Conformation and molecular packing of n-alkyl side chains protruding from α-helical poly ( l-glutamates) as studied by 13C CP/MAS NMR spectroscopy

Abstract

13C NMR spectra of a series of poly( l-glutamates) with n-alkyl side chains of various lengths ( n = (number of carbon atoms in the alkyl group) = 8, 12, 14, 16 and 18) were recorded by the cross polarization/magic angle spinning method, in order to elucidate conformational features and molecular packing through the observation of 13 13C chemical shifts. From these experimental results, it was found that the main chain of the poly(γ- n-alkyl l-glutamates) takes the α-helical conformation irrespective of side chain length, and the n-alkyl side chains can participate in the crystallization if they are long enough. In these side chain crystallites, all- trans zigzag conformation of alkyl chain was clarified and the type of molecular packing was presumed based on the reference data of n-alkanes. Further, we discuss the conformation of n-alkyl side chains of poly( n-stearyl l-glutamate) in isotropic solution and liquid crystalline solution.