Physiological Effects of Light

Abstract

The destructive effects produced in living systems by ultraviolet light are in many cases quite similar to effects produced by visible light in living systems sensitized by dyes (photodynamic action). As examples may be cited: the hemolysis of red blood cells; the production of erythema, pigmentation, and destructive lesions of the skin of mammals; muscle responses, etc. These similarities suggest a common causal mechanism.

The writer 1 has called attention to the fact that the photodynamic and ultraviolet effects can be differentiated, in so far as they have been studied, by the fact that the former take place only in the presence of oxygen, whereas the latter take place quite readily in the absence of oxygen. Nevertheless the thermodynamic considerations given below indicate the possibility of a common mechanism.

In investigations of the photochemical reactions of fluorescein dyes in this laboratory it has been found that hydrogen peroxide is formed when aqueous solutions of these dyes are irradiated with visible light in the presence of molecular oxygen, and as has been shown by the writer, 2 it is most probable that the production of peroxide is a step in the destruction of living systems sensitized by these dyes.

The principal reaction involved would appear to be:

(1) 2 H₂O (1.) + O₂ = 2 H₂O₂ (aq.); ΔF₂₉₈ = 50180 cals.; ΔH = 46000 cals. 3

Obviously this reaction cannot proceed very far in the direction indicated, and at equilibrium the concentration of H₂O₂ present would be approximately 10^-18 M. Since in the above mentioned experiments H₂O₂ was produced in concentrations up to 10^-3 M, it is obvious that we are not dealing with a thermal reaction, the rate of which is increased by the action of light, but one in which the energy is supplied directly by the light.