Stereospecificity of chiral salt complexes of chitosan with L- and D-aspartic acid under model conditions of intracellular regulation

Chiral polymeric matrices with controlled sites of complementary-specific interactions are considered innovative substrates for the production of highly selective medicinal and pharmaceutical drugs. To solve this problem, information is necessary about the stereospecific features of the chemical substance used to obtain such biomaterials, including those under model conditions of intracellular communication and regulation.
This work considers the acid–base and chiro-optical properties of chiral biologically active salt complexes of chitosan and L-(D-)aspartic acid (CS∙L-(D-)AspA), which are promising for the design of stereofunctional biomaterials and pharmaceuticals. Special attention is paid to studies of the attenuation of optical anisotropy intensity during the process of model biomolecular condensation of L-menthol in the CS∙L-(D-)AspA medium.
It has been established that an increase in the [AspA]/[CS] molar ratio is accompanied by a decrease in pH and a decrease in the protonation degree of CS amino groups in salt complexes. Aqueous solutions of CS·L-(D-)AspA exhibit a positive and negative, respectively, Cotton effect in the UV region, which is the result of the π-π transition in the aminoaspartic chromophore. In the visible region, CS L- and D-aspartate is characterized by a left-handed rotation of the plane of polarization and monotonic dispersion of specific optical rotation. The phase separation of an ethanolic L-menthol solution in an aqueous CS∙L-(D-)AspA solution proceeds through the mechanism of selective extraction crystallization and combines two types of phase separation, namely, liquid–liquid and liquid–crystal. The “emulsion–dispersion” phase transition is accompanied by the disappearance of the birefringence effect in the condensed phase of the substance.
The results obtained show the promise of using CS∙L-(D-)AspA in studies of biomolecular condensation (the formation of highly labile associates of macromolecules due to liquid phase separation) and the biogenesis of membraneless organelles in physiological/pathological cellular processes.

Acknowledgment
This research was funded by the Russian Science Foundation No. 24-16-00172, https://rscf.ru/project/24-16-00172/.