Search

The reaction coordinates of a molecular photo-switch 2-(4'-diethylamino-2'-hydroxyphenyl)-1 H -imidazo-[4,5- b ]pyridine (DHP) was tuned with a nanoparticle and surfactant. DHP undergoes excited state intramolecular proton transfer (ESIPT) and emits normal and tautomer emissions in N,N -dimethylformamide. Silver nanoparticles suppress the ESIPT and induce twisted intramolecular charge transfer (TICT). Further addition of surfactants alters the process. Interestingly, different surfactants cause different effects. Accordingly, the luminescence characteristics are altered. The anionic surfactant sodium dodecyl sulfate (SDS) restores the ESIPT process by completely detaching the molecule from the nanoparticle. The nonionic surfactant Triton X-100 (TX-100), at lower concentration, enhances the TICT emission and the ESIPT process is also observed due to the release of some fluorophore from the nanoparticle complex. But at higher concentration the fluorophores are released completely and the ESIPT process is restored. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB), at lower concentration, simply restores the ESIPT process by releasing the fluorophore. But at higher CTAB concentration, DHP enters the metalparticle-CTAB aggregate and shows enhanced ESIPT.

The proton transfer of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA), a fluorophore proficient in self-aided twin proton transfer, has been explored in the presence of dimethylformamide (DMF). The proton transfer is completely altered in the presence of DMF (compared to that in other solvents). Bis-HPTA forms a hydrogen bonded complex with DMF molecules, which shift the conformer equilibrium. Theoretical calculations predicted that the DMF complexes of bis-HPTA-II and bis-HPTA-III are more stable than the complexes of other conformers. Upon excitation, bis-HPTA-II transfers a proton to a DMF molecule to form an anion and bis-HPTA-III transfers a proton to a ring nitrogen to form a keto form. Silver particles reverse the effect of DMF. They regenerate bis-HPTA-I by removing the intermolecular hydrogen bond and thereby they reestablish the proton transfer triggered proton transfer (PTTPT).

The double excited state intramolecular proton transfer (ESIPT) of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA), a molecule possessing two intramolecular hydrogen bonded donor-acceptor pairs, has been investigated. The molecule undergoes not only a single ESIPT, but also a rare twin ESIPT. The most interesting fact is that initially, only one acid-base pair is ESIPT active and the other pair is ESIPT inactive. The first proton transfer triggers the proton transfer in the second acid-base pair by creating appropriate conditions. This new type of sequential proton transfer is labeled as 'proton transfer triggered proton transfer' (PTTPT). The PTTPT has been demonstrated by steady state and time resolved fluorescence techniques. It was further substantiated by theoretical data and using a chemically modified system 3-(2-hydroxyphenyl)-5-(2-methoxyphenyl)-1H-1,2,4-triazole.

Protein folding/unfolding transitions between the native (N) and unfolded (U) states are usually describable as two-state, only because of the dominant presence of the N and/or U states, because of which high energy intermediates remain undetected. Delineation of the cooperativity underlying these transitions, and characterization of high energy intermediates that are populated sparsely, have been difficult challenges, especially under equilibrium conditions, and require the use of a sensitive probe that reports on both the structures and population distributions of the partially unfolded intermediates. In this study, the use of multisite time-resolved FRET to monitor structural change in five specific segments of the small protein monellin, has brought out local deviations from two-state behavior during unfolding. It is shown that in some segments of the protein structure, denaturant-induced unfolding proceeds first by gradual expansion of the N state, then by an all-or-none transition from the N state ensemble to the U state ensemble, followed finally by expansion of the U state. Segments encompassing the sole helix appear, however, to unfold completely through a gradual transition from the N to U states. Finally, it is shown that equilibrium unfolding of monellin is not only heterogeneous, but that the degree of non-cooperativity differs between the sole α-helix and different parts of the β-sheet.

