Showing papers in "Bulletin of the Chemical Society of Japan in 1982"

Synthesis, Luminescence Quantum Yields, and Lifetimes of Trischelated Ruthenium(II) Mixed-ligand Complexes Including 3,3′-Dimethyl-2,2′-bipyridyl

Abstract: New five complexes of the type of [RuL3−x(dmby)x]X2(x=1,2,3, L=2,2′-bipyridyl or 1,10-phenanthroline, dmby=3,3′-dimethyl-2,2′-bipyridyl, X=halide ion) have been synthesized in order to investigate the effects of two methyl groups of dmby on the absorption and emission spectra, luminescence quantum yields, and lifetimes. Values of the radiative and nonradiative rate constants have been calculated from these data at 77 K. Although the absorption and emission maxima and the lifetimes are not much affected by the dmby ligand substitution, the molar extinction coefficients and emission quantum yields are decreased compared with trischelated complexes of the parent bipyridyl or phenanthroline ligands. At 25 °C the emission yields of the complexes containing dmby decrease by 3–4 orders of magnitude than at 77 K. Possible causes of the decrease in the quantum yields are discussed.

Palladium-catalyzed New Carbon-Phosphorus Bond Formation

Abstract: Dialkyl arylphosphonates are prepared by the palladium-catalyzed reaction of aryl bromides with dialkyl phosphite in the presence of triethylamine The similar treatment of vinyl bromides gives dialkyl vinylphosphonates stereoselectively

Catalysis by Heteropoly Compounds. III. The Structure and Properties of 12-Heteropolyacids of Molybdenum and Tungsten (H3PMo12−xWxO40) and Their Salts Pertinent to Heterogeneous Catalysis

Abstract: The fundamental properties of 12-heteropolyacids of Mo and W, such as the structure, the surface area, the acidity, and the reduction-oxidation mechanism, have been studied in their solid state in relation to heterogeneous catalysis The primary structure (heteropolyanion) was rather stable, but the secondary structure was very variable according to the interaction with various molecules, like water, alcohols, ketones, and pyridine This structural characteristic is important for the understanding of the catalysis of these compounds; it may be called the “pseudo-liquid phase” The heteropolyacids were all strong protonic acids, as revealed by the interaction with pyridine The redox cycle was reversible within a limited extent of reduction The redox mechanism is also discussed in this paper

Association of Inclusion Compounds of β-Cyclodextrin in Aqueous Solution

Abstract: Inclusion compounds in the systems naphthalene–β-cyclodextrin (CDx) and 2-methoxynaphthalene (MN)–o-dicyanobenzene (DB)–β-cyclodextrin in aqueous solution were studied by means of absorption and fluorescence spectra. In the former system a naphthalene excimer fluorescence was observed. By the analyses of the concentration dependence of the absorption spectra and the excimer fluorescence intensities, the species responsible for the excimer fluorescence is identified as a 2:2 inclusion compound which is formed by association of two 1:1 naphthalene–CDx inclusion compounds, not a 2:1 inclusion compound in which two naphthalene molecules enter into the cavity of one CDx molecule. In the latter system, absorption and fluorescence spectra due to a charge transfer complex of MN with DB were observed. By analyses similar to the naphthalene case, the existence of a 1:1:2 association compound formed between a MN–CDx inclusion compound and a DB–CDx inclusion compound is confirmed. Fluorescence quenching by I− and IO3...

Highly Selective Carbon-Carbon Bond Forming Reactions Mediated by Chromium(II) Reagents

Abstract: A low valent chromium reagent is generated from chromium(III) chloride and a half mol of lithium aluminum hydride in tetrahydrofuran. The reagent behaves similarly to anhydrous chromium(II) chloride, which is commercially available, and reduces allylic halides to produce unisolable allylchromium species which add efficiently to aldehydes or ketones with high degree of stereo- and chemoselectivity. Particularly, high threo selectivity is observed in the reaction of aldehydes and 1-bromo-2-butene and is ascribed to a chair-like six-membered transition state. Simple reduction of allylic and benzylic halides produces biallyls and bibenzyls, while gem-dibromocyclopropanes afford the corresponding allenes in excellent yields.

