Conformational Effects on Alanine Induced Induced by Various Alcohol Cosolvents

Abstract

The conformational ensemble of alanine, in the model peptide GAG, in aqueous solution, is known to exhibit a largely two-state equilibrium between polyproline II (pPII) and β-strand conformations with a high pPII population. If solvation by water is indeed pivotal for pPII stabilization as suggested in the literature, the addition of alcohol co-solvents such as ethanol and propanol could be expected to de-stabilize pPII in favor of β-strand conformations. Through the use of HNMR and UV circular dichroism (CD) spectroscopy, the conformationally sensitive 3J(HNHα) coupling constant and the dichroism at 215nm (Δɛ215, a pPII indicator) were obtained as a function of temperature. A two-state (pPII-β) thermodynamic analysis did not reproduce the temperature dependence of the J-coupling constant, indicating that alanine samples an increased turn-like population upon addition of co-solvent. The obtained Δɛ215 values were found to depend nonlinearly on the co-solvent concentration. This observation is likely to reflect different phases of the non-ideal mixture of water and primary alcohols. In order to further explore the relationship between peptide conformation and solvent mixture, we performed vibrational analyses and simulated the amide I’ band in IR, VCD and polarized Raman using an excitonic coupling algorithm and variable 2D conformational distributions reflecting sub-populations (pPII, β, various turns). Preliminary results of this analysis, which has not yet been completed, suggest that the addition of alcohol co-solvents affect both the relative population of sub-distributions assignable to pPII, β-strand and turn-like conformations as well as their specific locality of these distributions in the Ramachandran space.