Riboflavin-sensitized photooxidation of ascorbic acid: kinetics and amino acid effects

The study on the effects of riboflavin contents and light intensity on the riboflavin-sensitized decomposition of ascorbic acid indicated that light and riboflavin accelerated the decomposition of ascorbic acid. The relative photosensitizing activity of riboflavin, methylene blue and protoporphyrin IX was 21:15:1 at 1–2 ppm. The rate constants for reaction of ascorbic acid with singlet oxygen at pH 7·5, 6·0 and 4·5 were 6·63 × 108, 5·77 × 108 and 5·27 × 108 M−1 s−1, respectively. Cysteine showed strong antioxidant activity on both riboflavin- and methylene blue-sensitized photooxidation of ascorbic acid and its antioxidant effect was concentration-dependent. Alanine and phenylalanine at 0·1% concentration showed antioxidant effects on the riboflavin-sensitized photooxidation of ascorbic acid but prooxidant effects on the methylene blue-sensitized photooxidation. Tyrosine (0·1%) exhibited strong prooxidant activity on both the riboflavin- and methylene blue-sensitized photooxidation of ascorbic acid. However, tryptophan (0·1%,) showed antioxidant or prooxidant activity on the photooxidation of ascorbic acid depending on the storage time.