The reversible switching of a water-soluble spiropyran compound is recorded over 1 ns by means of femtosecond vis-pump/vis- and IR-probe spectroscopy under aqueous conditions. Our investigations reveal that the photochemical conversion from the closed spiropyran to the open merocyanine takes 1.6 ps whereas the reversed photoreaction is accomplished within 25 ps. The combination of time-resolved and steady-state observations allows us to reveal central parts of the reaction pathway leading to either form. The enhanced water solubility, its fast and efficient switching behavior, and its stability against hydrolysis over a time range of several weeks make this compound an attractive and versatile tool for biological applications.

Abstract3,4-Tetramethylpyridiniumporphyrazines bind strongly and selectively to human telomeric Gquadruplex DNA, inducing the formation of an antiparallel quadruplex in a process that mimics molecular chaperones.Telomerase is a reverse transcriptase which is responsible for the synthesis of telomeres, and is upregulated in about 80-85% of human cancer cells.1 Human telomeric DNA (Htelo) is composed of tandem repeats of the TTAGGG sequence with a single stranded 3′-end overhang, where the guanine-rich strand can fold into a four-stranded G-quadruplex structure. Telomeric DNA quadruplexes can form at the chromosome extremities.2 The formation and stabilisation of intramolecular telomeric G-quadruplex structures by quadruplex binding molecules can inhibit telomerase activity in vitro. Thus, there is considerable interest in ligand-mediated strategies for the interference of telomere maintenance that can induce cell death.3-5 As telomerase activity is low in human somatic cells, it is a very promising target for anticancer drug development. Furthermore, it has been demonstrated that G-quadruplex motifs are prevalent throughout the genome.6Cationic porphyrins, in particular tetramethylpyridiniumporphyrin (TMPyP4), are well known for their ability to bind to different types of G-quadruplexes and, in some cases, to facilitate G-quadruplex formation.7-11 We now report that tetramethylpyridiniumporphyrazines (TMPyPz) bind strongly to quadruplexes, selectively inducing the antiparallel conformer.Tetrapyridinoporphyrazines (TPyPz) are non-symmetrical phthalocyanine azo-analogues in which four pyridine moieties substitute the four benzene groups in the macrocycle periphery. They differ from porphyrins by having nitrogen atoms in the meso positions linking the individual pyrrole units. The pyridyl groups of the 3,4-TPyPz compounds can readily be methylated to give 3,4-TMPyPz which is water soluble. The syntheses of the 3,4-TPyPz, 3,4-TPyPz zinc(II), 3,4-TMPyPz and 3,4-TMPyPz zinc(II) porphyrazines ( Fig. 1) have already been reported,12,13 but, to the best of our knowledge, no TMPyPz-Gquadruplex binding properties have been reported.The UV-Vis titration spectra of both 3,4-TMPyPz and 3,4-TMPyPz zinc(II) porphyrazines with annealed Htelo in a 50 mM TRIS-HCl (pH 7.4), 150 mM KCl buffer (Fig. 2) show a red-shift of the respective Soret bands and a decrease in the hypochromicity; these † Electronic supplementary information (ESI) available: Binding plots and CD data. The binding stoichiometry extracted from the Scatchard plot and confirmed by a Job plot (vide in ESI † ) is 1 : 1 for 3,4-TMPyPz; however we find that the Htelo G-quadruplex binds four molecules of 3,4-TMPyPz zinc(II). TMPyP4 also shows high binding stoichiometries but, to the best of our knowledge, no structural rationalisation has yet been presented.16 According to results obtained by Haq et al., using similar experimental conditions, the TMPyP4 porphyrin shows a K D of 14 μM,16 which is almost two orders of magnitude weaker than the observed binding for the porph...

We report herein the successful design of the first molecule that induces the folding of the parallel human telomeric Gquadruplex from single-stranded DNA in the absence of added cation.[1] Ammonium ions stabilize quadruplex structures, [2] and K + ions have been shown to be separated by approximately 3.3 in the crystal structure of the human telomeric quadruplex.[3] Therefore, we synthesized 1 (Scheme 1) based on the hypothesis that the anthracene moiety would stack onto guanines, forming one of the external quartets and allowing the ammonium centers of 1, which are separated by approximately 3.4 , to induce the DNA folding through hydrogen bonding and cation-dipole interactions, mimicking K + ions within the central quadruplex channel. [4] Upon the addition of 1 to a solution containing the human telomeric DNA d [TTAGGG] 4 (telo24) at room temperature and free of added cation, we observed positive and negative CD signals at 263 and 240 nm, respectively, within 30 s (Figure 1). This resultant CD spectrum is characteristic of a parallel quadruplex.[5] The human telomeric quadruplex is highly polymorphic and can exist as parallel, [3] antiparallel, [6a] and mixed-type parallel/antiparallel [6b] structures depending on strand orientation. However, ligand 1 appears to induce a single, parallel, quadruplex conformation. After addition of 10 equivalents of 1, an induced CD signal at the absorbance of achiral 1 (253 nm) was observed as a result of 1 sensing the DNA chirality in the complex. In control experiments, we observed that the addition of 20 equivalents (200 mm) of diethylene triamine (DETA) to a solution free of added cation, but containing telo24, did not induce quadruplex structures as seen by CD spectroscopy. Furthermore, ligand 2, in which an amide linkage reduces the protonation state and geometrical freedom of the molecule, also did not induce quadruplex formation under similar conditions (see the Supporting Information). These results suggest that the polyamine component of 1 is involved in a rather specific interaction with the DNA quadruplex. Quadruplex-specific stabilization by cations has an order of preference of K + > NH 4 + > Na + > Li + as a result of the relative free energies, hydration state, and ionic radius for optimal sphere coordination within the quadruplex cavity.[2] The more-general cation-induced stabilization owing to shielding of charge repulsion of the phosphate backbone is less sensitive to the nature of the monovalent cation. To explore the proposal that the polyamine of 1 is involved in a threading mode of interaction with the quadruplex, we studied the competition of 1 with specific monovalent cations. The presence of a large excess of K + ions Scheme 1. Ligands studied for G-quadruplex induction.

