Dehydration and Dopant Effects in Nucleic Acid Hydrates
Koch, Elizabeth Scott
Swift, Jennifer A.
The dehydration mechanism and the inclusion of molecular dyes (dopants) in two nucleic acid hydrates, thymine and cytosine, were investigated. Thymine hydrate (TH) was identified as a modulated system which undergoes a phase change at low temperature (230-240 K) from a monoclinic to a triclinic superlattice as the partially occupied (77%) water positions order into a supercell. This work identified two new anhydrous thymine phases: one grown by sublimation at ~180°C (Ts), and another prepared by dehydration (~50°C) of thymine hydrate (Td). Ts and Td structures and morphologies were determined by synchrotron XRD and SEM. Under isothermal conditions, pure TH dehydrates with a Tmax=58.1°C to a polycrystalline material, Td. The dehydration occurs via a one dimensional diffusion mechanism. Macroscopically, this appeared as anisotropic propagation of opaque regions along the c-axis (water channel direction) of the crystal. Td could be partially rehydrated to TH by exposure to high relative humidity conditions.TH crystals were doped with a selection of 20 cationic, anionic, and neutral dyes, which displayed much higher average dehydration temperatures than the pure materials. TH-dye crystals dehydrate via a three dimensional diffusion mechanism. Td-dye did not rehydrate to TH-dye under high relative humidity conditions, which may be attributed to their differences in particle morphologies. TH-dye crystals also undergo the monoclinic to triclinic supercell change, but at significantly lower temperatures (~170 K) than pure TH. Additionally, a cocrystallization study of thymine and coformers with complementary hydrogen bonding motifs yielded three new phases: thymine:2,4-diaminopyrimidine, thymine:2,4,6-triaminopyrimidine monohydrate, and thymine:2,4-diamino-6-hydroxypyrimidine trihydrate.Dehydration of a second molecular hydrate, cytosine monohydrate (CM), was determined to follow a one dimensional phase boundary mechanism with a Tmax=89.7°C. The dehydration resulted in a polycrystalline anhydrous material, Cd, which has a PXRD pattern similar to a known anhydrous form. SEM indicated that the dehydration process results in large cracks on the (100) CM surface. In doping studies with six molecular dyes, optical microscopy showed a non-uniform distribution in the CM-dye crystal matrix. The relationship between [dye]soln and included wt% in the crystals also appeared to be quite different for different dyes depending on their size and charge.
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