Spectroscopic Investigation into the Binding of Ferulic Acid with Sodium Deoxycholate: Hydrophobic Force Versus Hydrogen Bonding

The binding of ferulic acid (FA) with sodium deoxycholate (NaDC) has been investigated using fluorescence and absorption measurements. The fluorescence probe technique of pyrene reveals that the presence of FA favors the micellization of NaDC, leading to the decreased critical micelle concentrations for the formation of NaDC micelles. As NaDC molecules change gradually from monomers via primary micelles into secondary micelles, the intensities of absorption and fluorescence spectra of FA increase at low NaDC concentrations, but decrease suddenly at intermediate NaDC concentrations, and finally increase again at high NaDC concentrations. These results corroborated well with FA fluorescence lifetime data suggesting that the aryl ring of FA hydrophobically binds to the convex surface of NaDC monomers, whereas the hydrogen bonding between FA and NaDC is significantly involved in NaDC primary micelles, which is gradually overcome by the hydrophobic interaction between FA and NaDC secondary micelles. The absorption and fluorescence spectra as well as the binding constant value of FA indicate the strong binding of FA in the large hydrophobic core of NaDC secondary micelles. At low FA concentrations, the measurement of FA anisotropy suggests that FA can increase the packing order of hydrophobic surfaces in NaDC secondary micelles, whereas the high amount of FA can greatly disrupt the packing structure of NaDC secondary micelles which is ascribed to the formation of FA dimers. The spectroscopic experiments outlined here present the binding events of FA with NaDC monomers and primary and secondary micelles, which are significantly related with the hydrophobic force and hydrogen bonding as well as the unique structural characteristics of bile salt.