Showing papers on "Schiff base published in 1991"
TL;DR: D96 confers an overall kinetic advantage because when this residue becomes anionic in the N state its electric field near the cytoplasmic surface lowers the free energy barrier of the capture of a proton in the next step.
Abstract: During the M in equilibrium with N----BR reaction sequence in the bacteriorhodopsin photocycle, proton is exchanged between D96 and the Schiff base, and D96 is reprotonated from the cytoplasmic surface. We probed these and the other photocycle reactions with osmotically active solutes and perturbants and found that the M in equilibrium with N reaction is specifically inhibited by withdrawing water from the protein. The N----BR reaction in the wild-type protein and the direct reprotonation of the Schiff base from the cytoplasmic surface in the site-specific mutant D96N are much less affected. Thus, it appears that water is required inside the protein for reactions where a proton is separated from a buried electronegative group, but not for those where the rate-limiting step is the capture of a proton at the protein surface. In the wild type, the largest part of the barrier to Schiff base reprotonation is the enthalpy of separating the proton from D96, which amounts to about 40 kJ/mol. We suggest that in spite of this D96 confers an overall kinetic advantage because when this residue becomes anionic in the N state its electric field near the cytoplasmic surface lowers the free energy barrier of the capture of a proton in the next step. In the D96N protein, the barrier to the M----BR reaction is 20 kJ/mol higher than what would be expected from the rates of the M----N and N----BR partial reactions in the wild type, presumably because this mechanism is not available.
171 citations
TL;DR: The gene coding for bacteriorhodopsin was modified in vitro to replace Asp212 with asparagine and expressed in Halobacterium halobium and results are consistent with the existence of a complex counterion for the Schiff base proposed earlier and suggest that Asp85, Asp 212, and at least one other protonable residue participate in it.
150 citations
TL;DR: The lambda max values and solute anion dependencies of the Glu-113 mutants indicate that interactions between Schiff base and its counterion play a significant role in determining the lambda max of rhodopsin.
Abstract: Glu-113 serves as the retinylidene Schiff base counterion in bovine rhodopsin Purified mutant rhodopsin pigments were prepared in which Glu-113 was replaced individually by Gln (E113Q), Asp (E113D), Asn (E113N), or Ala (E113A) E113Q, E113N, and E113A existed as pH-dependent equilibrium mixtures of unprotonated and protonated Schiff base (PSB) forms The Schiff base pKa values determined by spectrophotometric titration were 600 (E113Q), 671 (E113N), and 570 (E113A) Thus, mutation of Glu-113 markedly reduced the Schiff base pKa The addition of NaCl promoted the formation of a PSB in E113Q and E113A An exogenously supplied solute anion replaced Glu-113 to compensate for the positive charge of the PSB in these mutants The lambda max values of the PSB forms of the mutants in NaCl were 496 nm (E113Q), 506 nm (E113A), 510 nm (E113D), and 520 nm (E113N) To evaluate the effect of different types of solute anions on lambda max values, mutants were prepared in sodium salts of halides, perchlorate, and a series of carboxylic acids of various sizes and acidity The lambda max values of E113Q and E113A depended on the solute anion present and ranged from 488 nm to 522 nm for E113Q and from 486 nm to 528 nm for E113A The solute anion affected the lambda max values of E113N and E113D to lesser degrees The reactivities of the mutants to hydroxylamine were also studied Whereas rhodopsin was stable to hydroxylamine in the dark, E113N reacted slowly and E113Q reacted rapidly under these conditions, indicating structural differences in the Schiff base environments The lambda max values and solute anion dependencies of the Glu-113 mutants indicate that interactions between Schiff base and its counterion play a significant role in determining the lambda max of rhodopsin
132 citations
TL;DR: The pigment formed from K296G and the n-propylamine Schiff base of 11-cis-retinal was found to activate transducin in a light-dependent manner, with 30 to 40% of the specific activity measured for the wild-type protein.
