Retinal

Retinal is the photoactive component in various biologically relevant receptor proteins. It triggers perception, ion pumping or energy production in rhodopsins, a family of seven-helical transmembrane proteins. The initial reaction features a light induced change of the chromophore configuration: retinal isomerizes from 11-cis to its all-trans form in mammalian rhodopsin, whereas archaeal rhodopsins invole all-trans to 13-cis photoisomerization. This initial step takes place on the femtosecond timescale and drives further, thermally activated rearrangements in the protein to fulfill its function. We investigate the initial and ultrafast photoisomerization of retinal chromophores in their protein environments using combined quantum mechanical/molecular mechanical (QM/MM) approaches. With atomistic molecular dynamics simulations using the COBRAMM software,¹⁠ we follow the motions of the molecules on the excited and ground state potentials to obtain information on reaction channels, product distributions (i.e. quantum yields) and reaction times

Vision in vertebrates is steered by 11-cis to all-trans photo-isomerization of retinal with ca. 65% quantum yield. The first spectroscopically detectable photoproduct is batho-rhodopsin, where the chromophore entails a twisted all-trans configuration. The twists store energy, which is released in later thermal steps. A series of biochemical reactions finally lead to the excitation of the optical nerve. With CASSCF excited state trajectories including surface hopping,² we were able to follow the molecular motions and spectral traces of retinal during this photoprocess. The reaction involves a unidirectional bicycle-pedal-like motion of the C9=C10 and C11=C12 double bonds. The hydrogen-out-of-plane modes at the isomerizing double bond play a central role in the generation of the photoproduct. Bi-directional deactivation was found in the analogue iso-rhodopsin protein, that has a 9-cis chromophore.³⁠