Structural determinants of spectral tuning in retinal proteins-bacteriorhodopsin vs sensory rhodopsin II

The mechanism of spectral tuning in the rhodopsin family of proteins, that act as light-driven proton (ion) pumps and light detectors, has been invest igated by a combined ab initio quantum mechanical/molecular mechanical tech nique. Calculations are performed on two members of the family, bacteriorho dopsin (bR) and sensory rhodopsin II (sRII), for which crystal structures o f high resolution are available, to explore the physical mechanisms of spec tral tuning. Despite a high degree of similarity in the three-dimensional s tructure, electrostatic environments in bR and sRII differ sufficiently to shift absorption maxima of their common chromophore, a retinal bound to a l ysine via a protonated Schiff base, from 568 nm in bR to 497 nm in sRII. Th is spectral shift, involving the electronical ground state (SO) and first e xcited state (S-1) of retinal, is predicted correctly within 10 nm. The spe ctral shift can be attributed predominantly to a change in polarization of the S, state, and is induced predominantly by a shift of the G helix that r enders the distance between the Schiff base nitrogen of retinal and the Asp 201 counterion shorter in sRII than in bR. A second, weakly allowed excited state, S-2, is predicted to lie energetically close to S-1, at 474 nm. Its energetic proximity to the S, state suggests strong vibronic coupling and explains a shoulder observed at 457 nm in the sRII spectrum.