Origin and Consequences of Steric Strain in the Rhodopsin Binding Pocket

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To study the origin and the effects of steric strain on the chromophore conformation in rhodopsin, we have performed quantum-mechanical calculations on the wild-type retinal chromophore and four retinal derivatives, 13-demethyl-, 10-methyl-13-demethyl-, 10-methyl-, and 9-demethylretinal. For the dynamics of the whole protein, a combined quantum mechanics/molecular mechanics method (DFTB/CHARMM) was used and for the calculation of excited-state properties the nonempirical CASSCF/CASPT2 method. After relaxation inside the protein, all chromophores show significant nonplanar distortions from C10 to C13, most strongly for 10-methylretinal and least pronounced for 9-demethylretinal. In all five cases, the dihedral angle of the C10-C11=C12-C13 bond is negative which attests to the strong chiral discrimination exerted by the protein pocket. The calculations show that the nonplanar distortion of the chromophore, including the sense of rotation, is caused by a combination of two effects: the fitting of both ends to the protein matrix which imposes a distance constraint and the bonding arrangement at the Schiff base terminus. With both the counterion Glu113 and Lys296 displaced off the plane of the chromophore, their binding to N16 exerts a torque on the chromophore. As a result, the polyene chain, from N16 to C13, is twisted in a clockwise manner against the remaining part of the chromophore, leading to a C11=C12 bond with the observed negative dihedral angle. Shifts of the absorption maxima are reproduced correctly, in particular, the red shift of the 10-methyl and the strong blue shift of the 9-demethyl analogue relative to the wild type. Calculated positive rotatory strengths of the alpha-CD bands are in agreement with the calculated absolute conformation of the mutant chromophores.