Heterogeneous Photocatalytic Reactions on Semiconductor Materials. III. Effect of pH and Cu2+ Ions on the Photo-Fenton Type Reaction

Abstract: The effect of pH and Cu2+ ions on the heterogeneous photocatalytic oxidation of toluenes by H2O2 formed from dissolved O2 in the presence of illuminated TiO2 powders, i.e. “the photo-Fenton type reaction,” was investigated. At low and high pH, a total amount of products increased drastically compared with that of the additive free system (pH 7). In the acidic region (aqueous H2SO4), side-chain oxidation prevailed over cresol formation and benzaldehyde was formed quite selectively at pH 1. Oxidation of the side chain in preference to hydroxylation of aromatic ring was also observed in the alkaline region (aqueous NaOH). By adding Cu2+ ion to the aqueous H2SO4 (pH 1 and 2), the yield of benzaldehyde increased further and cresols, benzyl alcohol, and bibenzyl were formed newly in high yields. At high Cu2+ concentrations (pH 2), the cresol formation in preference to the side-chain oxidation was attained. The observation was in good agreement with the Fenton reaction reported in which Cu2+ and Fe3+ ions are ad...

Electrolytic Reduction of Carbon Dioxide at Mercury Electrode in Aqueous Solution

Abstract: Cathodic reduction of carbon dioxide at a mercury pool electrode was investigated in aqueous solutions of NaHCO3, NaH2PO4–Na2HPO4, NaCl, NaClO4, Na2SO4, LiHCO3, and KHCO3 as well as their combinations. The reaction product was confirmed to be HCOO− by a qualitative spot test. HCOO− was quantitatively analyzed by a permanganate titration. The partial current densities of HCOO− formation (ic) and H2 evolution (iH) were calculated. In the present paper, relationships between electrode behavior and solution properties, some of which have been qualitatively described in previous papers, are quantitatively clarified as follows. The electrode potential Ec for constant ic (0.5 mA/cm2) remains constant irrespective of pH and anions in the pH region of 2.4 to 7.8, whereas the potential EH for constant iH (0.5 mA/cm2) varies linearly with a slope of 125 mV/pH. The present experimental results verify Eyring et al.’s presumption that in a relatively high overvoltage region (region 2) the cathodic reduction of CO2 at H...

Reaction process of zirconium tetrachloride with ammonia in the vapor phase and properties of the zirconium nitride formed.

Abstract: On donne les produits de reaction obtenus aux diverses temperatures. Mecanisme: il y a d'abord formation de ZrCl 4 •2NH 3 , puis ZrClN. Au-dessus de 1000°C, ZrN x est forme

Fluoride Salts on Alumina as Reagents for Alkylation of Phenols and Alcohols

Abstract: The effectiveness of alkali metal fluorides impregnated on alumina as a reagent for promoting alkylation was optimized with respect to the metal cation, the amount of impregnation, and the reaction solvent. Potassium or caesium fluoride on alumina in acetonitrile or 1,2-dimethoxyethane was concluded to be the best reaction system for general use. O-Alkylation of substituted phenols, primary and secondary alcohols, and a glycol was carried out mostly in good yields under mild conditions with simple experimental procedures.

On the Ratio of Zwitterion Form to Uncharged Form of Glycine at Equilibrium in Various Aqueous Media

Abstract: The ratios of the zwitterion form to the uncharged form of glycine, KD=[NH3+CH2COO−]/[NH2CH2COOH], were determined in mixtures of water with methanol, ethanol, t-butyl alcohol, dimethyl sulfoxide, acetonitrile, 1,4-dioxane, N,N-dimethylformamide, and tetrahydrofuran. The value of In KD decreases and, therefore, ΔG298° increases, with the increase in the mole fractions of the organic solvents. The plots of ΔG° vs. reciprocal of the dielectric constants of the alcoholic mixtures are simply linear indifferent to the kind of alcohols, indicating that this relation obeys the Scatchard equation and that the free energy change of the process of the intramolecular protonation in glycine is controlled mainly by electrostatic interaction, especially by the entropy change associated with the preferential solvation around the charged amino and carboxyl groups.

