Cation binding characteristics of tetrandrine studied by UV-Vis absorption and fluorescence spectroscopies

The binding of monovalent (Li+, Na+, K+) and bivalent (Mg2+, Ca2+) metal cations to a macrocyclic alkaloid tetrandrine has been studied by UV-Vis absorption and fluorescence spectroscopies. The calculated binding constants are between 3654 and 282 613 dm3 mol−1 and the corresponding free energies from 4.777 to 7.308 kcal mol−1. The effect of the solvent on the complexation strength as well as the role of the counter-ion are discussed. Small changes are observed in the UV-Vis spectrum after complexation in respect to free ligand. The fluorescence response increases significantly in intensity and a blue shift appears by adding of calcium perchlorate to tetrandrine (6,6′,7,12-tetramethoxy-2,2′-dimethyl—berbaman; TET) as a consequence of conformational restriction induced by cation binding.