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The lumiflavin sensitized photooxidation of substituted phenylalanines and tyrosines

, : The lumiflavin sensitized photooxidation of substituted phenylalanines and tyrosines. Photobiochemistry & Photobiophysics 10(3): 149-162

Steady-state kinetic and flash photolysis techniques were used to study the relationships between the structures of substituted phenylalanines and tyrosines and their susceptibility to lumiflavin-sensitized photooxidation. Phenylalanine is oxidized at a moderate rate as measured by oxygen uptake; in most cases, substitution of progressively better electron donating (activating) groups such as -OCH3 (as in 4-methoxyphenylalanine), -OH (as in tyrosine) and -NH2 (as in p-aminophenylalanine) to the benzenoid ring results in increased rates of photooxidation. Substitution of additional activating groups to the ring gives even greater rates. Substitution of a deactivating group such as -NO2 (as in p-nitrophenylalanine) results in a decreased photooxidation rate; additional deactivating groups produce a further decrease. Thus, in most cases, the rate determining step in the photooxidation mechanism is largely dependent on the presence and position of an electron donating substituent on the benzenoid ring. A previous study of the eosin Y-sensitized photooxidation of substituted phenylalanines gave analogous results with the major difference being that eosin Y does not sensitize the photooxidation of phenylalanine and 4-methoxyphenylalanine. The flash photolysis studies show that triplet lumiflavin is a better oxidizing agent than triplet eosin since phenylalanine and 4-methoxyphenylalanine react efficiently with triplet flavin but not with triplet eosin; this may account for the major differences observed between the eosin Y- and lumiflavin-sensitized photooxidations.

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