Catalytic Properties of Perovskite-type Mixed Oxides, La1−xSrxCoO3

Abstract: The reactivity and related properties of oxygen both in the bulk and on the surface has been investigated for perovskite-type mixed oxides (La1−xSrxCoO3), in regard to the effects of Sr2+-substitution and calcination temperature. The reducibility and the readiness of oxygen desorption increased with the Sr2+ content, x, but the re-oxidation became slower with x. These results have been explained on the basis of the change in the chemical potential of lattice oxygen. The diffusivity of oxygen in the bulk and the ability to activate the oxygen molecule also increased with x. Oxygen vacancies in the bulk and on the surface, which tended to increase with x, are likely responsible for these reactions. The release of oxygen from the bulk became more difficult as the calcination temperature increased, in conformity with the trend of the catalytic activity.

Solubility Measurements of Benzene and the Alkylbenzenes in Water by Making Use of Solute Vapor

Abstract: Aqueous solubilities of benzene and of alkylbenzenes, toluene, ethylbenzene, propyl- and isopropylbenzene, (o-, m-, and p-) xylene, and (1,2,3-, 1,2,4-, and 1,3,5-) trimethylbenzene, have been measured. An apparatus is described which provides a convenient means of obtaining aqueous solutions saturated with solute vapor. The method involves introducing solute vapor, which is in equilibrium with a liquid solute, into water and circulating it in a closed system. This method has three principal advantages: (1) The solubility equilibrium can be attained within 3 min, (2) it can be determined whether the solubility obeys Henry’s law, and (3) based on Henry’s law, solutions of any desired concentrations of solute at a given temperature of water can be easily prepared.

Selective Dimerization of Aldehydes to Esters Catalyzed by Hydridoruthenium Complexes

Abstract: RuH2(PPh3)4 and other hydridoruthenium complexes catalyze selective conversion of aldehydes into esters in high yields. The method is applicable to most aliphatic aldehydes as well as to aromatic aldehydes. The purity of aldehydes is critical for achieving high conversions, since the presence of carboxylic acid completely inhibits the reaction and alcohol and triphenylphosphine reduce the yields of esters. RuH2(PPh3)4 is converted into Ru(CO)3(PPh3)2 through the reaction indicating the occurrence of decarbonylation of aldehyde. A mechanism involving the acyl-H cleavage of aldehyde is proposed to account for the catalysis and formation of compounds accompanying the reaction. The mechanism is compared with an alternative one which comprises of consecutive insertions of two aldehyde molecules into Ru–H bond followed by β-hydrogen abstraction from an alkoxo intermediate formed. Addition of water changes the reaction course to give carboxylato carbonyl complexes Ru(OCOR)2(CO)m(PPh3)2 (m=1 and 2). Cross esterif...

Correlation between critical solution phenomena and ultralow interfacial tensions in a surfactant/water/oil system.

Abstract: A three-phase region consisting of surfactant (D), water (W), and oil (O) phases appeared due to the superposition of two critical phenomena of D–W and O–D in a nonionic surfactant/water/oil system. Three interfacial tensions, γO–W, γO–D and γD–W, among the three phases were determined at various temperatures. The generalized Antonoff’s rule (γO–W=γO–D+γD–W) holds approximately in the three-phase region. γO–D and γD–W disappear at the respective critical solution temperatures, and ultralow interfacial tensions are attained in the vicinity of both critical temperatures. The oil–water interfacial tension, γO–W, reaches its minimum in the midst of the temperature range of the three-phase region, and the narrower the temperature range of the three-phase region, the lower the interfacial tension. The correlation between the interfacial tensions and the types of emulsions is also discussed.

Solvent Effect on the Nonradiative Deactivation of the Excited State of Tris(2,2′-bipyridyl)ruthenium(II) Ion

Abstract: Emission quantum yields and lifetimes of the [Ru(bpy)3]2+ ion were measured in various solvents at 25 °C. Nonradiative rate constants (knr) were calculated from these data. A plot of knr vs. the Gutmann’s acceptor number was found to be linear. This result can be explained in terms of the contributions of charge transfer to solvent (CTTS).

The Preparation of Optically Pure 3-Hydroxyalkanoic Acid. The Enantioface-differentiating Hydrogenation of the C=O Double Bond with Modified Raney Nickel. XXXVII.

