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The interactions of two stereoisomeric antioxidant flavonoids, catechin (C) and epicatechin (EC) with bovine serum albumin (BSA) and human serum albumin (HSA), have been investigated by steady state and time resolved fluorescence, phosphorescence, circular dichroism (CD), FTIR and protein–ligand docking studies. The steady-state fluorescence studies indicate a single binding site for both the ligands. FTIR spectra suggest that in both the albumins, C and EC stabilize the α-helix at the cost of a corresponding loss in the β-sheet structure. CD studies have been carried out using (±)C, and both the epimers (+)C and (−)C. The low temperature phosphorescence and protein–ligand [(+), (−) and (±) forms of C and EC] docking studies indicate that the ligands bind in the proximity of Trp 134 of BSA and Trp 214 of HSA, thereby changing their solvent accessible surface areas (ASA). Asn 158 and Glu 130 side chains are found to be within the hydrogen bonding distance from the phenolic –OH groups of C and EC in the case of BSA complex. C and EC are located within the binding pocket of sub-domain IIa of HSA.

The enhancement of the excited state intramolecular proton transfer (ESIPT) emission of 3-hydroxy-2-naphthoic acid (3HNA) in presence of poly N-vinyl-2-pyrrolidone (PVP) has been investigated by time resolved fluorescence and anisotropy measurement. In other water soluble polymers and monomers there is no enhancement of the ESIPT emission of 3HNA. The microenvironment of the probe in PVP–water mixture is investigated by comparing the ESIPT emission of 3HNA in mixed solvents of acetonitrile–toluene. The anisotropy, rotational correlation time and the blue shift observed in the ESIPT emission in PVP–water mixture have been ascribed to both the polarity and the motional restriction of the probe imposed by the bound water region of PVP.

The enhanced fluorescence and anisotropy of the ESIPT emission of 3-hydroxy-2-naphthoic acid (3HNA) in the complexes of 3HNA with bovine serum albumin (BSA) and human serum albumin (HSA) showed the formation of 1:1 complex [binding constant = 5.3 × 105 M−1 for BSA and = 2.2 × 105 M−1 for HSA]. The ESIPT emission of the probe in non-polar, polar protic, polar aprotic and mixed solvents indicate that the position and the quantum yield of the emission are governed by the intermolecular hydrogen bonding ability and the polarity/polarizability of the solvents. Rotational correlation time of 3HNA (2.4 ns and 5.2 ns in BSA and HSA, respectively) obtained from the time resolved anisotropy decay of the ESIPT emission suggests motional restriction of the probe. Docking studies reveal H-bonding of some residues with the probe and loss of accessible surface area of several residues located near binding site.