Abstract: Rhodopsin and the visual pigments are a distinct group within the family of G-protein-linked receptors in that they have a covalently bound ligand, the 11-cis-retinal chromophore, whereas all of the other receptors bind their agonists through noncovalent interactions. The retinal chromophore in rhodopsin is bound by means of a protonated Schiff base linkage to the epsilon-amino group of Lys-296. Two rhodopsin mutants have been constructed, K296G and K296A, in which the covalent linkage to the chromophore is removed. Both mutants form a pigment with an absorption spectrum close to that of the wild type when reconstituted with the Schiff base of an n-alkylamine and 11-cis-retinal. In addition, the pigment formed from K296G and the n-propylamine Schiff base of 11-cis-retinal was found to activate transducin in a light-dependent manner, with 30 to 40% of the specific activity measured for the wild-type protein. It appears that the covalent bond is not essential for binding of the chromophore or for catalytic activation of transducin.
116 citations
100 citations
75 citations
TL;DR: The complexes formed between titanium tetraalkoxide and chiral Schiff's bases make excellent catalysts for enantioselective trimethylsilylcyanation of aldehydes to optically active cyanohydrins in high optical yield as discussed by the authors.
Abstract: The complexes formed between titanium tetraalkoxide and chiral Schiff's bases make excellent catalysts for enantioselective trimethylsilylcyanation of aldehydes to optically active cyanohydrins in high optical yield.
75 citations
TL;DR: Asp-85 is the retinylidene Schiff base counterion in wild-type bacteriorhodopsin, and exogenous anions serve as surrogate counterions to the protonated Schiff base, when the intrinsic counterions have been neutralized by mutation or by protonation.
72 citations
TL;DR: In this article, the Schiff base ligand was used to obtain the 13C NMR, absorption, and circular dichroism spectra, and conductivity measurements of Titanium(IV) complexes, [TiCl(Schiff base)]2O.
Abstract: Titanium(IV) complexes, [TiCl(Schiff base)]2O, with quadridentate Schiff base ligands derived from optically active 1,2-diamines and salicylaldehyde were prepared by treating TiCl4 with an equimolar amount of the Schiff base ligand in wet pyridine. The complexes were characterized by their 13C NMR, absorption, and circular dichroism spectra, and conductivity measurements. The complexes in dichloromethane take a μ-oxo dinuclear structure, and in methanol the complexes break up into a mononuclear species, [Ti(OMe)(MeOH)(Schiff base)]Cl. The complexes catalyze the asymmetric oxidation of methyl phenyl sulfide into the sulfoxide with organic hydroperoxides; the highest enantiomeric excess (ee) was ca. 60%. The kinetics conform to a rate law which is first order in the concentration of each sulfide, hydroperoxide, and complex catalyst.
69 citations
62 citations
TL;DR: In this paper, a nuclear copper(II) complexes of ligands derived from condensation of 2,6-diformyl-4-methylphenol with various aromatic mono-and diamines have been synthesized.
TL;DR: The energy barrier to proton transfer reinforces the ability of changes in angle and dielectric coupling to induce a protontransfer.
TL;DR: In this paper, Dicopper(II) complexes of unsymmetrical dinucleating Schiff-base ligands bearing chemically distinct co-ordination sites have been prepared and their properties studied.
Abstract: Dicopper(II) complexes of unsymmetrical dinucleating Schiff-base ligands bearing chemically distinct co-ordination sites have been prepared and their properties studied. The ligands were derived from the condensation of 4-bromo-2-formyl-6-(4-methylpiperazin-1-ylmethyl)phenol and 4-bromo-2-[(2-diethylaminoethyl)ethylaminomethyl]-6-formylphenol with 2-(aminomethyl)pyridine, 2-(2-aminoethyl)pyridine, and 2-(aminomethyl)-4-nitrophenol.
TL;DR: An acylic dipeptide ester, 2-hydroxy-1-naphthylideneimino-(S)-valyl-(S-trp-OMe), whose terminal amino group is modified to a Schiff base, is designed as a chiral auxiliary of an asymmetric catalyst.
TL;DR: A series of homo-dinuclear lanthanide(III) complexes of the type: Ln2(LA)(X)2·.n(H2O) (H4LA and H4LB are the Schiff bases obtained by condensation of 3-formylsalicylic acid and 4-N-dodecyldiethylenetriamine or 1,5-diamino-3-azapentane, respectively; X=NO3−, Cl−; n= 3−8;) has been prepared and characterized RE
TL;DR: In this article, the crystal structure of ReOL2 was determined by single crystal X-ray methods and the structure has been refined by full-matrix least-squares methods to R=0.061.