Abstract: The enantioface-differentiating hydrogenation of methyl 3-oxoalkanoate (CH3(CH2)nCOCH2COOCH3, n=0, 6, 8, 10, 12) over the (R,R)-tartaric acid–NaBr–modified Raney nickel catalyst ((R,R)-TA–NaBr–MRNi) gave methyl (R)-3-hydroxyalkanoate (CH3(CH2)nCH(OH)CH2COOCH3, n=0, 6, 8, 10, 12) in an average optical yield of 85%. After the methyl ester had been converted to dicyclohexylammonium salt of 3-hydroxyalkanoic acid, the salt was recrystallized three times from acetonitrile and then treated with acid to give optically pure (R)-3-hydroxyalkanoic acid (CH3(CH2)nCH(OH)CH2COOH, n=0, 6, 8, 10, 12) in a reasonable yield. From the hydrogenation product with (S,S)-TA–NaBr–MRNi, optically pure (S)-3-hydroxyalkanoic acid was obtained by the same process as above.

The Enantioselective Michael Addition of Thiols to Cycloalkenones by Using (2S, 4S)-2-Anilinomethyl-1-ethyl-4-hydroxypyrrolidine as Chiral Catalyst

Abstract: Catalytic asymmetric addition of thiols to 2-cycloalkenone was studied by using the chiral amino alcohols, derived from L-hydroxyproline or (S)-proline, as base catalysts. Detailed investigation was carried out on the effects of the structure of the catalyst, the reaction medium, the temperature, and the concentration on the enantioselectivity. Good optical yields (47–88%) were achieved by the reaction of arenethiols and 2-cyclohexen-1-one in toluene at −5 °C, by using the catalyst (2S, 4S)-2-anilinomethyl-1-ethyl-4-hydroxypyrrolidine.

The Effect of Polyhydric Alcohols on the Thermal Denaturation of Lysozyme as Measured by Differential Scanning Calorimetry

Abstract: The thermal denaturation of lysozyme in aqueous solutions of polyhydric alcohols at pH 2 was investigated by differential scanning calorimetry (DSC). The polyhydric alcohols employed were inositol, sorbitol, xylitol, erythritol, glycerol, and ethylene glycol. These alcohols tended to enhance the thermal stability of lysozyme; both the temperature and the enthalpy of denaturation increased almost linearly with increasing alcohol concentration. The increase in denaturation temperature at a given alcohol concentration was a linearly increasing function of the number of hydroxyl groups per alcohol molecule. The denaturation enthalpy at identical temperature increased only slightly by the addition of polyhydric alcohol. The hypothesis may be supported that the dominant mechanism by which polyhydric alcohols stabilize proteins to thermal denaturation is through their effect on the water structure, which determines the strength of the hydrophobic interactions between the nonpolar groups of the protein.

The Dehydrogenation of Cyclohexane by the Use of a Porous-glass Reactor

Abstract: The dehydrogenation of cyclohexane was studied by the use of a double tubular reactor; the inner tube was made of porous glass, and the outer tube, of non-porous glass. The Pt/Al2O3 catalyst was placed inside the inner tube. It was demonstrated that dehydrogenation proceeded beyond the equilibrium conversion to be expected in the absence of a porous tube, by separating the hydrogen produced from the central reaction zone through the porous tube. When the reaction rate was so high that the reactant gas reached equilibrium near the inlet of the reactor, the dehydrogenation conversion became greater (more than twice the equilibrium conversion) as the flow rate in the inner path decreased and the flow rate in the outer path increased. The experimental data obtained by using the porous reactor were well reproduced by a computer simulation based on a simple model, in which such parameters as the flow rate, the gas permeability, and the reaction rate, were considered. The potential application of the present por...

Direct Observation of the Rate for Cis→Trans and Trans→Cis Photoisomerization of Stilbene with Picosecond Laser Photolysis

Abstract: The rates for direct cis→trans and trans→cis photoisomerizations of stilbene in solution were measured with a newly developed monitor light source for picosecond laser photolysis in the UV region. The rate constant of the trans→cis isomerization is 1.2×1010 s−1 in hexane at 295 K. The fluorescence decay rate constant of the S1 state of trans-stilbene has the same value. Thus, the decay of the S1 state is the rate limiting step and the lifetime of an intermediate perpendicular state (1p) has to be very short. The cis→trans isomerization is very fast and the half life of the excited cis-isomer is less than a few picosecond. These observations strongly indicate that the isomerization reaction proceeds through the potential surface of the singlet excited states.

