F. Rodante

Thermal analysis of some α-amino acids with similar structures

Abstract The thermal analysis of some α-amino acids using simultaneous TG-DSC measurements and kinetic calculations by the dynamic TG technique have been carried out. With this technique, compounds with similar structures can be grouped by their thermograms which have the same shape, although this does not give enough information on their thermal structural variation. The thermodynamic and kinetic data are of different significance, except in the case of series where the rings of the side chains influence the thermal decomposition of the compounds.

Thermodynamics of the “standard” α-amino acids in water at 25° C

Abstract A thermodynamic study of 23 α-amino acids (19 “standard” α-amino acids plus 4 derivatives) is the subject of this work. Thermodynamic quantities of the first, second and third ionization processes are correlated with the features of the four classes into which the “standard” α-amino acids are classified. A detailed thermodynamic analysis of the following effects influencing the ionization processes is presented. (i) The solvation effects on a molecule having the same atom in different positions on its skeleton. (ii) The solvation effects on different classes of α-amino acids. (iii) The thermodynamic effects on different classes of α-amino acids. (iv) The thermodynamic effects on α-amino acids having similar structures.

Thermodynamics and kinetics of decomposition processes for standard α-amino acids and some of their dipeptides in the solid state

Abstract Thermodynamic and kinetic data of the thermal decomposition process of the “standard” α-amino acids in the solid phase are presented in this work and correlated with the features of the four classes into which these compounds are classified in the liquid phase. The thermal stability of the components of the various classes is mainly linked to the decarboxylation process which, in turn, is influenced by the side-chains. The heat effects of the chemical processes and physical transitions, and the kinetic quantities of the decomposition processes of these compounds are presented. Three series of dipeptides have also been studied in the solid phase. In the first series, the mutual influence of two α-amino acids makes the dipeptides less stable than the independent free components. In the second, the methyl group plays a fundamental role in the thermal stability just as it does in the liquid phase where it is the key factor in the mutual influence of the different dipeptide structures. Finally, in the third class, a symmetrical system averages the thermodynamic quantities of the components.

Thermal analysis of some α-amino acids using simultaneous TG-DSC apparatus. The use of dynamic thermogravimetry to study the chemical kinetics of solid state decomposition

Abstract A study of the thermal stability of some α-amino acids by simultaneous TG-DSC measurements and kinetic calculations with the dynamic TG technique was carried out. Three different scales of thermal stability were found from the thermodynamic and kinetic data.

Thermodynamic study of some α-amino acids bearing different groups in their side-chains

Abstract A study of the behaviour of some α-amino acids bearing different groups in their side-chains is presented. α-Amino acids with similar structures are compared on the basis of the thermodynamic quantities related to the first and second ionization processes. Three pairs of compounds are considered: methionine/cysteine, glutamine/aspargine and lysine/arginine. The entropy values play a major role in explaining the different behaviour of the constituents of the pairs in both the first and second ionization processes.

Thermodynamics of the second proton dissociation processes of nine α-amino-acids and the third ionization processes of glutamic acid, aspartic acid and tyrosine

Abstract The thermodynamics of the second proton dissociation processes for nine α-amino-acids were evaluated using electrochemical and calorimetric data. A calorimetric acid strength scale in water is presented. The acidity order is explained as a function of the solvation of the zwitterions and anions. The differences between the thermodynamics of the first and second ionization processes of the compounds result from the fact that the two processes involve different solvation processes. The anions of the compounds containing a hydroxyl group are less solvated than the anions of the α-amino-acids having a hydrophobic group.

Thermodynamics of dipeptides in water: I. Calorimetric study of the influence of α-amino acid structures on the free α-aminic and free α-carboxyl groups

Abstract A calorimetric study has been made of the dissociation processes in water of the free α-aminic groups of the amino terminal residuum and the free α-carboxyl groups of the carboxy terminal residuum of various dipeptides. The influence of the structure of valine, which was one of the components of each of the dipeptides, upon a number of other α-amino acids as second component, and the influence of these other α-amino acids upon the structure of valine were investigated using valil-valine and the individual free α-amino acids as reference structures.

Thermodynamics of proton transfer processes for some α-amino-acids with different side-chain groups

Abstract The first and second proton ionization processes of two classes of α-amino-acids have been examined. Variation in the thermodynamic properties of the first and second proton transfer processes for the α-amino-acids were compared, using glycine as a reference compound. The structures of the two classes considered have side-chains formed by hydrophobic groups and polar groups without charge respectively. The compounds from the second of these classes (with the exception of proline in the first ionization process) ionize more readily than the compounds from the first, in both the processes of ionization.

Calorimetric study of the first and second proton dissociation processes of fluoro and hydroxyl derivatives of phenylalanine

Abstract A calorimetric study of the first and second proton dissociation processes of fluoro and hydroxyl derivatives of phenylalanine in water is presented. The calorimetric acid strength scales in water for both of these processes are also presented. The effects which influence the acidity scales are the solvation processes of the undissociated molecules, the zwitterions and the ionic forms. The presence of a fluorine atom affects all the processes considered. The large effect of the hydroxyl groups on the second proton dissociation process is due to the large solvation of the XPhCHNH 2 COO − ionic form. The total proton dissociation process (sum of the two proton dissociation processes) confirms that the hydroxyl group greatly favours this process for tyrosine, while the effect of the fluorine atom is negligible.

Calculation computer program for a quasi-adiabatic calorimeter

Abstract A computer program (in the language BASIC) for the determination of solution and reactions heats in a quasi-adiabatic calorimeter has been presented. The menu of the program is made up of the following steps: (1) calibration curve; (2) cooling curve; (3) reaction curve; and (4) equilibrium temperature. There are also three subroutines which are able to modify the values of time-voltage pairs for steps (1)–(3), to change the data of the reaction curve ( K / C , instantaneous power and mole number) and to reorganize the order of time-voltage pairs for steps (1)–(3) as a function of increasing time vector. The program was tested using the standard thermal reaction of solid tris(hydroxymethyl) mino methane (THAM) with 0.1 M aqueous HCl. The value obtained is in good agreement with that obtained in the literature.