Conformational Stability of V-Amyloses and Their Hydration-Induced Conversion to B-Type Form as Studied by High-Resolution Solid-State 13C NMR Spectroscopy

Abstract

Abstract We have recorded 13C NMR spectra of several preparations of amorphous samples, V-amyloses and their hydrates to gain insight into their relative stability and subsequent hydration-induced conformational change. It was found that the 13C NMR line width of the iodine complex prepared as a precipitate in aqueous solution was the narrowest among these preparations, including the iodine complex formed from cast film. It is noteworthy that these V-amyloses are not always stable when they are equilibrated in an atmosphere of 96% R. H. in a desiccator and converted to B-type form, consistent with previous observation by Senti and Witnauer (J. Am. Chem. Soc., 70, 1438 (1948)). 2H NMR line shape and spin-lattice relaxation times were measured for partially deuterated V and B-type amylose samples containing DMSO-d6 and deuterium oxide, respectively, to explore the dynamics of the DMSO and water molecules bound to the polymer chains. It is unlikely that any drastic conformational change such as unfolding followed by refolding and plausible change of handedness is caused by such mild physical treatment as humidification. Therefore, it appears that the resulting B-type form should be ascribed to a left-handed single helix as a consequence of a slight modification of V-amylose, in contrast to the previous interpretation based on the data of X-ray diffraction and NMR studies. Gelation mechanism of amylose gel based on this view is also discussed.