Binding interaction and conformational changes of human serum albumin with ranitidine studied by spectroscopic and time-resolved fluorescence methods

This study was designed to examine the interaction of histamine H-2-receptor antagonist drug ranitidine (RTN) with human serum albumin by multi-spectroscopic methods. The experimental results showed the involvement of dynamic quenching mechanism which was further confirmed by lifetime spectral studies. The binding constants (K (a)) at three temperatures (288, 298, and 308 K) were 2.058 +/- A 0.020, 4.160 +/- A 0.010 and 6.801 +/- A 0.011 x 10(4) dm(3) mol(-1), respectively, and the number of binding sites (m) were 1.169, respectively; thermodynamic parameters Delta H (0) (44.152 +/- A 0.047 kJ mol(-1)), Delta G (0) (-26.214 +/- A 0.040 kJ mol(-1)), and Delta S (0) (236.130 +/- A 0.025 J K-1 mol(-1)) were calculated. The distance r between donor and acceptor was obtained (r = 3.40 nm) according to the Forster theory of non-radiative energy transfer. Synchronous fluorescence, CD, AFM and 3D fluorescence spectral results revealed the changes in secondary structure of the protein upon interaction with RTN. A molecular modeling study further confirmed the binding mode obtained by the experimental studies.