The masking model: a possible explanation for various effects of near-UV radiation.

Abstract

Abstract— A general model is proposed to explain certain phenomena observed upon the near‐UV irradiation of biological materials. The model states that the ratio of two action spectra (e.g. in the presence or absence of histidine) does not accurately portray the absorption spectrum of the underlying chromophore. Rather, such a ratio is dictated by the relative amounts of two photoproducts. Both photoproducts are induced most efficiently at or below 300 nm. One photoproduct, sensitive to the presence of the variable agent (e.g. histidine), has a broader production spectrum than the other, which is induced independently of the variable agent. The first photoproduct is quantitatively more important at shorter wavelengths while the second dominates at longer wavelengths. An equation has been derived which defines the model in mathematical terms. Such a model might explain: (1) increased phage and bacterial inactivation in the presence of sublethal hydrogen peroxide concentrations; (2) the discrepancy between the inactivation action spectrum and absorption spectrum of psoralen; (3) histidine protection of transforming DNA inactivation; (4) phage T7 inactivation; (5) bacterial inactivation under anoxic conditions; (6) inactivation of DNA repair enzymes; and (7) photomutagenesis at low fiuence rates.