Dityrosine, 3,4-Dihydroxyphenylalanine (DOPA), and Radical Formation from Tyrosine Residues on Milk Proteins with Globular and Flexible Structures as a Result of Riboflavin-Mediated Photo-oxidation

Riboflavin-mediated photo-oxidative damage to protein Tyr residues has been examined to determine whether protein structure influences competing protein oxidation pathways in single proteins and protein mixtures. EPR studies resulted in the detection of Tyr-derived o-semiquione radicals, with this species suggested to arise from oxidation of 3,4-dihydroxyphenylalanine (DOPA). The yield of this radical was lower in samples containing beta-casein than in samples containing only globular proteins. Consistent with this observation, the yield of DOPA detected on beta-casein was lower than that on two globular proteins, BSA and beta-lactoglobulin. In contrast, samples with beta-casein gave higher yields of dityrosine than samples containing BSA and beta-lactoglobulin. These results indicate that the flexible structure of beta-casein favors radical-radical termination of tyrosyl radicals to give dityrosine, whereas the less flexible structure of globular proteins decreases the propensity for tyrosyl radicals to dimerize, with this resulting in higher yields of DOPA and its secondary radical.