A phenolic Schiff base for highly selective sensing of fluoride and cyanide via different channels

A chemosensor based on phenolic Schiff base bearing a pyrene group (sensor 1) has been synthesized and demonstrated. Sensor 1 showed a highly selective colorimetric response to fluoride anions based on a deprotonation process and fluorescent response to cyanide anions (606-fold fluorescence quantum yield enhancement) based on a cyclization process. Moreover, the cyclization of phenolic Schiff bases induced by cyanide could be used as a new way to synthesize 2-substituted benzoxazoles.     

Effect of base sequence and deprotonation of Guanine cation radical in DNA

The deprotonation of guanine cation radical (G+*) in oligonucleotides (ODNs) was measured spectroscopically by nanosecond pulse radiolysis. The G+* in ODN, produced by oxidation with SO4-*, deprotonates to form the neutral G radical (G(-H)*). In experiments using 5-substituted cytosine-modified ODN, substitution of the cytosine C5 hydrogen by a methyl group increased the rate constant of deprotonation, whereas replacement by bromine decreased the rate constant. Kinetic solvent isotope effects on the kinetics of deoxyguanosine (dG) and ODN duplexes were examined in H2O and D2O. The rate constant of formation of G(-H)* in dG was 1.7-fold larger in H2O than D2O, whereas the rate constant in the ODN duplex was 3.8-fold larger in H2O than D2O. These results suggest that the formation of G(-H)* from G+* in the ODN corresponds to the deprotonation of the oxidized hydrogen-bridged (G+*-C) base pair by a water molecule. The characteristic absorption maxima of G+* around 400 nm were shifted to a longer wavelength in the order of G相似文献   

Benzophenone Schiff bases of glycine derivatives: Versatile starting materials for the synthesis of amino acids and their derivatives

This review focuses on the introduction and early development, in solution, of phase-transfer catalyzed (PTC) reactions to afford racemic or enantioenriched natural and unnatural amino acids. To form monosubstituted amino acids alkylation reactions are performed on the benzophenone Schiff base of glycine. For α,α-disubstituted amino acids the activated intermediate is an aldimine derivative of the monosubstituted amino acid. Enantioenriched products are produced by organocatalysis using derivatives of Cinchona alkaloids as the phase-transfer catalyst. Selectivity for monoalkylatation and lack of product racemization depend on the acidities of the glycine imines, and dialkylated products are formed from aldimine esters of monoalkyl amino acids. The racemic and catalytic enantioselective reactions of a cationic glycine equivalent with organoboranes, organometallics and malonate anion are discussed as are other reactions of these versatile Schiff bases derivatives.     

Intrinsic primary and secondary hydrogen kinetic isotope effects for alanine racemase from global analysis of progress curves

The pyridoxal phosphate dependent alanine racemase catalyzes the interconversion of L- and D-alanine. The latter is an essential component of peptidoglycan in cell walls of Gram-negative and -positive bacteria, making alanine racemase an attractive target for antibacterials. Global analysis of protiated and deuterated progress curves simultaneously enables determination of intrinsic kinetic and equilibrium isotope effects for alanine racemase. The intrinsic primary kinetic isotope effects for Calpha hydron abstraction are 1.57 +/- 0.05 in the D --> L direction and 1.66 +/- 0.09 in the L --> D direction. Secondary kinetic isotope effects were found for the external aldimine formation steps in both the L --> D (1.13 +/- 0.05, forward; 0.90 +/- 0.03, reverse) and D --> L (1.13 +/- 0.06, forward; 0.89 +/- 0.03, reverse) directions. The secondary equilibrium isotope effects calculated from these are 1.26 +/- 0.07 and 1.27 +/- 0.07 for the L --> D and D --> L directions, respectively. These equilibrium isotope effects imply substantial ground-state destabilization of the C-H bond via hyperconjugation with the conjugated Schiff base/pyridine ring pi system. The magnitudes of the intrinsic primary kinetic isotope effects, the lower boundary on the energy of the quinonoid intermediate, and the protonation states of the active site catalytic acids/bases (K39-epsilonNH2 and Y265-OH) suggest that the pKa of the substrate Calpha-H bond in the external aldimine lies between those of the two catalytic bases, such that the proton abstraction transition state is early in the D --> L direction and late in the L --> D direction.     

