Interactions between 4-(2-dimethylaminoethyloxy)- N-octadecyl-1,8-naphthalimide and serum albumins: Investigation by spectroscopic approach

Abstract

A novel 4-(2-dimethylaminoethyloxy)- N-octadecyl-1,8-naphthalimide (DON) has been synthesized as a spectrofluorimetric probe for the determination of proteins. Photophysics of DON in different solvents has been delineated in this paper. Progressive redshift with polarity of solvents in emission and absorption spectra hints at intramolecular charge transfer. The interactions of DON with serum albumins (i.e., human serum albumin (HSA) and bovine serum albumin (BSA)) were studied by fluorescence and absorption spectroscopy. Fluorescence data revealed that the quenching of HSA/BSA by DON were static quenching and the DON–HSA/BSA complexes were formed. The binding constant ( K b) for HSA and was found to be 8.44×10 −4 and 60.26×10 −4 M −1 and the number of binding sites ( n) were 1.00 and 1.40, respectively. The thermodynamic parameters, Δ H and Δ S, for the DON–HSA system was calculated to be −14.83 kJ mol −1 and 23.61 J mol −1 K −1, indicating the hydrogen bonds and hydrophobic interactions were the dominant intermolecular force. Δ H and Δ S for the binding of DON with BSA was −60.08 kJ mol −1 and −90.7441 mol −1 K −1, suggesting the hydrogen bonds and van der Waals force played the main role in the interaction. The results of displacement experiments showed that DON bound HSA/BSA occurred at the Trp-214 proximity, located in subdomain IIA of the serum albumin structure (the warfarin binding pocket). The effect of DON on the conformation of HSA was also analyzed by synchronous and three-dimensional fluorescence spectra. The fluorescence of DON could be quenched by HSA, based on which, a fluorometric method for the determination of microamount protein using DON in the medium of HCl−Tris buffer solution (pH=7.4) was developed. The linear range of the calibration curves was 0.1–10.0 μM for HSA, 0.1–11.2 μM for BSA and 0.2–9.7 μM for egg albumin (EA). The detection limit (3 σ) for HSA was 1.12×10 −10 M, for BSA it was 0.92×10 −10 M and for EA it was 4.33×10 −10 M. The effect of metal cations on the fluorescence spectra of DON in ethanol was also investigated. The method has been applied to detect the total proteins in human serum samples and the results were in good agreement with those reported by the hospital.