It is known that electron donating groups have quite a different effect on the π-delocalization of a conjugate system when bonded at ortho and para as compared to meta positions in the phenyl ring. In the present work, the BF2 complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione (1), a molecule with two methoxy groups in one of the phenyl rings at meta positions, was prepared. Compound 1 exists as two polymorphs having different mutual orientations of the two methoxy groups: in polymorph A away from each other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn-anti). In both crystals, the molecules which are antiparallel (the subPh rings as well as dioxaborine are on opposite sides) form stacks through face-to-face π-π interactions, while in polymorph A the crystal packing is further stabilized by intermolecular C(phenyl)-H···F and C(methoxy)-H···F hydrogen bonds. Solid A possesses numerous chromic effects, including mechano-, thermo-, and chronochromism, though the latter to a lesser extent, as well as the effect of rearrangement of the amorphous phase into a more stable crystalline phase A, associated with crystallization-induced emission enhancement (CIEE). The solid-state emission can be repeatedly switched regarding its color and efficiency with excellent reversibility by external stimuli. On the other hand, crystalline solid B undergoes thermal interconversion of syn-anti to the anti conformer. Compound 1 shows a solvatochromic effect (SE), is aggregation-induced emission (AIE) active, and through the sublimation process displays self-assembling crystalline platelike microstructures or microfibers that reveal an obvious optical waveguide effect.

A four-repeat human telomere DNA sequence without the 3'-end guanine, d[TAGGG(TTAGGG)2 TTAGG] (htel1-ΔG23) has been found to adopt two distinct two G-quartet antiparallel basket-type G-quadruplexes, TD and KDH(+) in presence of KCl. NMR, CD, and UV spectroscopy have demonstrated that topology of KDH(+) form is distinctive with unique protonated T18⋅A20(+) ⋅G5 base triple and other capping structural elements that provide novel insight into structural polymorphism and heterogeneity of G-quadruplexes in general. Specific stacking interactions amongst two G-quartets flanking base triples and base pairs in TD and KDH(+) forms are reflected in 10 K higher thermal stability of KDH(+) . Populations of TD and KDH(+) forms are controlled by pH. The (de)protonation of A20 is the key for pH driven structural transformation of htel1-ΔG23. Reversibility offers possibilities for its utilization as a conformational switch within different compartments of living cell enabling specific ligand and protein interactions.

A four‐repeat human telomere DNA sequence without the 3′‐end guanine, d[TAGGG(TTAGGG)2TTAGG] (htel1‐ΔG23) has been found to adopt two distinct two G‐quartet antiparallel basket‐type G‐quadruplexes, TD and KDH+ in presence of KCl. NMR, CD, and UV spectroscopy have demonstrated that topology of KDH+ form is distinctive with unique protonated T18⋅A20+⋅G5 base triple and other capping structural elements that provide novel insight into structural polymorphism and heterogeneity of G‐quadruplexes in general. Specific stacking interactions amongst two G‐quartets flanking base triples and base pairs in TD and KDH+ forms are reflected in 10 K higher thermal stability of KDH+. Populations of TD and KDH+ forms are controlled by pH. The (de)protonation of A20 is the key for pH driven structural transformation of htel1‐ΔG23. Reversibility offers possibilities for its utilization as a conformational switch within different compartments of living cell enabling specific ligand and protein interactions.

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BackgroundSeveral papers described the structure of curcumin and some other derivatives in solid and in solution. In the crystal structure of curcumin, the enol H atom is located symmetrically between both oxygen atoms of the enolone fragment with an O···O distance of 2.455 Å, which is characteristic for symmetrical H-bonds. In the solution, the geometry of the enolone fragment is attributed to the inherent disorder of the local environment, which solvates one of the basic sites better than the other, stabilizing one tautomer over the other. In this paper, how the position of methoxy groups in dimethoxy curcuminoids influence the conformation of molecules and how the halogen atoms change it when they are bonded at α-position in keto-enol part of molecules is described.ResultsSix isomers of dimethoxy curcuminoids were prepared. Conformations in solid state, which were determined by X-ray single crystallography and 1H MAS and 13C CPMAS NMR measurements, depend on the position of methoxy groups in curcuminoid molecules. In solution, a fast equilibrium between both keto-enol forms exists. A theoretical calculation finding shows that the position of methoxy groups changes the energy of HOMO and LUMO. An efficient protocol for the highly regioselective bromination and chlorination leading to α-halogenated product has been developed. All α-halogenated compounds are present mainly in cis keto-enol form.ConclusionsThe structures in solid state of dimethoxy curcuminoids depend on the position of methoxy groups. The NMR data of crystalline solid samples of 3,4-diOCH3 derivative, XRD measurements and X-ray structures lead us to the conclusion that polymorphism exists in solids. The same conclusion can be done for 3,5-diOCH3 derivative. In solution, dimethoxy curcuminoids are present in the forms that can be described as the coexistence of two equivalent tautomers being in fast equilibrium. The position of methoxy groups has a small influence on the enolic hydrogen bond. Theoretical calculations show that the energy gap between HOMO and LUMO depend on the position of methoxy groups and are lower in solution. Chlorination and bromination on α-position of 1,3-diketone moiety do not change the preferential form being cis keto-enol as in parent compounds.