Oxidation of Lipids. I. Quantitative Determination of the Oxidation of Methyl Linoleate and Methyl Linolenate

Abstract: The extent of oxidation of methyl linoleate and methyl linolenate has been measured quantitatively by several methods such as oxygen uptake, substrate disappearance, and formation of conjugated diene, TBA-reactive material and of hydrocarbon gas. In the early stage of oxidation of methyl linoleate, oxygen uptake, substrate disappearance, peroxide and conjugated diene formation were all quantitative and their results agreed well each other. Little TBA-reactive material and hydrocarbons were observed. In the oxidation of methyl linolenate, however, no product was formed quantitatively, and the extent of oxidation estimated decreased in the order, oxygen uptake>substrate disappearance>peroxide>conjugated diene>TBA-reactive material.

Photocatalytic dehydrogenation of 2-propanol over TiO2 and metal/TiO2 powders.

Abstract: A series of TiO2 catalysts loaded with transition metals were prepared by photodeposition The activity order for these catalysts in the title reaction was as follows: Pt/TiO2>Rh/TiO2>Pd/TiO2>Ru/TiO2>Ir/TiO2>TiO2 Upon heat treatment the activities of all the metal/TiO2 catalysts were enhanced, but the above sequence was unaltered

Optical Resolution of 2-Amino-1,2-diphenylethanol by Preferential Crystallization and Its Utilization in Fractional Crystallization and Enantioselective Reduction of Prochiral Ketones

Abstract: (±)-erythro-2-Amino-1,2-diphenylethanol prepared from benzoin oxime by catalytic reduction was successfully resolved into a pair of optically active forms by preferential crystallization. The optically active amino alcohol was found to be useful as a basic resolving agent for the optical resolution of tartaric acid, trans-2,3-oxiranedicarboxylic acid, 2-hydroxy-2-phenylpropionic acid, and 3-endo-benzamido-5-norbornene-2-endo-carboxylic acid. Chiral hydrides prepared from lithium aluminium hydride and optically active threo- or erythro-2-amino-1,2-diphenylethanol derivatives were applied to the enantioface differentiating reduction of prochiral ketones to give the corresponding optically active alcohols in 26–72% optical purities.

Studies on 2-Aziridinecarboxylic Acid. VI. Synthesis of β-Alkoxy-α-Amino Acids via Ring-opening Reaction of Aziridine

Abstract: The reaction of aziridine derivatives having a urethane-type protecting group with several alcohols in the presence of boron trifluoride etherate afford the corresponding optically pure O-alkylserine and O-alkylthreonine derivatives via a ring-opening reaction of aziridine in good yield.

The Crystal and Molecular Structure of Staurosporine, a New Alkaloid from a Streptomyces Strain

Abstract: The molecular sturcture of staurosporine, an alkaloid from a Streptomyces strain, has been determined by means of an X-ray crystallographic analysis of its methanol solvate (C28H26N4O3·CH3OH). The crystals are monoclinic, with four formula units in a unit cell with dimensions of a=23.487(6), b=7.636(3), c=15.638(4) A, and β=116.71(5)°; the space group is C2. 2532 unique intensity data were collected on a four-circle diffractometer with LiF-monochromated Cu Kα radiation. The structure was solved by the Monte Carlo direct method, using the 20 strongest reflections as the starting set; the 72nd random phase set led to the correct solution. The R value reached 0.047 by block-diagonal least-squares refinements. Staurosporine has been found to contain a unique dimeric indole structure. Because of the steric hindrance, the indolo[2,3-a]carbazole aromatic system in the molecule is slightly bent; the two terminal benzene rings make an angle of 7.5° with each other.

Interaction between Hydrophilic Proteins and Nonionic Detergents Studied by Surface Tension Measurements

Abstract: The surface tensions of the systems of hexa(oxyethylene) dodecyl ether (C12E6) vs. lysozyme (Lyz) or bovine serum albumin (BSA) were measured for constructing the binding isotherms, since the surface tension method is more convenient for the systems of nonionic detergents and proteins than is equilibrium dialysis. The obtained binding isotherms depend on the protein concentration; this effect is much more remarkable for Lyz–C12E6 systems than for BSA–C12E6 ones. The binding isotherms of BSA–C12E6 systems obey the Scatchard equation, and the maximum number and estimated free energy for C12E6 binding to BSA are comparable to those for the system of Triton X-100 and BSA determined by equilibrium dialysis. The agreement suggests that the surface tension method is sound and has advantages over equilibrium dialysis for the estimation of the number of detergent molecules bound to protein. In the systems of Lyz–C12E6, the binding isotherms follow the Freundlich-type equation rather than the Scatchard equation. Th...