The effect of cucurbit-7-uril (CB-7) on a cationic mixture with same charge has been investigated by studying monocationic mixtures of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b) and 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-c]pyridine (DMAPIP-c). The pK a of both the guests increases in CB-7. DMAPIP-b forms all three monocations in the ground and the excited states in aqueous as well as in CB-7 solution. However, CB-7 shifts the equilibrium more towards the less polar MC2 and MC3. On the other hand, DMAPIP-c exists only as MC1 and MC3 in aqueous solution, however, in CB-7 it exists not only as MC1 and MC3 but also as MC2 in CB-7 in the ground state. In the excited state, as in aqueous solution MC1 forms MC2 by biprotonic phototautomerism in CB-7. The association constants of monocations suggest a pyridyl nitrogen position dependence. All the MC-CB-7 complexes are optimized by density functional theory (DFT).

The anion sensitivity and the deprotonation nature of the nitrogenous analogues of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) are investigated in a polar aprotic medium. It is observed that the substitution of pyridyl nitrogen enhances the anion sensitivity. However, despite the enhanced sensitivity of the nitrogenous analogues the deprotonation of these molecules in the presence of strong anions is less favored as compared to HPBI. This anomalous trend observed for the nitrogenous analogs is discussed and explained using theoretical calculations and experimental findings. It is also found that the sensitivity towards anions and the formation of anions also depend on the position of the pyridyl nitrogen.

Despite the fact that 2-(2'-hydroxyphenyl)benzimidazole (HPBI) exists as the cis and trans enol in neutral form, it was reported to exist only in the trans form upon deprotonation in an aqueous medium. It was also shown that the dianion formed in the ground state can be re-protonated to form the trans-anion in the excited state. In the present study, the cis-anion and dianion could be obtained upon proper stabilization in suitable environments. Switching between the cis and trans-anion was demonstrated to be possible by changing the environment. Theoretical calculations were performed to substantiate the existence of the cis-anion and dianion. In contrast to a literature report, it was shown that the 'OH' group deprotonates before the 'NH' group to form a monoanion not only in protic solvents but also in aprotic solvents.

Logic gates with different radixes have been constructed using a biologically active molecule, 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b). Taking advantage of the multiple binding sites of the fluorophore, a series of different molecular logic gates are developed using fluorescence intensities at different wavelengths. The high emission of the molecule is drastically quenched in the presence of Fe(3+). It is regained by the addition of an equivalent amount of F(-). The fluorescence On-Off nature has been used to construct molecular full subtractor and molecular keypad lock system with Boolean logic. A ternary system is generated by considering three defined fluorescence intensities at particular wavelengths. The smooth dependency of emission intensities with analyte concentration is utilized to construct an infinite-valued fuzzy logic system. The fuzzy logic system is further coupled with a neuro-adaptation method to predict more accurately the dependency of molecular intensity on external inputs.

The binding interactions of Cu(2+), an essential trace metal ion, and Cd(2+), a deleterious metal ion, with 2-(4'-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine (DMAPOP) were studied in acetonitrile and sodium dodecyl sulphate (SDS). The studies show that the surfactant can formulate the metal ions to bind at a specific binding site. In acetonitrile, the Cu(2+) binds at the dimethylamino nitrogen and pyridine nitrogen to form two different types of complexes with DMAPOP. On the other hand, Cd(2+) coordinates through the dimethylamino nitrogen and oxazole nitrogen to form two different types of complexes with DMAPOP in acetonitrile. But SDS effectively controls the binding site and both metal ions bind at the same ring nitrogen in SDS. The binding affinities of both metal ions with DMAPOP vary with the concentration of SDS.

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.

We present an electrokinetic label-free biomolecular screening chip (Glass/PDMS) to screen up to 10 samples simultaneously using surface plasmon resonance imaging (iSPR). This approach reduces the duration of an experiment when compared to conventional experimental methods. This new device offers a high degree of parallelization not only for analyte samples, but also for multiplex analyte interactions where up to 90 ligands are immobilized on the sensing surface. The proof of concept has been demonstrated with well-known biomolecular interactant pairs. The new chip can be used for high throughput screening applications and kinetics parameter extraction, simultaneously, of interactant-protein complex formation.

Symmetrically substituted styrenes photochemically gave unstable conformers in low temperature organic glass in a manner consistent with the recently postulated Hula-twist mechanism of photoisomerization.