An analysis of two spectral properties of vertebrate visual pigments

Spectral data are scrutinized on two properties: (1) the chromophore-induced wavelength shift, i.e. the variation in absorbance maximum (λ max) upon exchanging the vitamin A 1-based chromophore with one based on vitamin A 2 in the same opsin; (2) the half-band width (HBW) variation as a function of the reciprocal of the peak absorbance (λ max) −1. It is shown that in an extended spectral range that includes the UV and when data are plotted in wavenumbers, the chromophore-induced shifts can be approximated with a parabola, whose minimum occurs near 23,000 cm −1 (≈ 430nm). Similarly, HBW variations can also be fitted with parabolas, however, these show maxima near 430 nm. The theoretical implications of the two phenomena concerning λ max tuning in vertebrate visual pigments are discussed.