Dynamics of Palmitic Acid Complexed with Rat Intestinal Fatty Acid Binding Protein

Dynamics of palmitic acid (PA), isotopically enriched with 13C at the second, seventh, or terminal methyl position, were investigated by 13C NMR. Relaxation measurements were made on PA bound to recombinant rat intestinal fatty acid binding protein (I-FABP) at pH 5.5 and 23 °C, and, for comparison, on PA incorporated into 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (MPPC) micelles, and dissolved in methanol. The 13C relaxation data, T 1, and steady-state nuclear Overhauser effect (NOE) obtained at two different magnetic fields were interpreted using the model-free approach [Lipari, G., and Szabo, A. (1982) J. Am. Chem. Soc. 104, 4546−4559]. The overall rotational correlation time of the fatty acid·protein complex was 2.5 ± 0.4 ns, which is substantially less than the value expected for the protein itself (>6 ns). Order parameters (S 2), which are a measure of the amplitude of the internal motion of individual C−H vectors with respect to the PA molecule, while largest for C-2 and smallest for the methyl carbon, were relatively small (