Solvent effects on protonation equilibria of peptides and quinolones by factor analysis applied to the correlation between dissociation constants and solvatochromic parameters in acetonitrile-water mixtures

Dissociation constant values of series of peptides and quinolones in 5.54, 10, 16.3, 25 and 50% (w/w) acetonitrile-water mixed solvents at 298.15 K were, firstly, submitted to factor analysis in order to obtain the number of factors which affects the variation of the whole data sets and, afterwards, to target factor analysis to identify these factors. The pK values were correlated with the Kamlet and Taft solvatochromic parameters (α, β and π∗) of acetonitrile-water mixtures according to the results obtained through these chemometric techniques. The equation proposed allowed the calculation of pK values of peptides in any acetonitrile-water mixture up to 50% (w/w) and, therefore, the acid-base behaviour of these compounds in widely used acetonitrile-water media is to be known. Further, an overview of the effect of preferential solvation of electrolytes in acetonitrile-water mixtures on the values of the protonation constants in these media was also obtained.