Showing papers on "Schiff base published in 1982"
323 citations
TL;DR: It is suggested that a light-induced pKa change of at least nine units takes place during the photocycle of light-adapted bR, implying that the Schiff base is titratable in the dark, but its accessibility to external OH- ions is limited.
Abstract: Aqueous suspensions of dark-adapted bacteriorhodopsin (bR560) in the purple membrane of Halobacterium halobium are exposed to rapid jumps to high pH. Optical and resonance Raman measurements are carried out by using flow and stationary methods. Above pH congruent to 11.5 bR560 starts to be reversibly converted to a species absorbing at 460 nm (bR460) characterized by an unprotonated Schiff base chromophore. Above pH congruent to 13.0 bleaching takes place, first reversibly and subsequently irreversibly, to a species absorbing around 365 nm (bR365). This process competes with the formation of bR460. The pKa corresponding to the equilibrium (equation in text) is determined as 13.3 +/- 0.3. The value of the corresponding association rate constant determined from the reverse jumps (from pH 12.67 to pH 10 and 9.2) is ka = (3.5 +/- 0.5) X 10(11) M-1 s-1. Thus, starting with bR at pH 12.67 the reprotonation process is diffusion controlled as observed for homogeneous acid-base equilibria. The observed rate of dissociation when jumping from pH 6.5 to 12-13 is slower than that predicted by including the equilibrium (equation in text) The results imply that the Schiff base is titratable in the dark, but its accessibility to external OH- ions is limited. The limitations in the significance of the "apparent" value of pKa = 13.3 observed for the Schiff base titration are discussed in light of possible alterations in the structure of bR resulting from the parallel titration of other protein groups. It is suggested that a light-induced pKa change of at least nine units takes place during the photocycle of light-adapted bR.
143 citations
TL;DR: FTIR difference spectral changes in the bR570-to-K transition clearly indicate that bR 570 contains a protonated Schiff base, which is consistent with a simple model of the first step in the photocycle which involves a movement of the Schiff base proton away from a counterion.
Abstract: It is possible, by using Fourier-transform infrared (FTIR) difference spectroscopy, to detect the conformational changes occurring in both the protein and the chromophore of bacteriorhodopsin during the photocycle. In contrast to Raman spectroscopy, a laser is unnecessary and hence the problem of a perturbing probe beam is eliminated. Furthermore, the relatively high signal-to-noise ratio obtainable with FTIR enables measurements to be made in minutes over a large spectral range. In the study reported in this paper, we used this method to examine the state of protonation of the retinylidene Schiff base in light-adapted bR570 and in K, the first intermediate in the photocycle. Resonance Raman spectroscopy provides evidence that bR570 is protonated, but these results have been questioned on the basis of theoretical and experimental grounds. FTIR difference spectral changes in the bR570-to-K transition clearly indicate that bR570 contains a protonated Schiff base. In contrast, the K intermediate displays a Schiff base that is altered but still is associated to some degree with a proton. Because the low-temperature FTIR difference spectrum of bR570 and K is similar to the recently reported low-temperature resonance Raman spectra of bR570 and K [Braiman, M. & Mathies, R. (1982) Proc. Natl. Acad. Sci. USA 79, 403-407], we can assign most, but not all, vibrational changes in the bR570-to-K transition to the chromophore. These results are consistent with a simple model of the first step in the photocycle which involves a movement of the Schiff base proton away from a counterion.
108 citations
83 citations
65 citations
TL;DR: A simple method for the elimination of labile glycohemoglobin in the chromatographic quantitation of glycosylated hemoglobin is described, made of the instability of Schiff base adducts in acidic solution.
Abstract: A simple method for the elimination of labile glycohemoglobin in the chromatographic quantitation of glycosylated hemoglobin is described. Use is made of the instability of Schiff base adducts in acidic solution. Erythrocytes are lysed with a pH 5 buffer. At this pH dissociation reaches completion during sample preparation.
51 citations
48 citations
TL;DR: In this article, the α-methyl amino acids, p-chlorophenylalanine and DOPA have been prepared in good yields from amino ester hydrochlorides using catalytic phase-transfer alkylation of Schiff base derivatives of mono alkyl amino acids.