Synthesis, structure, and reactivity of a stabilized calcium carbene: R(2)CCa

Attempted 2-fold deprotonation of the bis(iminophosphorano)methane ligand, H(2)C(Ph(2)P=NSiMe(3))(2) (4-H(2)), with a calcium amide led only to mono-deprotonation. The crystal structure of (4-H)(2)Ca shows two tridentate ligands with short Ca-N and a rather long Ca-C bond. Reaction of 4-H(2) with a dibenzylcalcium complex gave the desired 2-fold deprotonation and formation of 4-Ca, which crystallized as a dimeric complex. Analysis of the calculated atomic and group charges in 4-H(2), (4-H)(2)Ca, and [4-Ca](2) showed that the negative charge at the imine nitrogens only slightly increases upon successive deprotonation of 4-H(2). The electron density at the central carbon, however, increases considerably: the charge on the carbene carbon in [4-Ca](2) is ca. -1.8. The negative charge in 4(2)(-) is therefore mainly located on the carbon. Reaction of [4-Ca](2) with benzophenone in benzene gave the remarkably stable adduct [4-Ca](2) x O=CPh(2), which was characterized by X-ray diffraction. Reaction of [4-Ca](2) with adamantylcyanide gave exclusive formation of the adduct [4-Ca](2) x (N identical withCR)(2), which did not react further, even at higher temperatures. Addition of cyclohexyl isocyanate to a benzene solution of [4-Ca](2) gave immediate [2 + 2]-cycloaddition and formation of a dianionic tetradentate ligand that binds to Ca(2+) through two nitrogens, the central carbon, and an oxygen. This product crystallized as a dimer with bridging oxygen atoms.     

Coupling of functional hydrogen bonds in pyridoxal-5'-phosphate-enzyme model systems observed by solid-state NMR spectroscopy

We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.     

The first NH aldimine organometallic compound. Isolation and crystal structure

The first NH aldimine organometallic derivative is unexpectedly formed by the cleavage of the nitrogen-carbon bond of the amino acid fragment of the Schiff base 2,4,6-Me3C6H2CH=NCH(CH2Ph)COOEt when the imine is treated with palladium acetate.     

The effect of the protein environment on the structure and charge distribution of the retinal Schiff base in bacteriorhodopsin

The effects of the amino acid side chains of the binding pocket of bacteriorhodopsin (bR) and of a water molecule on the structure of the retinal Schiff base have been studied using Becke3LYP/6-31G* level of density functional theory. A model protonated Schiff base structure including six conjugated double bonds and methyl substituents was optimized in the presence of several amino acid side chains and of a water molecule, separately. The Schiff base structure was also calculated in the form of a neutral species. At each optimized complex geometry the atomic charges of the model Schiff base were calculated using Mulliken population analysis. In agreement with previously proposed counterion(s) of the protonated retinal Schiff base in bR, the results show that Asp₈₅ and Asp₂₁₂, which are present in the form of negatively charged groups, have significantly large effects on the structure and electronic configuration of both unprotonated and protonated model Schiff bases. The presence of a water molecule in the vicinity of the Schiff base demonstrates significant effects which are comparable to those of aspartate groups. Other side chains studied did not show any significant effect in this direction. Apart from the aspartate groups and the water molecule, in none of the other complexes studied are the atomic charges and the bond alternation of the model Schiff base significantly influenced by the presence of the neighboring amino acids. Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 10 December 1998     

Deuterium isotope effects on 13C NMR chemical shifts of some α-(2-hydroxyaryl)-N-phenylnitrones (Schiff base N-oxides)

The deuterium isotope effect on the ¹³C NMR chemical shifts of some α-2-hydroxyaryl-N-phenylnitrones (Schiff base N-oxides) was studied. The existence of an intramolecular hydrogen bond with the proton localized on the phenolic oxygen atom was evidenced. Exceptionally large isotope effects ΔC-2(D) and ΔC-α(D) suggest that the substitution of the proton of the OH group by deuterium leads to a weakening of the hydrogen bond and some conformational changes in the molecule. This conclusion was drawn on the basis of a comparison of the deuterium isotope effects of Schiff base N-oxides and parent Schiff bases. Copyright © 2001 John Wiley & Sons, Ltd.     

Deamination of the amino acid fragment in imine metallacycles: unexpected synthesis of an NH-aldimine organometallic compound

We report that the action of Lewis bases, such as triphenylphosphine, pyridine, or trimethylamine, on imine metallacycles derived from amino acids leads to the formation of the first organometallic compound of an NH aldimine, a highly reactive organic species, and the corresponding alpha-ketoester, in a deamination reaction that mimics the metabolism of alpha-amino acids. The synthesis of different cyclopalladated compounds by a reaction between palladium acetate and the Schiff bases 2,4,6-Me(3)C(6)H(2)CH=NCH(R(1))COOR(2) (R(1) = CH(2)Ph, R(2) = Et and R(1) = Ph, R(2) = Me) is also reported.     

