EFFECT OF A HYDROPHOBIC ENVIRONMENT ON THE HYDROGEN-EXCHANGE KINETICS OF MODEL AMIDES DETERMINED BY H-1-NMR SPECTROSCOPY

In proteins, backbone amide hydrogen exchange rates can reveal important information about protein structure and dynamics. In order to assess the possible effects of detergent on the hydrogen exchange rates of detergent-solubilized proteins, we have synthesized a series of model aliphatic amides and measured their amide proton exchange rates in water and sodium dodecyl sulfate (SDS) micelles. Hydrogen exchange was measured using steady-state saturation-transfer proton nuclear magnetic resonance (NMR) spectroscopy. The extent of interaction of the model compounds with SDS was determined by measuring the longitudinal relaxation times, chemical shifts, and temperature coefficients of the amide protons. The sensitivity of the amide proton chemical shift to hydrogen bonding was found to be a particularly useful indicator of the extent of interaction of the amides with the hydrophobic core of the micelle. It is argued that the measured hydrogen exchange parameters reflect the dynamics of exchange of the molecules between bulk solvent and the surface and core of the micelle. Two major effects of the micelle on hydrogen exchange were measured: First, an electrostatic effect due to the negatively charged sulphate groups of SDS causes a decrease of the local pH at the micellar surface. This effect increases with the affinity of the amides for micelle and enhances acid-catalyzed exchange and decreases base-catalyzed exchange. Second, a hydrophobic effect of the core of the micelle causes a depression of the minimum rate of exchange, which, for the most nonpolar molecule, is 25-fold. This effect is similar in magnitude to the slowing of exchange by hydrogen bonding reported by Perrin et al. (J. Am. Chem. Soc. 1990, 112, 3122-3125). The hydrophobic effect is likely to be an important factor in the slowing of exchange in the solvent-excluded interior of water-soluble proteins as well as in the exchange of detergent-solubilized peptides and proteins.