Exploring the Effects on the Conformational Propensity of Alanine in the Unblocked Tripeptide Glycyl-Analyl-Glycine in Water/Ethanol Mixtures

Abstract

Short peptides have been shown to be model systems for obtaining the conformational propensities of individual amino acids. Recent studies on cationic GxG (x: different guest amino acid residues) peptides in water revealed the Ramachandran plots for 16 amino acid residues. Out of the natural amino acids, alanine in GAG has been shown to stand out owing to its unusually high propensity for polyproline II (pPII) conformations (0.72). Thus, its conformational distribution differs substantially from the classical ones of Ramachandran and Flory. Peptide-water interactions have been proposed as the major determinant of alanine's preference for pPII. To elucidate how peptide hydration affects conformational preferences of alanine we used FTIR, vibrational circular dichroism, polarized Raman and NMR spectroscopy to determine Ramachandran plots of GAG in aqueous solutions with 3, 12, and 42mol% ethanol. We observed a modest decrease of the pPII fraction from 0.72 to 0.65 and an increase in β-strand population from 0.15 to 0.20 from pure water to 12 mol% ethanol. Interestingly, the corresponding changes of the enthalpic and entropic differences between pPII and β -strand by far exceed changes in Gibbs energy at room temperature owing to enthalpy-entropy compensation effects. Our thermodynamic data reveal that the cosolvent stabilizes pPII enthalpically (−10 kJ/mol in water to −18 kJ/mol in 12 mol% ethanol) but destabilizes it even more entropically (−23 to −50 J/mol∗K). Comparisons of IR and NMR spectra of GAG and N- methylacetamide in the same ethanol/water mixtures reveal changes of peptide hydration which causes the observed destabilization of pPII of alanine and the concomitant increase of its conformational entropy.