Hydrogen Bonded Schiff Bases Research Articles

Intramolecular dual hydrogen bonded fluorescent “turn-on” probe for Al3+ and HSO4- ions: Applications in real water samples and molecular keypad lock

Dual hydrogen bonded Schiff base containing unsymmetrical double proton transfer sites, one with imine bond (CN) and hydroxyl group (OH), and the other with benzimidazole and hydroxyl groups has been successfully synthesized. Probe 1 displayed intramolecular charge transfer and acts as a potential sensor for Al3+ and HSO4- ions. Probe 1 displayed two absorption peaks at 325 nm and 340 nm and an emission band at 435 nm upon excitation at 340 nm. Probe 1 behaves as a fluorescence “turn-on” chemosensor for both Al3+ and HSO4- ions in H2O-CH3OH solvent system. The proposed method allows the determination of Al3+ and HSO4- ions up to 39 nM and 23 nM at emission wavelength 385 nm and 390 nm, respectively. The binding behavior of probe 1 towards these ions is determined by the Job’s plot method and 1H NMR titrations. Probe 1 is used to construct a molecular keypad lock where the absorbance channel can be opened only in the presence of the correct sequence. Further, it is used for the quantitative determination of HSO4- ion in different real-field water samples.

Strongly Hydrogen-Bonded Schiff Base and Adjoining Polyene Twisting in the Retinal Chromophore of Schizorhodopsins.

A transmembrane proton gradient is generated and maintained by proton pumps in a cell. Metagenomics studies have recently identified a new category of rhodopsin intermediates between type-1 rhodopsins and heliorhodopsins, named schizorhodopsins (SzRs). SzRs are light-driven inward proton pumps. Comprehensive resonance Raman measurements were conducted to characterize the structure of the retinal chromophore in the unphotolyzed state of four SzRs. The spectra of all four SzRs show that the retinal chromophore is in the all-trans and 15-anti configuration and that the Schiff base is protonated. The polyene chain is planar in the center of the retinal chromophore and is twisted in the vicinity of the protonated Schiff base. The protonated Schiff base in the SzRs forms a stronger hydrogen bond than that in outward proton-pumping rhodopsins. We determined that the hydrogen-bonding partner of the protonated Schiff base is not a water molecule but an amino acid residue, presumably an Asp residue in helix G. The present observations provide valuable insights into the inward proton-pumping mechanism of SzRs.

Unsymmetrical tetracatenar liquid crystals containing 2-phenylbenzoxazole: Synthesis and characterisation

A series of 2-phenylbenzoxazole derivatives containing an intramolecular hydrogen-bonded Schiff's base linkage have been synthesised. All are liquid crystalline, displaying only the smectic phase transition, apart from one compound which seems to be non-mesogenic. These compounds may be the first examples reported of tetracatenar liquid crystals containing the 2-phenylbenzoxazole moiety. The optical properties of the target compounds were evaluated in solution.

2-phenylbenzoxazole-containing calamitic liquid crystals: synthesis and characterisation

A series of 2-phenylbenzoxazole liquid crystals having intramolecular hydrogen-bonded Schiff's base linkers have been synthesised. All of them exhibit liquid crystalline phase behaviour. They showed only SmC phase transition. The phase is stabilised over a wide temperature range due to the presence of intramolecular H-bonding.

Deprotonation of hydrogen bonded Schiff bases by three strong nitrogen bases

Three Schiff bases obtained from substituted salicylaldehydes and 2-hydroxy-1-naphthaldehyde and aliphatic amines were investigated in terms of possible withdrawal of tautomeric proton from intramolecular hydrogen bridge. Three strong nitrogen bases: 1,8-bis(dimethylamino)naphtalene (DMAN), 1,1,3,3-tetramethylguanidine (TMG) and 1,8-bis(tetramethylguanidino)naphthalene (TMGN) were used as deprotonating agents in acetonitrile solution at room temperature. In the specified conditions it was found that only in the case of 5-nitrosalicylaldehyde and isopropyl amine derivative this process could be performed using TMG and TMGN as a base. The other derivatives, where bridged proton is shifted to oxygen or nitrogen atom, do not undergo such reaction. The deprotonation process was monitored by nitrogen and proton NMR measurements.