Supramolecular assemblies and molecular recognition of amphiphilic schiff bases with barbituric acid in organized molecular films

A bolaform Schiff base, N,N'-bis(salicylidene)-1,10-decanediamine (BSC10), has been synthesized and its interfacial hydrogen bond formation or molecular recognition with barbituric acid was investigated in comparison with that of a single chain Schiff base, 2-hydroxybenzaldehyde-octadecylamine (HBOA). It has been found that while HBOA formed a monolayer at the air/water interface, the bolaform Schiff base formed a multilayer film with ordered layer structure on water surface. When the Schiff bases were spread on the subphase containing barbituric acid, both of the Schiff bases could form hydrogen bonds with barbituric acid in situ in the spreading films. As a result, an increase of the molecular areas in the isotherms was observed. The in situ H-bonded films could be transferred onto solid substrates, and the transferred multilayer films were characterized by various methods such as UV-vis and FT-IR spectrosopies. Spectral changes were observed for the films deposited from the barbituric acid subphase, which supported the hydrogen bond formation between the Schiff bases and barbituric acid. By measuring the MS-TOF of the deposited films dissolved in CHCl3 solution, it was concluded that a 2:1 complex of HBOA with barbituric acid and a 1:2 complex of BSC10 with barbituric acid were formed. On the other hand, when the multilayer films of both Schiff bases were immersed in an aqueous solution of barbituric acid, a similar molecular recognition through the hydrogen bond occurred. A clear conformational change of the alkyl spacer in the bolaform Schiff base was observed during the complex formation with the barbituric acid.     

Synthesis and spectroscopic and electrochemical studies of chitosan Schiff base derivatives

Schiff derivatives were prepared by the reactions of salicylaldehyde and its derivatives (5-chloro, 5-methoxy, 5-fluoro, 5-methyl, 5-nitro) with the amino group of chitosan. The Schiff bases were studied by Fourier IR spectroscopy and by UV-visible spectroscopy. The cyclic voltammograms of the Schiff bases were analyzed and compared to those of chitosan and salicylaldehyde. The formal potential of the chitosan Schiff base derivative correlates with the Hammett parameters. The oxidation potential increases and the optical density decreases with enhancement of the electron-acceptor properties of the functional group R in the m-position to the -N=CH-group. Chitosan (Chi) is a polysaccharide whose chains consist of recurrent units of acetamido-2-deoxy-D-glucode linked by the 1,4-β-glycoside bond. This polysaccharide was widely studied as drug carrier [1, 2], because it is nontoxic, biodegradable, and well biocompatible [3].     

Selective conversion of C=N bonds to their corresponding carbonyl compounds by the tribromoisocyanuric acid/wet SiO2 system as a novel reagent

Abstract  

Tribromoisocyanuric acid/wet SiO₂ was used for the conversion of C=N bonds to their corresponding carbonyl compounds in oximes, semicarbazones, azines, and Schiff bases. The interesting feature of this system is that in those oximes, semicarbazones, azines, and Schiff bases which have conjugated or unconjugated C=C bonds, the C=N bond will selectively change to the relevant C=O bond while the conjugated or unconjugated C=C bond will remain intact.     

DMAP-catalyzed cyclization of Schiff bases with α-halo ketones: Synthesis of 1,4-benzoxazines

We demonstrate that 1,4-benzoxazines can be efficiently synthesized using 4-dimethylaminopyridine (DMAP) as a catalyst. We found that the DMAP catalyst can successfully promote the incorporation of the activated carbon atoms of α-halo ketones into the Schiff bases. The 1,4-benzoxazines can be synthesized through the formation of C–O and C–C bonds in high yield, using DMAP as the catalyst, α-halo ketones and phenolic Schiff bases as the reactants. Furthermore, a possible mechanism is proposed. This work provides a simple and efficient route to prepare the 1,4-benzoxazines.     

19F and 1H NMR of aldimines of fluoroamino acids with pyridoxal-5′-phosphate

¹H and ¹⁹F NMR spectra were obtained for six Schiff bases (aldimines) formed by pyridoxal-5′-phosphate (PLP) with four fluorinated or their two parent non-fluorinated α-amino acids (phenylalanine and α-aminobutyric acid). pK_A Values were derived from ¹H and ¹⁹F titration curves. The imine nitrogen of the aldimines is very basic (～13) and sensitive to the electron withdrawing effect of fluorine. The pyridine nitrogen is less basic in the aldimines (～7) than in PLP (8.12) and is less sensitive to the electron withdrawing effect of the fluorine than is the imine nitrogen. The phosphate group has a pK in the same range (～6) and the chemical shifts of some nuclei are sensitive to both pK values. Protonation of the aldimine causes the ¹H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high pK value (except for the aldimines prepared from the β-fluorophenylalanine), but upfield at the aromatic proton and at the aldehydic proton of the aldimine for the low pK. Protonation of the aldimine causes the ¹⁹F signal of an aryl fluorine to shift downfield but gives an upfield shift at a β-fluorine. These data are related to the highly conjugated electronic structure of the Schiff bases.     

