First and Second Dissociation Enthalpies in Bi-Component Crystals Consisting of Maleic Acid and L-Phenylalanine

The energetics of the stepwise dissociation of a A:B2 bi-component crystal, according to A:B2(cr) → A:B(cr) + B(cr) and A:B(cr) → A(cr) + B(cr), was investigated using MA:Phe2 and MA:Phe (MA = maleic acid; Phe = L-phenylalanine) as model systems. The enthalpy changes associated with these sequential processes and with the overall dissociation reaction A:B2(cr) → A(cr) + 2B(cr) were determined by solution calorimetry. It was found that they are all positive, indicating that there is a lattice enthalpy gain when MA:Phe2 is formed, either from the individual precursors or by adding Phe to MA:Phe. Single-crystal X-ray diffraction (SCXRD) analysis showed that MA:Phe2 is best described as a protic salt containing a maleate anion (MA−) and two non-equivalent L-phenylalanine units, both linked to MA− by NH···O hydrogen bonds (H-bond): one of these units is protonated (HPhe+) and the other zwitterionic (Phe±). Only MA− and HPhe+ molecules are present in the MA:Phe lattice. In this case, however, NH···O and OH···O H-bonds are formed between each MA− unit and two HPhe+ molecules. Despite these structural differences, the enthalpy cost for the removal of the zwitterionic Phe± unit from the MA:Phe2 lattice to yield MA:Phe is only 0.9 ± 0.4 kJ mol−1 higher than that for the dissociation of MA:Phe, which requires a proton transfer from HPhe+ to MA− and the rearrangement of L-phenylalanine to the zwitterionic, Phe±, form. Finally, a comparison of the dissociation energetics and structures of MA:Phe and of the previously reported glycine maleate (MA:Gly) analogue indicated that parameters, such as the packing coefficient, density, hydrogen bonds formed, or fusion temperature, are not necessarily good descriptors of dissociation enthalpy or lattice enthalpy trends when bi-component crystals with different molecular composition are being compared, even if the stoichiometry is the same.

Self-assembly of four generations of RNA dendrimers for drug shielding with controllable layer-by-layer release

Multifunctional RNA dendrimers have been developed as novel drug delivery platforms, which show shielding effect of loaded cargos and temperature triggered stepwise dissociation.

POLYPROTIC ACIDS IN SOLUTION: IS THE INVERSION OF THE CONSTANTS OF STEPWISE DISSOCIATION POSSIBLE?

The paper is devoted to the problem of the ratio of the constants of stepwise dissociation of polyprotic acids in solution. Special attention is paid to the problem of close dissociation constants and in particular to the possibility of the inversion of constants, for example, Ka1 < Ka2 (or pKa1 > pKa2) for equilibria H2A ⇄ HA– ⇄ A2–, H2A+ ⇄ HA ⇄ A– or H2A2+ ⇄ HA+ ⇄A. Equilibria of inorganic and organic acids are successively considered. Some cases of inversion are considered. In particular, such relations are sometimes observed for fluorescein and phthalein compounds, and for porphyrins. In the last case, as well as in some other systems the acid-base reactions proceed relatively slow. An overview of approaches to estimation of the Ka1/ Ka2 ratio is presented. Namely, these approaches use the concepts of the detailed equilibrium scheme, dissociation microconstants, statistical and electrostatic factors, and the influence of intra-molecular hydrogen bonds. The variety of reasons for additional stabilization of the forms H2A (or H2A+, or H2A2+) and A2– (or A–, or A) and destabilization of the form HA– (or HA, or HA+) is regarded, including salt effects and ionic association. Peculiarities of stepwise protolytic equilibria of macrocyclic compounds, such as calixarenes, porphyrins, and cryptands, are considered. Dispersed systems such as polyelectrolytes, micelles of functionalized surfactants, monolayers, silica and modified silica, other oxides, nanodiamonds decorated by carboxylic groups, represent a peculiar type of polyprotic acids. In this case, a “spectrum” of pKa values appears because of influence of ionized functional groups on the properties of un-dissociated ones. It was demonstrated that such unusual ratio of constants, Ka1 < Ka2, may be caused by a set of factors, among which most important are (i) the tautomerism in true solutions and (ii) selective binding of different equilibrium forms in organized solutions, i.e., in micellar solutions of surfactants, suspensions of phospholipid liposomes, and related systems.

STANDARD FORMATION ENTHALPIES OF HYDROXYETHYLIDENEDIPHOSPHONIC ACID AND PRODUCTS OF ITS DISSOCIATION IN AQUEOUS SOLUTION

This study aimed the measuring the standard enthalpies of formation of hydroxyethylidenediphosphonic acid and the products of its dissociation in aqueous solutions at 298.15 K. In the literature there is a large amount of data on the constants of stepwise dissociation of the acid. The works were done at different temperatures and the values of the ionic strength of the solution on the background of the different nature of the supporting electrolytes. In order to compare the values of the constants of stepwise dissociation of the acid, obtained by different authors, we recalculated the values of рK1, рK2, рK3, рК4, рК5 at zero ionic strength by the Davies equation. The most probable values of thermodynamic constants of dissociation of hydroxyethylidenediphosphonic acid were the following: рК10 = 1.7±0.03; рК20 = 2.47±0.05; рК30 = 7.78±0.05; рК40 = 10.29±0.05 and pK50 = 11.13±0.05. Calorimetric measurements were performed on an ampoule calorimeter equipped with an isothermal shell and a KMT-14 thermistor temperature sensor, and the automatic recording the temperature changes over time. The operation of installation was verified with the integral enthalpies of dissolution in water of crystalline potassium chloride and was considered suitable for measurements, if certain it ∆solН298.15 (KCl ∞ H2O)=17.234± 0.018 kJ/mol, corresponding to the standard SRM 1655NBS. The heat value of the calorimeter water was 87±12 Дж/mol virtually coincided with the thermal value of the calorimeter for potassium hydroxide. The volume of the calorimetric liquid was 43.12 ml. The hydroxyethylidenediphosphonic acid samples were weighed on a VLR-200 balance with 2∙10-4 g accuracy. The confidence range of the average ΔН value was determined at 0.95 probability. The equilibrium compositions of solutions were calculated using RRSU software. The standard enthalpies of formation of hydroxyethylidenediphosphonic acid and the products of its dissociation in aqueous solutions were calculated.Forcitation:Lytkin A.I., Chernikov V.V., Krutova O.N., Skvortsov I.A. Standard formation enthalpies of hydroxyethylidenediphosphonic acid and products of its dissociation in aqueous solution. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 4-5. P. 37-42