NMR Investigation of the Interactions between 4‘-Fluoro-1‘-acetonaphthone and the Suwannee River Fulvic Acid

Abstract

The association of 4‘-fluoro-1‘-acetonaphthone (F-acetonaphthone) with the International Humic Substances Society (I.H.S.S.) Suwannee River fulvic acid (SRFA) standard in 55% CH3OD/45% D2O was investigated as a function of SRFA concentration (4.5−13.5 mg/mL) as well as solution pαH* (3.88−7.56). 19F NMR measurements of T1 and T2 relaxation times were used in conjunction with chemical shift and diffusion coefficient measurements to study the interaction, and the results were interpreted using the current association models reported in the literature (1). Analysis of diffusion coefficients measured as a function of SRFA concentration suggests that a small amount of the F-acetonaphthone is associated with SRFA. Unusual behavior of the T1 and T2 relaxation time constants, correlation times calculated from these parameters, and diffusion coefficients were observed for F-acetonaphthone in solutions of SRFA in 55% CH3OD/45% D2O at pαH* 6.10. Analysis of the pαH* dependence of the diffusion coefficients suggests two competing mechanisms of association between F-acetonaphthone and SRFA, hydrophobic interactions, and hydrogen bonding. The contribution of the fluorine label to the interaction of F-acetonaphthone with SRFA was investigated using 19F NOE measurements as well as measurements of competitive binding by unfluorinated acetonaphthone and fluorobenzene.