Quantum yield for the photo-degradation of vitamin D₃

Ultraviolet radiation can photodegrade vitamin D₃ to suprasterol I, superasterol II and 5,6-transvitamin D₃. The probability of the photodegradation of vitamin D₃, after the absorption of one photon, is called the quantum yield. We estimated the quantum yield ϕ for all the degeneration reactions together on the basis of the results reported by Webb et al. (1989) for a degradation experiment on pure, tritiated vitamin D₃ in methanol in sunlight. We find ϕ = (0.42 ± 0.1). This value of the quantum yield can be used to estimate the kinetics contribution from vitamin D₃ degradation in exposure experiments and model calculations.