PROBING THE HELICAL CONTENT OF GROWTH HORMONE-RELEASING FACTOR ANALOGS USING ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

A series of growth hormone-releasing factor analogs have been studied by both circular dichroism and electrospray ionization mass spectromet ry (ESI/MS). The peptides are 32 residues long and are known to adopt a random-coil structure in aqueous solution but become increasingly he lical as the proportion of organic solvent is increased. Deuterium exc hange was observed as an increase in mass of the peptide, as measured by ESI/MS. Rates of exchange were measured and half-lives calculated f or analogs containing amino acid substitutions designed to promote or discourage helix formation. Exchange was slower in peptides that are h elical (as shown by circular dichroism) than in randomly coiled peptid es. Solution conditions that favor helix formation also produced slowe r exchange rates. These studies suggest that ESI/MS can provide date a bout the extent and stability of helix formation.