Volumetric properties of amphionic molecules in water. Part 1. Volume changes in the formation of zwitterionic structures

The values of the limiting partial molar volumes, Φ°v, at 25°C of some typical amphionic molecules (α-amino acids, ω-amino acids and polypeptides) are reported. The methods followed to obtain the values of the volume change, ΔVzw, in the reaction for the formation of an amphionic molecule from the corresponding neutral molecule are critically reviewed and two ways of calculating these quantities are proposed. In both these methods the values of the neutral forms are calculated using group-contribution values obtained by two different procedures. In the first one, a criterion of best fitting is used between the experimental and calculated Φ°v values of a large number (220) of hydrocarbons and monofunctional compounds; in the second one, a model is used in which the Φ°v values in water are correlated with the van der Waals volumes, Vw, and the volumetric effects connected with the substitution of a hydrocarbon part of the molecule by a hydrophilic part are evaluated. The ΔVzw values are compared with the volume change associated with the reaction of proton exchange between primary amines and carboxylic acids; the correlation of the ΔVzw values with the distance between the NH+3 and COO- groups and with the nature of the chain separating them is also discussed.