The analysis of complex mixtures is a prominent area of research that spans many disciplines of science. Recently, diffusion-based nuclear magnetic resonance (NMR) has found wide applicability in the field of mixture analysis. Pulsed-field gradient NMR (PFG-NMR) is a well-established method for the determination of diffusion coefficients, which are indicative of molecular size and shape. Diffusion coefficients were measured with the bipolar pulse longitudinal eddy current delay pulse sequence for the <i>cis</i> and <i>trans</i> prolyl isomers of two simple peptides, phenylalanylproline and phenylalanylprolyalanine, and for arginine vasopressin, a more complex peptide hormone. This methodology was also applied to a dipeptide, glycylsarcosine, which exhibits <i>cis/trans</i> isomerism, but lacks a proline residue. Obtaining quantitative diffusion coefficients from PFG-NMR spectra of proline-containing peptides can be challenging due to the low content of the <i>cis</i> isomer in most peptides as well as the limitations imposed by spectral overlap. However, for each of the proline-containing peptides studied, statistically different diffusion coefficients were measured for the <i>cis</i> and <i>trans</i> isomers, and these diffusion coefficients reflect the smaller hydrodynamic radii of the <i>cis</i> isomers. In contrast, no significant difference was detected in the diffusion coefficients measured for the <i>cis</i> and <i>trans</i> isomers of glycyl-sarcosine.

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