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 cis and trans 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 cis/trans 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 cis 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 cis and trans isomers, and these diffusion coefficients reflect the smaller hydrodynamic radii of the cis isomers. In contrast, no significant difference was detected in the diffusion coefficients measured for the cis and trans isomers of glycyl-sarcosine.
Tiffany S. Derrick and Cynthia K. Larive, "Use of PFG-NMR for Mixture Analysis: Measurement of Diffusion Coefficients of Cis and Trans Isomers of Proline-Containing Peptides," Appl. Spectrosc. 53, 1595-1600 (1999)