Dielectric properties and AC conduction mechanism for 5,7-dihydroxy-6-formyl-2-methylbenzo-pyran-4-one bis-schiff base

The dielectric properties and the electrical conductivity of the tetradentate Schiff base, derived from the condensation of 5,7 dihydroxy-6- formyl-2-methylbenzo-pyran-4-one with ethylenediamine has been studied in the temperature range 300 K< T<420 K and the frequency range 0.1–20 KHz. At T>350 K, both the dielectric constant, ε ′ , and the dielectric loss, ε ″ , showed a decrease with increasing frequency and increasing temperature. The broad loss peak anomalies observed are attributed to intrinsic lattice dipoles in the hydrogen bonded Schiff base investigated. The discontinuity observed in the DC conductivity-temperature dependence behavior is correlated with structural changes in the network. The AC conductivity measurements as function of temperature exhibit a frequency dependent, temperature dependent behavior. Protonic conduction prevails at temperatures below 350 K. Decrease in the AC conductivity above 350 K is also indicative of structural changes within the hydrogen-bonded conjugated system. The frequency dependent-conductivity of the Schiff base is found to be proportional to ω s . Analysis of the AC conductivity experimental data obtained, and the frequency exponent s with theoretical models reveals that the correlated barrier hopping (CBH) model is the appropriate mechanism for conduction in the Schiff base system. The barrier height, W m , between charged defect states is calculated and its value is in good agreement with the theory of hopping of charged carriers over a potential barrier.

Functional characterization of sensory rhodopsin II from Halobacterium salinarum expressed in Escherichia coli

Sensory rhodopsin II (SRII) from Halobacterium salinarum is heterologously expressed in Escherichia coli with a yield of 3–4 mg of purified SRII per liter cell culture. UV/Vis absorption spectroscopy display bands characteristic for native SRII. The resonance Raman spectrum provides evidence for a strongly hydrogen-bonded Schiff base like in mammalian rhodopsin but unlike to the homologous pSRII from Natronobacterium pharaonis. Laser flash spectroscopy indicates that SRII in detergent as well as after reconstitution into polar lipids shows its typical photochemical properties with prolonged photocycle kinetics. The first functional heterologous expression of SRII from H. salinarum provides the basis for studies with its cognate transducer HtrII to investigate the molecular processes involved in phototransduction as well as in chemotransduction.

Hydrogen bonded Schiff bases; di-anil of 2-hydroxy-5-methyl-isophthaldehyde

Structural and spectroscopic properties of solid dianil of 2-hydroxy-5-methyl-isophthaldehyde have been investigated. This is a Schiff base containing intramolecular O–H…N hydrogen bonding. Single crystal X-ray diffraction shows that the possibly equivalent hard structural parameters (bond lengths and the valence angles) of the title molecule are hardly differentiated by an asymmetric hydrogen bonding and different conformations of the anil wings. 13C MAS NMR results are more sensitive and they clearly show the influence of [O–H…N] hydrogen bonding on the chemical shifts of the central ring carbon atoms.

X-Ray structure and force field calculations of a chiral benzaldehyde Schiff base

The crystal structure of the Schiff base formed from benzaldehyde and (R)-2-aminobutan-1-ol is compared to one obtained from force-field calculations employing a new parameter set; the probable structure of the hydrogen-bonded Schiff base dimer is discussed.

Stopped-Flow Kinetic Study of the Complex Formation between Pyridoxal 5′-Phosphate and Bovine Serum Albumin

Abstract The complex formation between pyridoxal 5′-phosphate (PLP) and bovine serum albumin at pH=7.0 and 25 °C was characterized by four relaxation processes. The dependence of the reciprocal relaxation times upon reactant concentrations was found to be consistent with a mechanism in which a rapid bimolecular binding step is followed by three sequential isomerization steps. The eight rate constants for this mechanism were determined and then combined to calculate an overall equilibrium constant which is in agreement with the literature value. Absorption spectra of the complexes formed by the two faster steps were determined from absorbance changes associated with them; the spectrum corresponding to the first complex has two bands at 330 nm and 360 nm and an additional 410 nm band, while the spectrum corresponding to the second complex has a large single band at 375 nm. Comparing these spectra with those reported for other PLP-enzyme systems, we speculate that four steps in the proposed mechanism may correspond to the rapid formation of a carbinolamine, which is in very rapid equilibrium with a Schiff base form, followed by the deprotonation of the ring nitrogen, the formation of an ionic hydrogen-bonded Schiff base, and the final formation of a nonionic hydrogen-bonded Schiff base.

The mode of binding of pyridoxal 5′-phosphate in glycogen phosphorylase

The absorption, fluorescence and chemical properties of the pyridoxal 5′-phosphate (PLP) site in glycogen phosphorylase at pH 7.0 can be simulated by Schiff base derivatives of PLP in non-aqueous solvents. It is proposed that at neutral pH,PLP in phosphorylase is embedded in a hydrophobic microenvironment and bound to an ε-amino group of a lysine residue in the protein through a hydrogen-bonded Schiff base structure.