Solvent effects on the conformation of the transmembrane peptide gramicidin A: Insights from electrospray ionization mass spectrometry

The binding of sodium ions to the transmembrane channel peptide gramicidin A has permitted the use of electrospray ionization mass spectrometry to stu dy its conformation in different solvent environments. The mass spectra of the peptide in the various solvents suggest that different conformations of gramicidin A differ in their ability to bind metal ions. The data are cons istent with monomeric behavior of gramicidin A in trifluoroethanol and dime thyl sulfoxide solutions, but reveal the presence of noncovalent intermolec ular interactions in ethanol solution through the observation of heterodime rs formed between the naturally occurring variants of the peptide. The addi tion of 50% v/v of water to the ethanolic solution causes changes in the ci rcular dichroism spectrum of the peptide, suggestive of a shift in the equi librium mixture of conformers present toward monomeric species, a result su pported by its mass spectrum. The structure of gramicidin A in trifluoroeth anol has also been investigated by hydrogen exchange measurements monitored by mass spectrometry. The observation of significant protection against ex change suggests that the monomeric peptide is highly structured in trifluor oethanol, The results indicate that mass spectrometry has the potential to probe the conformational behavior of neutral hydrophobic peptides in enviro nments that mimic their functional states.