Structure and Dynamics of the Acyl Chain of a Transmembrane Polypeptide

We have used acylated analogs of gramicidin as a model to study the interaction between a covalently coupled fatty acid and the hydrophobic part of a membrane-spanning protein in a bilayer environment. The acyl chain was covalently coupled to the C-terminal ethanolamine group of gramicidin which is located near the membrane interface, mimicking a situation found in acylated proteins. Either perdeuterated palmitic acid or palmitic acid deuterated at only C2, C3, C5-6, C7-8, C9, or C13 was coupled to gramicidin and examined by 2H-NMR in oriented bilayers of dimyristoylphosphatidylcholine. In this way, quadrupolar splittings of deuterons at specific carbons were assigned. The quadrupolar splittings and T1 values were compared to those of free palmitic acid in oriented bilayers, with and without gramicidin. The results indicate that the covalently coupled fatty acid is highly immobilized near the carboxyl terminus because double quadrupolar splittings and very low T1 values (4 ms) were found for the -CD2- deuterons at carbon atoms C2 and C3. Control experiments with free fatty acid showed single quadrupolar splittings and higher T1 values for this segment of the fatty acid. Molecular modeling of the carboxy-terminal segment of the covalently coupled acyl chain suggested that it has a defined structure with a bend near its attachment site. In contrast, the methyl end (C10-C16) of the covalently coupled fatty acid had quadrupolar splittings and T1 values very similar to those found for free fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)