Comment to 13C-NMR studies of alpha and gamma polymorphs of glycine.

Glycine provided to research laboratories by chemical companies can contain different polymorphs and/or mixture of polymorphs. The alpha- and gamma-glycine differ slightly in 13C shielding parameters and very much in dynamic properties. The isotropic values of chemical shifts for carboxyl groups of alpha- and gamma-glycine are found to be delta(iso) = 176.50 ppm and delta(iso) = 174.60 ppm, respectively. By using cross-polarization contact time equal to 20 ms, the intensity of carboxyl signal arising from gamma-glycine is reduced almost to zero, while that of alpha-glycine is only slightly depleted compared to the intensity of signal recorded at contact time of 1 ms. In contrast, for alpha-form, the 13C T1 relaxation time of carboxyl carbon is five times shorter compared to gamma-form.