Synthesis and spectroscopic characterization of 4-butoxyethoxy-N-octadecyl-1,8-naphthalimide as a new fluorescent probe for the determination of proteins

A novel 4-butoxyethoxy-N-octadecyl-1,8-naphthalimide (BON) was synthesized as a fluorescent probe for the determination of proteins. The interactions between BON and bovine serum albumin (BSA) were studied by fluorescence spectroscopy and UV–vis absorption spectroscopy. Fluorescence data revealed that the fluorescence quenching of BSA by BON was likely the result of the formation of the BON–BSA complex. According to the modified Stern–Volmer equation, the binding constants of BON with BSA at four different temperatures were obtained. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be −23.27 kJ mol−1 and 10.40 J mol−1 K−1 according to van’t Hoff equation, indicating that the hydrogen bonds and hydrophobic interactions played a dominant role in the binding of BON to BSA. Furthermore, displacement experiments using warfarin indicated that BON could bind to site I of BSA. The effect of BON on the conformation of BSA was also analyzed by synchronous fluorescence and three-dimensional fluorescence spectra. A new fluorescence quenching assay of the proteins BSA using BON in the HCl–Tris (pH 7.4) buffer solution was developed with maximum excitation and emission wavelengths of 373 and 489 nm, respectively. The linear range was 0.1–10.0 × 10−5 mol L−1 with detection limits were determined to be 1.76 × 10−8 mol L−1. The effect of metal cations on the fluorescence spectra of BON in ethanol was also investigated. Determination of protein in human serum by this method gave results which were very close to those obtained by using Coomassie Brilliant Blue G-250 colorimetry.

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