Quantitative characterisation of the hydrogen bonding behaviour on the acceptor functions in oligopeptide derivatives with a fluorinated alcohol

The equilibrium constants for the association of a fluorinated alcohol, 1,1,1,3,3,3-hexafluoropropanol with amino acid and peptide derivatives dissolved in methylene chloride were calculated. The mathematical approach for the determination of the equilibrium constant KOH is based on the decrease of the integral intensity of the OH stretching signal in the infrared spectrum. Alternatively, the decrease of the intensities of the acceptor signals can be used for the calculation of individual equilibrium constants KZ, KPeptide and KEster, for the association of the alcohol on the urethane, peptide and ester function, respectively. The equilibrium constants obtained with both approaches for a number of amino acid, di- and tripeptide derivatives will be discussed. Generally, in no case was the ester function involved in a complex formation with the fluorinated alcohol. For most examples the association constant for the peptide function was larger than for the urethane group. The investigation of the effect of the variation of the concentration on the equilibrium constant shows that up to a ratio cA0/cB0 of the initial concentrations of the alcohol, cA0, and the acceptor compound, cB0, equal to 10, no formation of 1 ∶ 2 or 1 ∶ 3 associates need to be considered.