NMR studies of electrostatic fields around charged monosaccharides and related molecules

A new and simple technique has been developed to experimentally measure electrostatic fields around charged molecules in solution. Using a series of three charged or neutral [4-hydroxy (or carboxy or amino)-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)] spin probes, the apparent rate constants (k(+), k(o), and k(-)) for a particular nucleus can be calculated as the slope of 1/T-1, (i.e., the spin-lattice relaxation rate) vs, increasing radical concentration. Protons residing within a particular charged region suffer a decrease in their apparent rate constants due to repulsive interactions when the nitroxide spin probe bears the same charge-type as that of the local electrostatic environment. Accordingly, electrostatic forces of attraction increase the apparent rate constants for these same protons when the nitroxide spin probe bears the opposite charge to that present in the local electrostatic field. The apparent rate constants of the neutral nitroxide spin probe are an indication of the solvent accessibility to that particular site in the molecule being investigated. Ratios of these apparent relaxation rate constants (k(+)/k(-) > 1 or k(-)/k(+) > 1, etc.) thus provide a measurement of the sign and magnitude of the local electrostatic fields surrounding a particular proton in the target molecule. Three charged hexosamines [D-glucosamine.HCl (4), D-galactosamine.HCl (5), and D-mannosamine.HCl (6)1, a glucuronide metabolite of paracetamol [p-acetamidophenyl-beta -D-glucuronide sodium salt (7)1, and a mononucleotide [thymidine-3 ' -phosphate sodium salt,(8)1, were investigated in this study. Positively charged environments around protons in the hexosamines are clearly seen by k(o)/k(+) ratios greater than unity, while negatively charged surroundings about protons in the glucuronide and nucleotide are markedly evident from larger than unity k(+)/k(-) and k(o)/k(-) ratios. In the case of small charged target molecules such as 6-8, the electrostatic field seems to envelop the entire molecule. In general, a lack of statistically significant differences in the k(attraction)/k(repulsion) or k(neutral)/k(repulsion) ratios as a function of proton distance from the charged functionality was observed. This finding (with the possible exception of the methyl protons in 7) suggests that the present version of this method is not sensitive enough to map subtle differences in charged environments around small and conformationally flexible or dynamic target molecules.