Spectroscopic studies on the binding of bioactive phenothiazine compounds to human serum albumin

In this paper, the binding characteristics of human serum albumin (HSA) with phenothiazine derivatives (PDS) viz., thioridazine hydrochloride (TDH) and triflupromazine hydrochloride (TFP) have been studied by employing different spectroscopic techniques. The Stern–Volmer quenching constant values were found to decrease with increase in temperature thereby indicating the presence of static quenching mechanism in the interactions of PDS with HSA. The number of binding sites, n and the binding constant values, K were noticed to be 1.063 and (4.46 ± 0.040) × 104 L M−1 for TDH and 1.08 and (5.18 ± 0.071) × 104 L M−1 for TFP, respectively at 298 K. The binding distances and the energy transfer efficiency between PDS and protein were determined. The negative value of enthalpy change and positive value of entropy change in the present study indicated that both hydrogen bonding and hydrophobic forces played a major role in the binding of PDS to HSA. The circular dichroism data revealed the conformational changes in secondary structure of protein upon its interaction with PDS. The decreased binding constants of HSA–TDH/TFP in presence of common ions indicated the availability of higher concentration of free drug in plasma.