The ability of three different bifunctional azobenzene linkers to enable the photoreversible formation of a defined intermolecular two-tetrad G-quadruplex upon UV/Vis irradiation was investigated. Circular dichroism and NMR spectroscopic data showed the formation of G-quadruplexes with K(+)  ions at room temperature in all three cases with the corresponding azobenzene linker in an E conformation. However, only the para-para-substituted azobenzene derivative enables photoswitching between a nonpolymorphic, stacked, tetramolecular G-quadruplex and an unstructured state after E-Z isomerization.

No abstract

We report herein the successful design of the first molecule that induces the folding of the parallel human telomeric G-quadruplex from single-stranded DNA in the absence of added cation.[1] Ammonium ions stabilize quadruplex structures, [2] and K + ions have been shown to be separated by approximately 3.3 Å in the crystal structure of the human telomeric quadruplex.[3] Therefore, we synthesized 1 (Scheme 1) based on the hypothesis that the anthracene moiety would stack onto guanines, forming one of the external quartets and allowing the ammonium centers of 1, which are separated by approximately 3.4 Å, to induce the DNA folding through hydrogen bonding and cation-dipole interactions, mimicking K + ions within the central quadruplex channel. [4] Upon the addition of 1 to a solution containing the human telomeric DNA d[TTAGGG] 4 (telo24) at room temperature and free of added cation, we observed positive and negative CD signals at 263 and 240 nm, respectively, within 30 s (Figure 1). This resultant CD spectrum is characteristic of a parallel quadruplex.[5] The human telomeric quadruplex is highly polymorphic and can exist as parallel, [3] antiparallel,[6a] and mixed-type parallel/ antiparallel[6b] structures depending on strand orientation. However, ligand 1 appears to induce a single, parallel, quadruplex conformation. After addition of 10 equivalents of 1, an induced CD signal at the absorbance of achiral 1 (253 nm) was observed as a result of 1 sensing the DNA chirality in the complex. In control experiments, we observed that the addition of 20 equivalents (200 μM) of diethylene triamine (DETA) to a solution free of added cation, but containing telo24, did not induce quadruplex structures as seen by CD spectroscopy. Furthermore, ligand 2, in which an amide linkage reduces the protonation state and geometrical freedom of the molecule, also did not induce quadruplex formation under similar conditions (see the Supporting Information). These results suggest that the polyamine component of 1 is involved in a rather specific interaction with the DNA quadruplex.Quadruplex-specific stabilization by cations has an order of preference of K + >NH 4 + >Na + >Li + as a result of the relative free energies, hydration state, and ionic radius

A series of cationic porphyrins carrying 1-3 meso-N-pyridinium groups has been synthesised, and their binding to G-quadruplex DNA has been explored by surface plasmon resonance (SPR) and circular dichroism spectroscopy. Two trans substituents appear to be sufficient for tight binding; preferential binding to the anti-parallel intramolecular human telomeric DNA was observed for the A2trans and A3 porphyrins. The A2trans is able to induce the formation of an anti-parallel G-quadruplex in a K+ free solution, mimicking the effect of a molecular chaperone.

The vast majority of nanomaterials studied in light of their ability to transmit chirality to or amplify their chirality in a surrounding medium, constitute an achiral core with chirality solely installed at the surface by conjugation or encapsulation with optically active ligands. Here we present the inverse approach focusing on surface‐modified cellulose nanocrystals (CNCs) with core chirality at both the molecular and the morphological level to quantify transmission and amplification of core chirality through space using a host nematic liquid crystal (N‐LC) as reporter. We find that CNCs functionalized at the surface with achiral molecules, structurally related to the N‐LC, exhibit better N‐LC solubility, thereby serving as highly efficient chiral inducers. Moreover, functionalization with chiral molecules only marginally enhances the efficacy of helical distortion in the host N‐LC matrix, indicating the high propensity of CNCs to transfer chirality from an inherently chiral core.