3.1.2. Determination of the tryptophan fluorescence
α-La includes four residues of tryptophan and a high proportion of essential amino acids (Barbana & Pérez, 2011). A possibility of reaction of oxidised phenolic compound with tryptophan has also been discussed by Le Bourvellec and Renard (2012). Fig. 1 shows the changes of the tryptophan fluorescence of α-La, which depended on the reaction extent with CA or EGCG for 24 h at pH 8.0, 60 °C. The quenching of the fluorescence intensity, as an indicator of tryptophan changes, was studied after activation at 295 nm and measurement of the emission from 300 to 360 nm. A significant decrease in the fluorescence intensity of α-La–CA or EGCG covalent complex was generally observed (p < 0.05). This difference in fluorescence intensity indicated that the indole ring of tryptophan was most likely to be involved in the covalent reaction of α-La with CA or EGCG. Compared with CA, EGCG slightly decreased the fluorescence intensity of α-La. These changes in fluorescence intensity revealed that covalent interaction between α-La and CA or EGCG led to possible alteration in the structure of α-La, and the alteration of the fluorescence intensity was consistent with the result by Ishii et al. (2011), who reported the fluorescence intensity of human serum albumin to be gradually decreased with increase of EGCG level.