Low-molecular-weight amino-acid-based derivatives: from organogels to single crystals and mesocrystal

REY, JUAN MANUEL; DI SALVO, FLORENCIA; SANCHEZ MONTILVA, OLGA CAROLINA; MOVILLA, FEDERICO

Bahía Blanca

Congreso; XIII Reunión anual AACr; 2017

Amino acids are able to self-assemble into ordered superstructures, also called mesocrystals, when they are subjected tocertain crystallisation conditions such as, pH, supersaturation level and the use of additives, being the last one the most popularstrategy [1, 2]. Besides, self-assembly strategies applied to low-molecular weight building blocks can conduct to supramoleculargels. Their properties can be modulated by variation of temperature or solvent, since these changes directly affect the strengthof the non-covalent interactions within the gel network [3]. The obtaining of both, supramolecular gels or crystalline materials, isgoverned by the presence of non-covalent interactions, resulting in spanning network that can immobilise solvents for the firstones or, the self-assembly between the building blocks, to give place to the others. Thus, there is a delicate balance for a lowweightmolecule to behave as a gelator or crystal.Herein, we present a new family of chiral L-amino-acid-based low-weight molecules which behave as excellent building blocksfor the construction of supramolecular gels or crystalline structures. Compounds 1-3 are obtained as zwitterions after thereaction between piperonal and L-Alanine, L-Phenylalanine and L-Tyrosine, respectively. Crystallisation experiments usingorganic solvents are only possible with dilute solutions of the amino acid derivatives due to their low solubility. Compounds 1and 3 conduct mainly to polycrystalline materials and 2 gives place to crystalline superstructures with spherical morphologies,results attributed to the different solubility and supramolecular properties conferred by the amino acid bone. All derivatives aresoluble in basic media after the deprotonation of the zwitterion. After a slow evaporation of the solvent of the basic solutions,spherical-like mesocrystals are obtained. Due to their structural features, compounds 1-3 are also soluble in acids. After slowdiffusion of hydrogen chloride vapours in basic solutions of 2, single crystals suitable for XRD studies are obtained. Thecrystallographic results confirm the presence of the protonated derivative. Although 1 and 3 do not give place to crystals ofsuitable quality under the same conditions, experiments using other acids are in progress. In order to increase the solubility ofcompounds 1-3 in their zwitterionic form in organic solvents, the temperature of the solutions can be increased. Only thederivative of phenylalanine is soluble in higher concentrations after increasing the temperature and then after cooling, it givesplace to supramolecular gels. The reversibility of the gelification process with temperature is also observed. In conclusion, it ispossible to suggest that the presence and directionality of certain functional groups and the intermolecular interactionsdeveloped, such as the existence or not of the OH and the phenyl group, are key factors in the mechanism of self-assembly intohierarchical structures, single crystals or even supramolecular organogels. Furthermore, the strong pH dependence allows theobtaining of single crystals or the assembly to crystalline superstructures. This new family of low-molecular-mass derivativesshows a huge versatility regarding supramolecular properties. Varying the aldehyde and/or the amino acid opens a broadperspective for the design of novel self-assembly architectures for the further development of functional soft and/or crystallinematerials.