The Effect of Aliphatic Alcohols on the Helix-Coil Transition of Poly-l-ornithine and Poly-l-glutamic Acid

The helical stability of poly-l-glutamic acid and poly-l-ornithine in aqueous alcohol solutions was studied by optical rotatory dispersion and potentiometric titration determinations. It was found that aliphatic alcohols enhance the stability of the helix according to the series methanol l ethanol l 1-propanol l 1-butanol. The role of secondary and tertiary alcohols was also investigated. Thermodynamic analysis of the thermal transition of uncharged poly-l-ornithine showed that the greater stability of the helix in aqueous alcohol solutions with respect to that in water arises from a more favorable value of the entropy change of the transition ΔS0. A similar effect of alcohols on the thermodynamics of the coil-helix transition in aqueous solution has already been observed for poly-l-glutamic acid. It is suggested that solvation of the peptide amide group in the randomly coiled conformation may be responsible for the observed trend of the variation of the thermodynamic quantities.