Fluorescence properties of insulin analogs with tryptophan substitutions

The fluorescence properties of four insulin analogs with tryptophane substitutions, [L-Trp]A1 insulin, [D-Trp]A1 insulin, [L-Trp]B1 insulin and desPheB1 [L-Trp]B2 insulin, have been studied. The effect of pH on the fluorescence behaviours of the three L-Trp insulin analogs are similar to only one fluorescence intensity peak at pH 9. [D-Trp]A1 insulin shows two fluorescence intensity peaks, a main peak at pH 9.8 and a small peak at pH 5.5. The fluorescence quenching on the acid side of the pH approximately 9 peak is caused by protonation of the A1 alpha-amino group. The difference in A1 alpha-amino group pK values of these derivatives, the presence of a second pH-fluorescence peak for the D-Trp derivative and other differences reflect differences in molecular conformation of the two A1-insulin analogs which could form the basis of the pronounced differences in their biological activities.