Absorption band shape and emission in rhodopsin.

Abstract

Abstract— –The Planck law relationship between absorption and emission spectra is applied to the spectral sensitivity curve of human rod vision, assumed to be equivalent to the absorption spectrum of the visual pigment rhodopsin, to compute a hypothetical emission spectrum for rhodopsin. When 17, 100 cm‐1 is used as the reflection axis, the mirror image of this hypothetical emission substantially matches the activation spectrum over 2500 cm‐1. The predicted emission increases exponentially at long wavelengths, in contrast to published observations of fluorescence at 580 nm; this discrepancy, and an estimate of excited state lifetime based on the Planck law theory, suggest that emission occurs from a meta‐stable vibrationally‐excited state. The exceptionally slow falloff in absorption at long wavelengths is explained as being due to the smaller dependence of potential energy on angle of twist about bonds in the polyene chain in the lowest π, π* singlet state of rhodopsin than in its ground state; a model assuming six or seven essentially flat vibronic modes in the excited state accurately fits the observed action spectrum.