47 citations
TL;DR: Schiff bases of p- and m-aminostyrenes with perfume aldehydes such as citral, cinnamaldehyde, piperonal, vanillin, and ethyl vanillin were synthesized in ethanol in more than 50% yield.
Abstract: Schiff bases of p- and m-aminostyrenes with perfume aldehydes such as citral, cinnamaldehyde, piperonal, vanillin, and ethyl vanillin were synthesized in ethanol in more than 50% yield. Water-soluble copolymers of these Schiff bases with N-vinyl-2-pyrrolidone or with N,N-dimethylacrylamide were obtained. The hydrolytic behavior of Schiff base monomers and copolymers to liberate perfume aldehydes was structure dependent, thereby affording chemical release control.
43 citations
38 citations
TL;DR: In this article, the pyridoxylidene-DL-valinato (DL-V-copper) compound is reported with a polymeric structure with square-pyramidal co-ordination to copper involving five donor atoms.
TL;DR: The Sciff base derived from glycine ethyl ester and p-chlorobenzaldehyde can be alkylated by the ion-pair extraction method as well as under catalytic liquid-liquid or solid-liquid phase transfer conditions as mentioned in this paper.
TL;DR: In this paper, resonance Raman spectra of the retinylidene Schiff base chromophore were obtained from aqueous suspensions of the purple membrane of Halobacteria and various hydrogen bending modes were identified.
Abstract: Bacteriorhodopsin the protein of the purple membrane of Halobacteria, involves a retinylidene Schiff base as its photochemically active chromophore. Resonance Raman spectra, which selectively probe the retinylidene Schiff base chromophore, were obtained from aqueous suspensions of the purple membrane. In the unphotolysed chromophore Bacteriorhodopsin, the Schiff base nitrogen carries a proton whose reversible release during a photo-induced cyclic process is essential for the ability of the purple membrane as a proton pump. It is expected that during this process the Schiff base group undergoes conformational and/or configurational changes, the elucidation of which is important for an understanding of the proton pump mechanism. We have looked for resonance Raman bands characteristic of the terminal group =C14HC15H=NH+- of the retinylidene Schiff base chromophore in BR570. Vibrational modes could be assigned on the basis of deuterium exchange at C15 and replacement of the Schiff baee proton by a deuteron. Resonance Raman spectra of BR603, a chromophore with an additional double bond in the retinal ring (3-dehydroretinal) but an identical terminal group, were also used for this analysis. Various types of hydrogen bending modes could be identified. From resonance Raman theory it was concluded that the intensities and frequencies of such modes reflect the conformation (bending and twisting angles) of the terminal group. A comparison of the resonance Raman spectra of BR570 (all-trans) and the 13-cis component BR548 from the dark-adapted purple membrane revealed that the conformation of the protonated Schiff base is significantly different in these two chromophores. This is ascribed to variable interactions with the protein. The relation between resonance Raman spectroscopic data and the conformational structure of the terminal group is discussed on the basis of a simple model. From the results of this paper it is suggested that important structural information from resonance Raman spectroscopic data can also be obtained for retinylidene Schiff base chromophores in intermediates of the photochemical cycle.
TL;DR: In this article, cyclic voltammetry of two NiII macrocyclic complexes [NiIIL]2+ in acetonitrile each shows two reversible one-electron reduction waves near 1E½=−1.0 V and 2E½ = −1.55 V.
Abstract: Reductive cyclic voltammetry of two NiII macrocyclic complexes [NiIIL]2+ in acetonitrile each shows two reversible one-electron reduction waves near 1E½=–1.0 V and 2E½=–1.55 V. Electrochemical reduction by controlled-potential electrolysis at these potentials and investigation of the reduced products by e.s.r. spectroscopy show the two reduction waves to correspond to the formation of [NiII(L˙–)]+ and [NiI(L˙–)]0 respectively. In the presence of axial ligands X = P(OMe)3 PPh3, or CO, cyclic voltammetry of [NiIIL]2+ shows the primary reduction wave to shift to lower negative potentials, the magnitude of 1E½ increasing in the order P(OMe)3 < CO < PPh3 < no ligand added; the magnitude of 2E½ was found to decrease along this series. Controlled-potential electrolysis of [NiIIL]2+ at the primary reduction potential in the presence of these ligands leads to the formation of the metal-reduced nickel(I) complexes, [NiILX]+. Likewise, addition of these ligands to solutions of [NiII(L˙–)]+ leads to the stabilisation of the same nickel(I) products. An intramolecular macrocyclic ligand-to-metal electron transfer is thought to be involved in the latter process and in the case of CO this electron transfer is found to be reversible. Reduction of [NiIIL]2+ at the secondary reduction potential in the absence or in the presence of CO, PPh3, or P(OMe)3 leads to the formation of nickel(I) ligand radical species; no nickel(0) products could be isolated.