TL;DR: A Schiff base metal complex with the molecular formula CuC17O13N2H27P has been obtained by magnetic, spectral, and X-ray structural studies as mentioned in this paper, which reveals pi-pi interaction of the aromatic side chain of the amino acid with the pyridoxal pi system.
Abstract: A Schiff base metal complex, [Cu(II)(PLP-DL-tyrosinato)(H2O)].4H2O (PLP = pyridoxal phosphate), with the molecular formula CuC17O13N2H27P has been prepared and characterized by magnetic, spectral, and X-ray structural studies. The compound crystallizes in the triclinic space group P1BAR with a = 8.616 (2) angstrom, b = 11.843 (3) angstrom, c = 12.177 (3) angstrom, alpha = 103.40 (2)degrees, beta = 112.32 (2)degrees, gamma = 76.50 (1)degrees, and Z = 2. The structure was solved by the heavy-atom method and refined by least-squares techniques to a final R value of 0.057 for 3132 independent reflections. The coordination geometry around Cu(II) is distorted square pyramidal with phenolic oxygen, imino nitrogen, and carboxylate oxygen from the Schiff base ligand and water oxygen as basal donor atoms. The axial site is occupied by a phosphate oxygen from a neighboring molecule, thus resulting in a one-dimensional polymer. The structure reveals pi-pi interaction of the aromatic side chain of the amino acid with the pyridoxal pi system. A comparative study is made of this complex with similar Schiff base complexes. The variable-temperature magnetic behavior of this compound shows a weak antiferromagnetic interaction.
TL;DR: In this article, the X-ray crystal structure of the dimeric Schiff's base ligand N,N′-bis(salicylidene)ethylenediaminato was reported.
Abstract: Visible light irradiation of [{Mn(L)(H2O)}2](ClO4)2·4H2O, which contains the tetradentate unsymmetric Schiff's dianionic ligand 4-[2-(2-hydroxyphenylmethyleneamino)ethylamino]pent-3-en-2-one, results in the formation of the dimeric [{Mn(salen)(H2O)}2](ClO4)2·H2O, which contains the symmetrical dianionic Schiff's base ligand N,N′-bis(salicylidene)ethylenediaminato; the X-ray crystal structure of the latter complex is reported.
TL;DR: The chromophores of all visual pigments, the rhodopsins, is the 11-cis form of retinal and its analogs bound covalently to opsin, the receptor protein through a protonated Schiff base bond.
Abstract: The chromophores of all visual pigments, the rhodopsins, is the 11-cis form of retinal and its analogs bound covalently to opsin, the receptor protein through a protonated Schiff base bond. Similarly, the pigments present in Halobacterium halobium, namely, bacteriorhodopsin (proton pump), halorhodopsin (chloride pump), and the two sensory rhodopsins (phototaxis receptor) contain the all-trans isomer of retinal bound to the apoprotein by a protonated Schiff base linkage. The chromophores are indeed uniquely designed to perform such vital functions. Why was retinal chosen as the chromophore of pigments, why is the chromophore a protonated Schiff base, and why 1 1-cis in some cases and all-trans in other cases?
TL;DR: In this article, the exact configurations of the chiral centres in the triphenylphosphine adduct have been established by an X-ray diffraction study [HL star, (S)-alpha-methylbenzylsalicylaldimine].
01 Jan 1991-International Journal of Radiation Applications and Instrumentation. Part A. Applied Radiation and Isotopes
TL;DR: The NCA asymmetric synthesis of l -6-[18F]fluorodopa starting from (1R,2R,5R)-[(+)-2-hydroxypinanyl-3-idene]glycine t-butyl ester as chiral agent has been developed as discussed by the authors.
TL;DR: In this article, an oximate-bridged binuclear chromium-III-copper-II complex (salen)Cr(pdmg)Cu]ClO41 and a trinuclearchromium(III)-copper(II)-chromium (III) complex [{(salenCr}2(dmg)2Cu]2 have been prepared and characterized.
