqueous environments at room temperature [2, 3]. These measurements test both the standard picture of rapid photoisomerization and also address the issue of whether proton translocation is also important in the initial step of vision [4]. A 500 nm 150 fs pump pulse initiates die reaction and a white light continuum probe is used to monitor absorbance transients in the 500 to 640 nm range. We observed two distinct kinetic components having 200 fs and 3 ps lifetimes. These data are well modeled by rate equations based on scheme depicted in Figure 1 which illustrates isomerization along the torsional coordinate of the 11-cis bond of the retinal chromophore. As illustrated, the 200 fs time can be associated with the appearance of a 90 degree twisted metastable intermediate by rotation around the double bond on the excited state surface and the 3 ps time with the decay of that intermediate to form the fully isomerized all-trans photoproduct known as bathorhodopsin. These times as well as the absorption spectrum (Figure 3) of the twisted metastable configuration agree well with the semiempirical energy level and molecular dynamics calculations of Tallent et al.[5].