TL;DR: Several sulfonamide-Schiff base complexes of selenium and tellurium were prepared and characterized using conductivity measurements, and infrared and proton magnetic resonance spectra as discussed by the authors.
TL;DR: In this article, the relationship between the macrocycle cavity size and the nature and mode of attachment of the bridging substrate is discussed, and the structure of a tetranuclear Cu(I) cluster containing phenylacetylide is described.
TL;DR: Amino acid Schiff base copper complexes of either salicylaldehyde or 2-hydroxy-1-naphthaldehyde with 1 -serine, 1 -histidine, histamine and 1 -tryptophan have been prepared by constituent combination.
TL;DR: Some new organotin(IV) complexes of Schiff bases containing sulfur and fluorine were tested for their activity on axenically grown Entamoeba histolytica trophozoites and one compound, tri‐n‐butyltin (2‐fluorobcnzaldehyde‐S‐benzyldithiocarba‐zate), showed remarkable activity at an extremely low dose even after 48 h.
Abstract: Some new organotin(IV) complexes of Schiff bases containing sulfur and fluorine were tested for their activity on axenically grown Entamoeba histolytica trophozoites (strain NIH‐20). Some of the complexes were found to be potentially active at low doses, and one compound, tri‐n‐butyltin (2‐fluorobcnzaldehyde‐S‐benzyldithiocarba‐zate), showed remarkable activity at an extremely low dose even after 48 h.
TL;DR: In this paper, the potentially heptadentate trianionic Schiff base ligands derived from tris(2-aminoethyl)amine and 3 mol of (substituted) salicylaldehyde react with lanthanide metal nitrates to form neutral 1:1 compounds of formula Ln(X-Saltren).
Abstract: The potentially heptadentate trianionic Schiff base ligands derived from tris(2-aminoethyl)amine and 3 mol of (substituted) salicylaldehyde react with lanthanide metal nitrates to form neutral 1:1 compounds of formula Ln(X-Saltren). Physicochemical measurements are described for the following complexes: Ln = La, Pr, Nd, Eu, Gd, Dy, Ho, Yb, X = 3,5-Cl2; Ln = La, X = 5-Cl and 5-NO2; and the analogous complexes, M = Sc, Y, Co, X = 3,5-Cl2. Discussion of the spectroscopic data focuses on far infrared ν(Ln—O) and ν(Ln—N) bands, 1H nmr pseudocontact shifts and hypersensitive f–f transitions.
TL;DR: In this paper, the salen-type Schiff base (SB) and 1-l-menthyloxy-3-benzoylacetone (H(l-moba) were synthesized, where H2salen stands for N,N′-disalicylideneethylenediamine.
Abstract: Cobalt(III) mixed-chelates, Co(SB)(l-moba), containing a salen-type Schiff base (SB) and 1-l-menthyloxy-3-benzoylacetone (H(l-moba)) have been synthesized, where H2salen stands for N,N′-disalicylideneethylenediamine. These complexes were shown to have a cis-β-octahedral structure, based on electronic and infrared spectra. They showed circular dichroism in the visible region, demonstrating an induction of an asymmetry around the metal ion. On the basis of NMR and CD spectra and in comparison with the CD spectrum of Λ-Co(sal-(S, S)-ch)(bazc), it was shown that the Co(SB) (l-moba) complexes prefer the Λ-cis-β1-configuration. The reaction of racemic N,N′-disalicylidene-trans-1,2-cyclohexanediaminatocobalt(II) (Co(salch)) and racemic N,N′-disalicylidene-1,2-propanediaminatocobalt(II) (Co(salpn)) with H(l-moba) in a 2 : 1 mole ratio yielded Λ-cis-β1-Co(sal-(S, S)-ch)(l-moba) and Λ-cis-β1-Co(sal-(R)-pn)(l-moba), respectively, as amain species. The stereoselectivity in the present complexes has been discussed in ...
