The protective action of a series of organic sulfur compounds and amino acids has been investigated by using glycine and glycylglycine as substrates. A linear relationship is shown to exist between the reciprocal of the G-m value for the substrate in the presence of the protecting agent, and the concentration of this protecting agent. The "protecting activity" of the agents, expressed as the ratio of the rate constants for the reaction of radicals with the protecting agent and the substrate, is determined.

The Photolysis of glycine and alanine containing peptides in oxygenated solution has been shown to result in the formation of glyoxylic and pyruvic acids, through oxidative cleavage of the peptide bond[1]. The same products were formed in the radiolysis of such peptides in the presence of oxygen[2]. In a peptide containing aromatic amino‐acids the absorption of light results in the excitation of the aromatic ring and it has been postulated[3] that energy is transferred to the peptide bond, resulting in the formation of a radical on the carbon atom adjacent to the peptide bond. Radiolysis, however, involves the attack of hydroxyl radicals and other reactive species formed in the absorption of radiation by water[2]. This study compares the behavior of phenylalanylglycine to photolysis and radiolysis in aerated aqueous solution in relation to a possible energy transfer in photolysis.