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The dependence of the partial molar volumes of a number of a-amino acids, w-amino acids, dipeptides and diketopiperazines on temperature has been determined at various concentrations and for the temperature range 0-55 O C using an automatic dilatometer ; the limiting partial molar expansibilities, Og, have been calculated. Measurements of the sound velocity at 15 MHz and at various concentrations allowed us to determine the compressibility of the solutions at 25 *C and to calculate the limiting partial compressibilities, OO,.,, for some of the above mentioned compounds. Finally, by using the known values @ : and @OK. , together with the partial molar heat capacity of the solute and some physical properties of pure water, the limiting isothermal partial molar compressibilities, The values of 0% and @OK,, and the trends they show when the structure of the amphionic molecule is changed have been analysed by taking into account the features exhibited by 0; and @OK,, of related charged and non-charged compounds. The dependence on temperature and pressure of the internal and external proton-exchange reaction between a carboxyl and an amino group have been evaluated and compared.have been obtained.

The enthalpy, entropy and free energy changes have been determined for the transfer, at 25°C and 1 atm, of cyclic amines of the type CnHznNH (n = 4,5,6) and CnHZnNCH3 (n = 4,5), from either the ideal gas state (AXh, X = H,S,G) or the liquid state (AX,, X = H,S,G) to dilute aqueous solution. Heats of solution of compounds of the type CnHZnNH with n = 2,3,7 and heats of vaporization (AHv) for all the above compounds are also reported.In the CnHznNH series the heat of solution, which is exothermic, has a maximum at n = 4, whereas AH, values increase in the homologous series from 8.09 kcal mol-1 (n = 2) to 11.04 kcal mol-l (n = 7). The heats of solution of the N-methyl amines are larger than those evolved on solution of the corresponding secondary, unsubstituted amines, while the AH, values are practically equal. On the other hand, a sensible decrease of Ash is caused by introducing a methyl group on the nitrogen atom. Possible interpretations of the observed effects are suggested.

The values of the limiting partial molar volumes, @ :, at 25 O C of some typical amphionic molecules (a-amino acids, w-amino acids and polypeptides) are reported. The methods followed to obtain the values of the volume change, AV,,, 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 @: values of a large number (220) of hydrocarbons and monofunctional compounds; in the second one, a model is used in which the 00, values in water are correlated with the van der Waals volumes, V,, and the volumetric effects connected with the substitution of a hydrocarbon part of the molecule by a hydrophilic part are evaluated.The AV,, values are compared with the volume change associated with the reaction of proton exchange between primary amines and carboxylic acids; the correlation of the A V,, values with the distance between the NHZ and COOgroups and with the nature of the chain separating them is also discussed.In previous work172 we determined the values of the apparent molar heat capacities in aqueous solutions, aCp, for various amino acids, dipeptides and diketopiperazines (dkp). These substances were selected in order to obtain information on the interactions between water and the peptidic group, -CONH--, in cyclic neutral molecules and on the interactions of water with the charged centres NHZ and COOin those molecules in which these centres are separated by one or more methylenic or peptidic groups. We have also3 summarized the results of a study on the volumetric properties (partial molar volumes, mV, expansibilities, Q, E, and compressibilities, OK) of the above-mentioned systems.Recently, limiting apparent molar volumes, (D", of o-amino acids of the type +NH,(CH,),COO-(n = 1-10) were determined by Shahidi and Farrell,* who also calculated the volume change corresponding to the formation of zwitterionic forms, AV,,, by comparing the of o-amino acids with those of the corresponding hydroxyamides. Apparent molar volumes and adiabatic compressibilities of several a-amino acids in aqueous solutions were determined by Millero et aZ.,5 who calculated the number of water molecules bonded to the charged centres and also evaluated the values of A Gw by different methods. Values of A cw by group contributions were also