TL;DR: Two series of methyl and nonmethyl cobalt complexes involving closely related quadridentate Schiff bases have been investigated in this paper, where the use of nonsymmetrical or substituted ligands allows the reduction potential of the cobalt centre to be gradually modified.
Abstract: Two series of methyl and nonmethyl cobalt complexes involving closely related quadridentate Schiff bases have been investigated. The use of nonsymmetrical or substituted ligands allows the reduction potential of the cobalt centre to be gradually modified. The data thus obtained are considered in connection with the ability of the cobalt complexes to alkylate tin (IV).
TL;DR: In this paper, the properties of binuclear copper(II) complexes, such as 1,2,3,4-tetrakis(R-salicylideneamino), 2,3-dimethylbutane (abbreviated as H4(Rsata), 1.2-bis, 1.5-me, 5-me and 1.6-tablet base ligands, have been synthesized.
Abstract: New Schiff base ligands, 1,2,3,4-tetrakis(R-salicylideneamino)-2,3-dimethylbutane (abbreviated as H4(R-sata); R (substituent on the ring)=H, 5-Me, 5-But) and 1,2-bis(R-salicylideneamino)-1,2-bis(salicylideneaminomethyl)cyclohexane (H4(R-sacta)), which are capable of forming “strati-bis” type binuclear copper(II) complexes, have been synthesized. Magnetic, spectral and electrochemical properties of the binuclear copper(II) complexes, Cu2(R-sata) and Cu2(R-sacta), were investigated in comparison with those of mononuclear copper(II) complexes Cu(R-salpn) and Cu(R-sacda), where H2(R-salpn) is N,N′-bis(R-salicylidene)-1,2-propanediamine and H2(R-sacda) 1-(R-salicylideneamino)-1-(R-salicylideneaminomethyl)cyclohexane. ESR spectra of Cu2(sata), Cu2(sacta) and Cu2(5-Me-sacta) showed a seven-lined hyperfine structure whose hyperfine coupling constant (A||≈85 G, 1 G=10−4 T) was about one half of that of the corresponding mononuclear copper(II) complex. Each electronic spectrum of these binuclear complexes showed th...
TL;DR: In this paper, the influence of equatorial ligand on the electrochemical oxidation of the compounds [H3CCo(chel)B], where chel is bis (dimethylglyoximato), (DH)2; bis(salicylaldehyde)ethylenediimine, salen; bis (saliclyaldehyde) o-phenylenedimine (O-PEN), salophen; bis((salicaldehyde)cyclohexylenediIMine,salcn); bis(acetylacetone) ethylenediamine,
TL;DR: The product from the reaction of 2,6-diacetylpyridine with o-phenylenediamine contains one 12-membered and two 7membered inner rings as mentioned in this paper.
Abstract: The product from the reaction of 2,6-diacetylpyridine with o-phenylenediamine contains one 12-membered and two 7-membered inner rings and not a single 18-membered inner ring as proposed by others.
TL;DR: In this article, new dioxomolybdenum(VI) complexes MoO2L · H2O (LH2=Schiff base) derived from Salicylhydrazide and salicylaldehyde were synthesized and characterized.
Abstract: New dioxomolybdenum(VI) complexes MoO2L · H2O (LH2=Schiff base) derived from Salicylhydrazide and salicylaldehyde, 5-chloro-, 5-bromo-, 5-nitro-, 3-methoxy, 3-ethoxy-, 3,5-dichlorosalicylaldehyde and 2-hydroxy-1-naphthaldehyde have been synthesized and characterized, together with the MoO2L · THF adducts. The complexes are monomers, nonelectrolytes and diamagnetic, and contain acis O=Mo=O
Abstract: Ten new chelating complexes of the type R2SnL, where R represents methyl and phenyl groups and L is a dibasic tetradentate Schiff base, have been synthesized by the reaction of diorganotin dichloride and the Schiff bases in the presence of triethylamine The complexes have been characterized by elemental analyses and spectral data