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  • Title: NMR, Raman, and DFT Study of Lyotropic Chromonic Liquid Crystals of Biomedical Interest: Tautomeric Equilibrium and Slow Self-Assembling in Sunset Yellow Aqueous Solutions.
    Author: Kristinaitytė K, Maršalka AN, Dagys L, Aidas K, Doroshenko I, Vaskivskyi Y, Chernolevska Y, Pogorelov V, Valevičienė NR, Balevicius V.
    Journal: J Phys Chem B; 2018 Mar 29; 122(12):3047-3055. PubMed ID: 29489366.
    Abstract:
    Temperature and composition effects in Sunset Yellow FCF (SSY) aqueous solutions were studied by the 1H, 15N NMR as well as Raman spectroscopy passing through all phase transitions between isotropic phase (I) and chromonic phases-nematic (N) and columnar (M). It was shown that the tautomeric equilibrium in SSY is strongly shifted toward the hydrazone form. The corresponding equilibrium constant p KT = 2.5 was deduced using the density functional theory solvent model density model. The dominance of the hydrazone form was confirmed experimentally using the long-range 1H-15N correlation, widely known as heteronuclear multiple bond correlation. The peak found in the 1H NMR spectra at ca. 14.5 ppm can be attributed to the proton in the intramolecular N-H···O bond. The existence of this signal shows that (i) the growth of the SSY aggregates is accompanied by the segregation of water in the intercolumnar areas with no access for exchange with the N-H protons in the internal layers of the columnar stacks and that (ii) the lifetime of those aggregates is ≥10-8 s or even longer. The temperature dependences of H2O chemical shift and Raman O-H stretching band shape show that water confined in the intercolumnar areas behaves as in the neat substance. When the sample is heated and the transition from M phase to N phase occurs, the molecular motion of water is seen to change in a manner similar to that when water is melting. The equilibration time for N + M→ M is very long because of slow supramolecular restructuring, i.e., the growing of columnar stacks and building of hexagonal arrays. If the sample is cooled down to the temperature below N → M transition relatively fast, the structural changes are behind, and the system falls into supercooled state. In this case, the system evolves via long-lasting self-assembling from the supercooled state to the equilibrium. This process affects the shape of the 1H NMR signal and is easy to monitor.
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