Excitation of Biomolecules by Coherent vs. Incoherent Light: Model Rhodopsin Photoisomerization

Light-induced processes in biological molecules, which occur naturally in continuous incoherent light, are often studied using pulsed coherent light sources. With a focus on timescales, the relationship between excitation due to these two types of light sources is examined through a uniform minimal model of the photoisomerization of retinal in rhodopsin, induced by either coherent laser light or low level incoherent light (e.g. moonlight). Realistic timescales for both processes are obtained and a kinetic scheme involving rates for both coherent and incoherent light excitation is introduced, placing all timescales into a uniform framework. The rate limiting step in the natural light-absorption process is shown to be the low incoherent photon flux.