Abstract: An oximate-bridged binuclear chromium(III)–copper(II) complex [(salen)Cr(pdmg)Cu]ClO41 and a trinuclear chromium(III)–copper(II)–chromium(III) complex [{(salen)Cr}2(dmg)2Cu]2 have been prepared and characterized [salen2–=N,N′-ethylenebis(salicylideneiminate), pdmg2–= 3,9-dimethyl-4,8-diazaundeca-3,8-diene-2,10-dione dioximate and dmg2–= dimethylglyoximate]. Magnetic susceptibility measurements in the temperature range 4.2–300 K demonstrated the operation of a ferromagnetic interaction between the adjacent chromium(III) and copper(II) ions through oximate bridges in both 1 and 2. Based on spin Hamiltonians ℋ=–2JŜ1·Ŝ2(S1=, S2=½) and ℋ=–2J(Ŝ1·Ŝ2+Ŝ2·Ŝ3)(S1=S3=, S2=½), the exchange integrals (J) were evaluated as +12.6 and +13.0 cm–1 for 1 and 2, respectively.
TL;DR: In this article, a diastereoselective alkylation of a chiral Schiff base was used to synthesize homoserine derivatives, with the key step being the diastreoselectively alkylating of a Schiff base.
Abstract: Enantiomerically pure phosphonic analogues of homoserine derivatives are synthesized the key-step being the diastereoselective alkylation of a chiral Schiff base.
TL;DR: Tetranuclear and binuclear manganese(III) complexes with 1,5-bis(salicylideneamino)-3-pentanol (H3L), [Mn4(L)2(O) 2(CH3COO)2]·4H2O·2CH3OH and [mn2(L)(CH3O)(NCO)2
Abstract: Tetranuclear and binuclear manganese(III) complexes with 1,5-bis(salicylideneamino)-3-pentanol (H3L), [Mn4(L)2(O)2(CH3COO)2]·4H2O·2CH3OH and [Mn2(L)(CH3O)(NCO)2(H2O)2], have been prepared and characterized by X-ray structure analysis and magnetic susceptibilities (80–300 K).
TL;DR: In this article, the cyclic Schiff base was used to synthesize (2S, 3S, 4S)-3, 4-dihydroxyproline (1) and the new (2R, 3s, 4s)-isomer (7) from L-tartaric acid via cyanosilylation of the Schiff base.
Abstract: Natural (2S, 3S, 4S)-3, 4-dihydroxyproline (1) and the new (2R, 3S, 4S)-isomer (7) have been synthesized from L-tartaric acid via cyanosilylation of the cyclic Schiff base.
TL;DR: It is found that the above-mentioned Schiff bases in aqueous solutions of pH 7 adopted a conformation in which the pyridine ring is embedded in a very hydrophobic medium from which water is virtually completely excluded.
Abstract: We recorded the absorption spectra of the Schiff bases of pyridoxal 5'-phosphate (PLP) and 5'-deoxypyridoxal (DPL) with dodecylamine (DOD) at different pH values. By applying deconvolution techniques to the spectra and analysing their different components we found that the above-mentioned Schiff bases in aqueous solutions of pH 7 adopted a conformation in which the pyridine ring is embedded in a very hydrophobic medium from which water is virtually completely excluded. This conformation in the same as that adopted by PLP when it acts as coenzyme for some enzymes such as glycogen phosphorylase. The experimental results obtained also show such a conformation to be highly favoured but sensitive to the protonation of the pyridine nitrogen, which makes the aromatic ring more readily accessible to the solvent.
TL;DR: In this article, a series of neutral chelate compounds [Pd 2 L(AB) 2 ] has been obtained by reacting the precursor with sodium salts of acetylacetone, ethylacetoacetate, N-methylsalicylaldimine (HAB), and N,N′-dimethyldithiocarbamic acid (Hab).
TL;DR: Some complexes of thorium(IV) and dioxouranium(VI) with the Schiff base of 2-hydroxy-1-naphthaldehyde condensed with 4-aminoantipyrine (HNAAP) have been synthesized and characterized by elemental analyses, conductivity, molecular weight, magnetism and IR spectra.