Complexing of Riboflavin and Its 2‐substituted Analogs With Adenosine and Other 6‐substituted Purine Derivatives*

Fluorescence properties of riboflavin and its 2‐substituted analogs and the quenching of fluorescence which occurs upon complexing with adenosine and other 6‐substituted purine derivatives have been examined at different concentrations of interactants and hydrogen ion. The 6‐substituted purines and their ribosides show varying degrees of association with riboflavin. Successive N‐alkylation of the 6‐amino group of purine ribosides leads to more stable complexing in the order ethyl > methyl > propyl with riboflavin and its analog. These results suggest that such groups, including the 6‐amino group of adenine or adenine moiety of FAD, do not directly interact with flavins by hydrophobic or hydrogen bonding. Lowering the pH to above that required to protonate the isoalloxazine system disrupts the intermolecular complexes. Hence, only the unprotonated bases can form such complexes. The 2‐substituted aminoriboflavins, as well as FAD, exhibit fluorescence optima upon changing pH. The species involved appear to be a non‐fluorescent form with protonated isoalloxazine and amine or adenine portions at low pH, a fluorescent form with only the amine or adenine portions protonated at moderately acid pH, and a non‐fluorescent form which is intramolecularly quenched at higher pH. The general electron donating properties of the amines may be the common reason for their quenching of fluorescence in inter‐ and intramolecular complexes with flavins.