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

In proteins, backbone amide hydrogen exchange rates can reveal importa nt information about protein structure and dynamics. In order to asses s 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 wat er and sodium dodecyl sulfate (SDS) micelles. Hydrogen exchange was me asured using steady-state saturation-transfer proton nuclear magnetic resonance (NMR) spectroscopy. The extent of interaction of the model c ompounds with SDS was determined by measuring the longitudinal relaxat ion times, chemical shifts, and temperature coefficients of the amide protons. The sensitivity of the amide proton chemical shift to hydroge n bonding was found to be a particularly useful indicator of the exten t of interaction of the amides with the hydrophobic core of the micell e. It is argued that the measured hydrogen exchange parameters reflect the dynamics of exchange of the molecules between bulk solvent and th e surface and core of the micelle. Two major effects of the micelle on hydrogen exchange were measured: First, an electrostatic effect due t o the negatively charged sulphate groups of SDS causes a decrease of t he local pH at the micellar surface. This effect increases with the af finity 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 ex change, which, for the most nonpolar molecule, is 25-fold. This effect is similar in magnitude to the slowing of exchange by hydrogen bondin g 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 slow ing of exchange in the solvent-excluded interior of water-soluble prot eins as well as in the exchange of detergent-solubilized peptides and proteins.