An insight into pH-induced changes in FAD conformational structure by means of time-resolved fluorescence and circular dichroism.

Optical properties of flavin adenine dinucleotide (FAD) moiety are widely used nowadays for biotechnological applications. Given the fundamental role played by FAD, additional structural information about this enzymatic cofactor can be extremely useful in order to obtain a greater insight into its functional role in proteins. For this purpose, we have investigated FAD behaviour in aqueous solutions at different pH values by a novel approach based on the combined use of time-resolved fluorescence and circular dichroism spectroscopies. The results showed that pH strongly affects time-resolved fluorescence emission and the analysis allowed us to detect a three-component decay for FAD in aqueous solution with pH-depending lifetimes and relative amplitudes. Circular dichroism data were analyzed by a multi-Gaussian fitting procedure and the trends of properly chosen parameters confirmed pH-depending changes. The comparison between the results obtained by these two optical techniques allowed us to improve the significance of the outcome of circular dichroism. This combined approach may provide a useful tool for biotechnological investigation.