The labels naphthalene-2,3-dicarboxaldehyde (NDA), 1-phenylnaphthalene-2,3-dialdehyde (ΦNDA), and anthracene-2,3-dialdehyde (ADA) have been used as fluorigenic reagents. They formed fluorescent derivatives with proteins. The derivatives formed are in fact isoindoles. The fluorescence decay of the labels-antibody was found to extend over a period of 4, 8, and 10 h for ΦNDA, ADA, and NDA-derivative, respectively. Protein formed is comparatively less stable as compared to simple amino acids. In relation to innerfilter effect, the addition of cytochrome C, myoglobin, and ATP as absorbers to label-human albumin fluorophores appeared to have quenched the fluorescence. In the case of using NDA as label, the fluorescence was quenched roughly 70%, 24%, and 58% for addition of cytochrome C, myoglobin, and ATP, respectively. The labels used were found to give rapid, reproducible, and reliable results.

The effect of caffeine complexation with riboflavin on the kinetics of riboflavin photolysis in the pH range 2.0-10.5 has been studied. The photolysis of riboflavin solutions (5x10(-5) M) was carried out in the presence of caffeine (0.5-2.5x10(-4) M) using a visible radiation source. A specific multicomponent spectrophotometric method was used for the determination of riboflavin and photoproducts in photolysed solutions. The apparent first-order rate constants (k) for the photolysis reactions range from 2.71x10(-4) to 4.26x10(-2) min(-1). The values of the rate constants decrease with increasing concentrations of caffeine indicating its inhibitory effect on the reactions. The second-order rate constants (k') for the caffeine inhibited reactions lie in the range of 0.13 to 5.10x10(-3) M(-1) min(-1). The log k-pH profiles for the photolysis reactions at various caffeine concentrations involve multiple steps indicating a gradual increase in the rate up to pH 10. The lower rates at pH 2.0 and 10.5 are due to the ionization of riboflavin as evident from fluorescence measurements. The k'-pH profile for the interaction of riboflavin with caffeine represents a bell-shaped curve in the pH range 3-6 followed by a sigmoid curve in the pH range 7-10. The inhibition of photolysis of riboflavin in the presence of caffeine is a result of the monomeric interaction and complex formation of caffeine with riboflavin. The photochemical interaction of riboflavin with caffeine suggests that a pH around 6 is most appropriate for the stabilization of the vitamin. At this pH the complex shows the highest stability constant.