低价钛试剂促进下2-(2-硝基苯基)咪唑与席夫碱的反应

在低价钛试剂作用下,以四氢呋喃为溶剂,研究了2-(2-硝基苯基)咪唑与席夫碱的分子间还原偶联反应.结果发现席夫碱中C-N键的断裂优于其与硝基的偶联成环,生成了5,6-二氢化咪唑并[1,2-c]喹唑啉.     

Metal ion inhibition of nonenzymatic pyridoxal phosphate catalyzed decarboxylation and transamination.

Nonenzymatic pyridoxal phosphate (PLP) catalyzed decarboxylations and transaminations have been revisited experimentally. Metal ions are known to catalyze a variety of PLP-dependent reactions in solution, including transamination. It is demonstrated here that the rate accelerations previously observed are due solely to enhancement of Schiff base formation under subsaturating conditions. A variety of metal ions were tested for their effects on the reactivity of the 2-methyl-2-aminomalonate Schiff bases. All were found to have either no effect or a small inhibitory one. The effects of Al(3+) were studied in detail with the Schiff bases of 2-methyl-2-aminomalonate, 2-aminoisobutyrate, alanine, and ethylamine. The decarboxylation of 2-methyl-2-aminomalonate is unaffected by metalation with Al(3+), while the decarboxylation of 2-aminoisobutyrate is inhibited 125-fold. The transamination reaction of ethylamine is 75-fold slower than that of alanine. Ethylamine transamination is inhibited 4-fold by Al(3+) metalation, while alanine transamination is inhibited only 1.3-fold. Metal ion inhibition of Schiff base reactivity suggests a simple explanation for the lack of known PLP dependent enzymes that make direct mechanistic use of metal ions. A comparison of enzyme catalyzed, PLP catalyzed, and uncatalyzed reactions shows that PLP dependent decarboxylases are among the best known biological rate enhancers: decarboxylation occurs 10(18)-fold faster on the enzyme surface than it does free in solution. PLP itself provides the lion's share of the catalytic efficiency of the holoenzyme: at pH 8, free PLP catalyzes 2-aminoisobutyrate decarboxylation by approximately 10(10)-fold, with the enzyme contributing an additional approximately 10(8)-fold.     

Hydrolysis of schiff bases, 1: Kinetics and mechanism of spontaneous,acid, and base hydrolysis of N-(2/4-hydroxybenzylidene)-2-aminobenzothiazoles

The rate of hydrolysis of title schiff bases was studied in the pH range 4–13 in 10% dioxanewater system, CTAB, NaLS micellar solutions. The hydrolysis was found to be due to ‘water’ and ‘hydroxide’ reactions and hence the rate constants in different pH were computed and analyzed with an intermediate involving a water molecule being positioned with the help of 2-OH group and the thiazolyl C–N pi bond. The ‘hydroxide’ reaction was due to a direct nucleophilic attack of hydroxide ion at the aldimine linkage of the reactant. The influence of cationic and anionic micelles supports the mechanistic picture in the entire pH range.     

Reversible-irreversible approach to Schiff base macrocycles: access to isomeric macrocycles with multiple salphen pockets

We have developed methodology for the formation of a new family of metal-free Schiff base macrocycles utilizing the differential exchange rates of aldimines and ketimines. The more robust ketimine bond is kinetically inert under the milder conditions used for aldimine bond formation. In particular, this route enables access to the first conjugated macrocycles with four unsymmetrical N2O2 salphen-like pockets.     

Neutral and ionic hydrogen bonding in Schiff bases

Low-temperature, high-resolution X-ray studies of charge distributions in the three Schiff bases, the dianil of 2-hydroxy-5-methylisophthaldehyde, 3,5-dinitro-N-salicylidenoethylamine and 3-nitro-N-salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N(+)-H...O- and neutral O-H...N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral O-H...N hydrogen bond is accompanied by an increase in the length of the C(1)-O(1) bond, opening of the ipso-C(1) angle, elongation of the aromatic C-C bonds, shortening of the C(7)-N(2) bond and increased length of the C(1)-C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral O-H...N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double-bond character in the C-O bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3-nitro-N-salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O...H interactions in a centrosymmetric dimer unit.