The Interplay of Light and Heat in Bleaching Rhodopsin

Abstract

I mTRODVCTIONThe absorption of light by rhodopsin in the retina activates the pigment molecules and leads eventually to the stimulation of rod vision. The initial light reaction is followed by a sequence of thermal reactions occurring equally in light or darkness, by which the rhodopsin, a conjugated carotenoid protein, ultimately bleaches to retinenel and protein (Wald, Durell, and St. George, 1950). The work described here deals with the initial activation of rhodopsin.Rhodopsin can be bleached not only by light but by heat. At 50°C. or above it bleaches rapidly in the dark and at a rate that is very temperature-dependent. Lythgoe and Quilliam (1938) calculated an experimental activation energy of 44 kg. cal. per mole from their data on the temperature coefficient in neutral solutions. On the other hand, the rate of bleaching by white light is not temperature-dependent, showing that no thermal activation is needed (Hecht, 1920-21).The purpose of the following research was to study the transition between photic and thermal activation. The argument runs as follows. On going from shorter wave lengths to red light a point should be reached beyond which the incident quanta cannot supply the entire energy needed to activate the rhodopsin molecule. If bleaching is to occur nonetheless, the deficit will have to be met by a contribution of thermal energy from the internal degrees of freedom of the molecule. There should therefore be a critical wave length beyond which the bleaching by light becomes temperature-dependent.Measurement of the temperature coefficient of bleaching at intervals along the wave length scale showed that a temperature dependence does in fact appear in the orange at about 590 mu. Moreover, the quantum energy at this wave length differs very little from the activation energy observed in heat bleaching. At wave lengths longer than 590 mt~, the temperature coefficient increases regularly so that the sum of the energy of the absorbed quanta and the thermal ac-

A thermal component of excitation in the lateral eye of Limulus

“…The temperature dependence of the photosensitivity of extracted cattle and frog rhodopsin was shown to have a similar pattern of wave-length temperature interaction (St George, 1952). This similarity strongly suggests that the effect of temperature observed in this experiment is directly on the visual pigment.…”

“…A difference could arise if bleaching were not sufficient to produce excitation or if it were incidental to the excitation process. It is therefore of interest that the activation energy for bleaching of frog and cattle rhodopsin, 48 kcal mole-' (St George, 1952) does exceed the lower limit of the activation energy for excitation found for the lateral eye of Limulus.…”

Human infrared vision is triggered by two-photon chromophore isomerization

“…The spectral sensitivity of human dim light perception matches well with the absorption spectrum of the rod visual pigment, rhodopsin (2,3). Activation of visual pigments is temperature independent around their absorption peaks (λ max ), but at longer wavelengths, the lower energy photons must be supplemented by heat to achieve chromophore photoisomerization (4). Long wavelength-sensitive visual pigments of vertebrates exhibit maximal absorption at the ∼500-to ∼625-nm range.…”

“…Pigments with λ max > 700 nm are theoretically possible, but the high noise due to spontaneous thermal activation would render them impractical (5). At human body temperature and with 1,050-nm stimulation, the sensitivity of the peripheral retina to one-photon (1PO) stimulation is less than 10 −12 of its maximum value at 505 nm (4,6). Indeed, reports about human IR vision can be found in the literature, although they are fragmentary and do not describe the mechanism of this phenomenon.…”

The photosensitive pigments in the retinae of deep-sea fish

“…The rate at which human visual purple would have to break down in the dark in order to be the limiting factor in the detection of light, was calculated as being 0'0005 %/h by Denton & Pirenne. We know that in extracts with digitonin it breaks down at the much higher rate of I %/h (St George, 1952), whilst Barlow (1956Barlow ( , 1957 has calculated the consequences of the idea that retinal' noise' sets the limit to visual performance and has, moreover, shown experimentally that many of the predictions of his theory hold good. Reconsidering the possible gain in sensitivity of the deep-sea fish over the human with the idea of retinal 'noise' in mind, we have as before the same advantages because of the greater aperture and greater transparency of eye media, but the extra density of pigment would be much less advantageous because an increase in density is not only accompanied by an increase in signal given by the more effective absorption of the incident light quanta, but also by an increase in 'noise' consequent in the increase in the· amount of visual pigment which achieves this extra absorption.…”