Fluorescence studies of the binding of two natural alkaloids (cinchonine and cinchonidine) with β-lactoglobulin

The binding to the transport protein β-lactoglobulin (β-LG) of the natural alkaloids stereoisomers, cinchonine ana cinchonidine, has been investigated in a pH 7.5 buffer solution by fluorescence spectroscopy. The large decrease of β-LG fluorescence brought about by addition of cinchonine and cinchonidine has been correlated to an increase of the complex emission, indicating an efficient energy transfer. The complex affinity constants have been evaluated. The results clearly show that only one binding site is affected in the dimeric protein and ţhat the affinity of cinchonine for β-LG is much more important than that of cinchonidine.