Vera Deneva

The effect of the water on the curcumin tautomerism: A quantitative approach

The tautomerism of curcumin has been investigated in ethanol/water binary mixtures by using UV-Vis spectroscopy and advanced quantum-chemical calculations. The spectral changes were processed by using advanced chemometric procedure, based on resolution of overlapping bands technique. As a result, molar fractions of the tautomers and their individual spectra have been estimated. It has been shown that in ethanol the enol-keto tautomer only is presented. The addition of water leads to appearance of a new spectral band, which was assigned to the diketo tautomeric form. The results show that in 90% water/10% ethanol the diketo form is dominating. The observed shift in the equilibrium is explained by the quantum chemical calculations, which show that water molecules stabilize diketo tautomer through formation of stable complexes. To our best knowledge we report for the first time quantitative data for the tautomerism of curcumin and the effect of the water.

Exploiting Tautomerism for Switching and Signaling

Herein, we demonstrate a conceptual idea for a tautomeric switch based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol. The results show that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. The developed molecular switch, in spite of the simple host–guest system, has shown acceptable complexation ability towards small alkaliand alkalineearth-metal ions and can be a promising basis for further development of effective molecular sensors through implementation of azacrown ethers. Organic molecular materials are increasingly recognized as suitable molecular-level elements (such as switching, signaling, and memory elements) for molecular devices, because the wide range of molecular characteristics can be combined with the versatility of synthetic chemistry to alter and optimize molecular structure in the direction of desired properties. Virtually every molecule changes its behavior when acted upon by external fields or other stimuli. True molecular switches undergo reversible structural changes, caused by a number of influences, which give a variety of possibilities for control. Several classes of photoresponsive molecular switches are already known; these operate through processes such as bond formation and bond breaking, cis– trans isomerization, and photoinduced electron transfer upon complexation. A conceptual scheme of a molecular switch based on molecular recognition is shown in Scheme 1. The host–guest system represents, for instance, a crown ether that can bind ions or a cyclodextrin that can bind other small molecules. It is bound to a signal converter. The complexation behavior is monitored by the state of the signal converter, and in turn its optical or electronic properties are determined by the complexation state of the host–guest system. The main requirement in the design of new molecular switches is to provide fast and clean interconversion between structurally different molecular states (on and off). Tautomerism could be a possibility, because change in the tautomeric state can be accomplished by a fast proton transfer reaction between two or more structures, each of them with clear and different molecular properties. Therefore, our aim herein is to show how tautomerism can be exploited for signal conversion. The conceptual idea of such a device is presented in Scheme 1. In this structure, a change in tautomeric state, labeled A and B, is linked to changes in the complexation abilities of the host–guest system by modulating the propensity of the system to hydrogen bond to the antenna. At the same time, engagement of this antenna causes a change in the tautomeric state. The sensitivity of the electronic ground and excited states of the tautomeric forms to environment stimuli (light, pH value, temperature, solvent) and to the presence of a variety of substituents or to hydrogen bonding can be exploited in the design of flexible tools for control. Obviously, such a device should be based on a tautomeric structure with easy proton exchange between the tautomers, which means that they must coexist in solution. At the same time, a main feature of systems of tautomers coexisting in solution is that the overall optical response is a mixture of the optical responses of the individual tautomers. Consequently, in the design of tautomeric switches, conditions for obtaining pure end tautomer in the corresponding off and on states must be provided. Herein we report the properties of two tautomeric switches, namely 3 and 4 (Scheme 2), based on 4-(phenyldiazenyl)phenol (1) and 4-(phenyldiazenyl)naphthalen-1-ol (2). The parent compound 2 is the first dye that was shown to tautomerize by Zincke and Bindewald in 1884. It has been the object of many spectral and theoretical studies because its tautomeric forms coexist in solution and the equilibrium Scheme 1. Molecular switch based on molecular recognition (left) and conceptual idea for a tautomerism-based molecular switch (right).

Tautomeric transformations of piroxicam in solution: a combined experimental and theoretical study

Piroxicam tautomerism was studied in solution by using UV-Vis spectroscopy, NMR measurements and advanced chemometrics. It has been found that in ethanol and DMSO the enol-amide tautomer is present mainly as a sandwich type dimer. The addition of water leads to distortion of the aggregate and to gradual shift of the equilibrium towards the zwitterionic tautomer. Quantitative data for the aggregation and the tautomeric equilibria are presented. Quantum chemical calculations (M06-2X/TZVP) have been used to explain stability of the tautomers as a function of the solvent and concentration.

Controlled tautomerism – switching caused by an “underground” anionic effect

In a previous communication, we demonstrated a conceptual idea for a tautomeric switching system based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol (1). The results showed that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. However, the effect of the counter ion in the process of protonation was never considered. The crystallographic analysis of protonated cyano and nitro derivatives of 4-(phenyldiazenyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol have shown an interesting and unexpected feature: the counter ion is captured in the process of protonation and the shift in the position of the tautomeric equilibrium is achieved through a bridged complex formation. To the best of our knowledge this is a rare example when controlled shift in the position of tautomeric equilibrium is achieved through anion complexation. The results from the solid state analyses are confirmed by NMR spectroscopy in solution and by quantum-chemical calculations.

10-hydroxybenzo[h]quinoline: Switching between single and double-well proton transfer through structural modifications

Proton transfer in 10-hydroxybenzo[h]quinoline (HBQ) and structurally modified compounds was investigated experimentally (steady state UV-Vis absorption and emission spectroscopy, NMR and advanced chemometric techniques) and theoretically (DFT and TD-DFT M06-2X/TZVP calculations) in the ground and excited singlet state. We observed that the incorporation of electron acceptor substituents on position 7 of the HBQ backbone led to appearance of a keto tautomer in ground state and changes in the excited state potential energy surface. Both processes were strongly solvent dependent. In the ground state the equilibrium could be driven from the enol to the keto form by change of solvent. The theoretical calculations explain the substitution-determined transition from a single- to a double-well proton transfer mechanism.

Aggregation of 2‐Aminobenzimidazole—A Combined Experimental and Theoretical Investigation

An investigation of 2-aminobenzimidazole was carried out by calculations at HF, MP2, and DFT levels of theory and also by UV and IR spectroscopy. The quantum chemical calculations predict a full shift of the equilibrium towards the amino form, but the absorption spectra in different solvents distinctly show a two-component equilibrium system. Examination of possible equilibria in solution shows that an equilibrium between two dimeric forms of the amino tautomer of 2-aminobenzimidazole explains the spectral observations.