Quenching of the fluorescence of pterin derivatives by anions.

Steady-state and time-resolved studies of the fluorescence of pterins in aqueous solutions in the presence of different anions have been performed using the single-photon counting technique. In the pH range between 3 and 13, most pterins exist in a protonated and a deprotonated form. Results obtained for both acid and basic forms of five compounds belonging to the pterin family (pterin, 6-carboxypterin, 6-formylpterin, biopterin and neopterin) show that the fluorescence of the acid forms is dynamically quenched by phosphate and acetate, and the corresponding bimolecular rate constants of fluorescence quenching (k(q)) are reported. These results are of importance from the technical and analytical points of view because measurements of the fluorescence of pterin derivatives for a variety of purposes are often performed in the presence of salts, especially buffers, and significant quenching of the pterin fluorescence by the buffer might lead to errors in interpretation and erroneous conclusions. No quenching of the fluorescence of the acid forms by chloride, sulfate or nitrate was detected. The fluorescence of the basic forms was either not quenched by anions or any such quenching was negligible in comparison with that observed for the acid forms.