A molecular orbital study of the protein-controlled bathochromic shift in a model of rhodopsin

A molecular orbital study based on the Pariser-Parr-Pople method has been made of the effect of a negative charge on the long wavelength band of the protonated Schiff base of methylamine and retinal. The results show that the energy of the lowest transition, but not the oscillator strength, is highly dependent on the position of the negative charge. With the charge at infinity, the spectral maximum is found to be 524.7 nm. When the charge is moved closer to the Schiff base, the spectrum shows a blue shift from this value toward 440 nm. However, a red shift to about 556 nm is calculated when the charge is moved to the opposite end of the molecule. These results support the theories which hold that the position of a negative charge on the protein controls the observed spectral shifts on rhodopsin formation.