Potassium caffeate/caffeic acid co-crystal: the rat race between the catecholic and carboxylic moieties in an atypical co-crystal

Abstract

The vast literature concerning caffeic acid and its derivatives lacks any reference to the solid state structures of its inorganic salts as these crystals are quite difficult grow. Most of the already published works deal with computational studies of these compounds as well as investigations of their behaviour in solution. Having obtained good quality potassium caffeate/caffeic acid co-crystals, we solved their structure and used a robust approach, already applied to caffeic acid alone, to compare the X-ray structure with the one inferred by Molecular Dynamics (MD), focusing our attention on the structure-property relationships. The reliability of this method is confirmed by the overall agreement extended up to the anisotropic displacement parameters calculated, on one hand, by means of MD and the ones gathered, on the other hand, by X-ray measurements. Moreover, the lack of experimental evidence of an enthalpically favored polymorph, arising from the MD calculations, were explained on the basis of the Shannon's entropy.

Self-assembling of calcium salt of the new DNA base 5-carboxylcytosine

“…The two axial positions are occupied by a Cl − ion and a water molecule. In (caC)2 -Ca, the observed competition between the two binding sites in the ligand is not surprising, and it has been addressed by several studies in similar compounds [75].…”

Functional hybrid co-crystals of humic substances: a growth forecast

“…As already stated, the only reported structure is a hybrid co-crystal with the potassium ion that we grew, harvested and discussed in a previous paper. 30 By exploring the CSD, it is evident that there is instead a good availability of different types of crystals (as well as co-crystals) of PCA which therefore shows a relevant tendency to crystallize in several different forms. Moreover, given the fairly high solubility in water of PCA (∼30 g L −1 ), 40 which is almost an order of magnitude greater than that of CA (∼3 g L −1 ), 41 we can assume that PCA has an evidently stronger tendency to form co-crystals as compared to CA.…”

“…We started with the validation of the force field settings, used to describe CA 42 and its potassium co-crystal, 30 of PCA. This was done considering various structures of PCA from the CSD, as described in the ESI † in detail.…”

“…29 We are interested in the possibility of obtaining good quality hybrid co-crystals of the two above mentioned compounds in order to tune, accordingly to different needs, their different properties. The reported structure of CA with the K + ion 30 was considered as a hybrid co-crystal and not a "straight" co-crystal as we evidenced in this structure, in a 1 : 1 ratio, a fully protonated CA moiety together with a deprotonated one, once on the carboxylic function and then on the catecholic part, is coordinated, together with a molecule of water in a six-fold layout around the K + ion. This is the first, and yet to our knowledge, the only crystal structure of the caffeic acid coordinated with a metal.…”

“…This is the first, and yet to our knowledge, the only crystal structure of the caffeic acid coordinated with a metal. In that work 30 the X-ray structure (from now on A) was flanked by another computed and predicted one (from now on B), which is more stable from the enthalpic point of view but less stable in the overall thermodynamic balance.…”