Use of PFG-NMR for mixture analysis: Measurement of diffusion coefficientsof cis and trans isomers of proline-containing peptides

The analysis of complex mixtures is a prominent area of research that spans many disciplines of science. Recently, diffusion-based nuclear magnetic re sonance (NMR) has found wide applicability in the field of mixture analysis . Pulsed-held gradient NMR (PFG-NMR) is a well-established method for the d etermination of diffusion coefficients, which are indicative of molecular s ize and shape. Diffusion coefficients were measured with the bipolar pulse longitudinal eddy current delay pulse sequence for the cia and trans prolyl isomers of two simple peptides, phenylalanylproline and phenylalanylprolya lanine, and for arginine vasopressin, a more complex peptide hormone. This methodology was also applied to a dipeptide, glycylsarcosine, which exhibit s 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 peptide s as well as the limitations imposed by spectral overlap. However, for each of the proline-containing peptides studied, statistically different diffus ion coefficients were measured for the cis and tra,ls isomers, and these di ffusion coefficients reflect the smaller hydrodynamic radii of the ris isom ers. In contrast, no significant difference was detected in the diffusion c oefficients measured for the cis and trans isomers of glycyl-sarcosine.