Synthesis of 1,1'-, 2,2'-, 1,2'-, and 2,6'-biazulenes and their derivatives by Ullmann reaction.

Abstract: Four kinds of biazulenes, 1,1′- (1a), 2,2′- (2a), 1,2′- (3a), and 2,6′-biazulenes (4a), and their derivatives were synthesized by utilizing Ullmann-type coupling of haloazulene derivatives. Ullmann reaction of ethyl 3-iodo-(5a) and 2-iodoazulene-1-carboxylates (6b) gave diethyl 1,1′-biazulene-3,3′-dicarboxylate (1b) and diethyl 2,2′-biazulene-1,1′-dicarboxylate (2b), respectively, in excellent yields. The reaction of 2-iodoazulene gave 2a directly. Diethyl 2-chloroazulene-1,3-dicarboxy late (7a) and its 5-alkyl derivatives also reacted to give tetraethyl 2,2′-biazulene-1,1′,3,3′-tetracarboxylate (2d) and its 5,5′-dialkyl derivatives, respectively. A mixed Ullmann reaction of 5a and 6b afforded a mixture of 1b, 2b, and diethyl 1,2′-biazulene-1′,3-dicarboxylate (3b). Similarly, a mixed Ullmann reaction of 7a and diethyl 6-iodoazulene-1,3-dicarboxylate gave a mixture of 2d and tetraethyl 2,6′-biazulene-1,1′,3,3′-tetracarboxylate (4b). The parent hydrocarbons of biazulenes, 1a, 2a, 3a, and 4a, could be derive...

Mechanisms of enzymatic S-oxygenation of thioanisole derivatives and O-demethylation of anisole derivatives promoted by both microsomes and a reconstituted system with purified cytochrome P-450.

Abstract: Oxygenations of thioanisole derivatives have been shown to be promoted with phenobarbital induced rabbit liver microsomes. In order to understand the mechanistic details of the oxygenations, a kinetic study on the S-oxygenation promoted by a reconstituted system with purified cytochrome P-450 has been carried out. Log(Vmax) were correlated with the one electron oxidation potentials of the sulfides (Ep). The oxygenation is considered to proceed via one electron transfer from the sulfides to the iron-bound “oxenoid” intermediate of the enzyme. A similar O-demethylation of anisoles has been investigated. In the oxidative O-demethylation of p-methoxy-d3-anisole with rabbit liver microsomes, an intramolecular kinetic isotope effect of 3.4 was observed. Meanwhile, a large isotope effect of 5.1 was observed in the competitive O-demethylation of p-dimethoxybenzene and p-di(methoxy-d3) benzene. In order to clarify the oxidation pathway of O-demethylation with the enzymatic system of cytochrome P-450, the rates of ...

Direct synthesis of cyclic carbonates in the presence of organometallic compounds. Catalyses by systems from IVA, VA, and VIA group compounds and Lewis base.

Abstract: In the cycloaddition of carbon dioxide to oxiranes to form cyclic carbonates in the presence of organometallic halides of group IVA, VA, and VIA, catalytic activities of organometallic halides are in the order Sb>Te>Sn>>Bi, Ge, and Si. An improvement of their catalytic activities is achieved by addition of an equimolar amount of Lewis base. In binary catalyst systems, the order of catalytic activities is in the order Sn>Te>Sb>>Bi, Ge, and Si. Organotin halide–base systems show the highest catalytic activity.

Synthesis of threo-3-Methylcysteine from Threonine

Abstract: threo-3-Methyl-D-cysteine as a moiety of β-methyllanthionine in the peptide antibiotic nisin was synthesized stereospecifically. The reaction of (2R,3R)-3-methyl-2-aziridinecarboxylic acid derivative prepared from D-threonine with thiobenzoic acid gave the β-mercapto α-amino acid derivative of the same configuration as the starting material. A stereochemistry of the product was retained as a result of double inversion mechanism through the reactions. Thus, we could prepare threo-3-methyl-D-cysteine, i.e., (2S,3S)-2-amino-3-mercaptobutanoic acid which is required for the synthetic study of nisin.