Model Compound Data Research Articles

Determination of the Relative Configurations in the Side Chains of the Antibiotics Hedamycin and Pluramycin A; Synthesis and NMR. Data of Suitable Model Compounds

AbstractSeveral diepoxyhexanoates and epoxyhexenoates were prepared by epoxidation of methyl 2‐methyl‐2,4‐hexadienoate or by Darzens condensation. Their 1 H‐ and 13C‐NMR. spectra were measured and assigned. Comparison of these data with the spectra of the antibiotics hedamycin (1) and pluramycin A (2) allowed the determi‐nation of the relative configurations in the side chains of these antibiotics. They were found to be (14R*,16S*,17R*,18S*) for hedamycin (1) and (14R*,16S*,17Z) for pluramycin A (2).

Infrared spectra and conformation of substituted 3-benzylidene-2,4-pentanediones

Fundamental CO and CC stretching frequencies and carbonyl first overtones of substituted 3-benzylidene-2,4-pentanediones were measured in CCl 4 and CHCl 3. The data are correlated with substituent constants and carbonyl stretching frequencies of corresponding substituted acetophenones. By investigating the effect of substituents, solvents, temperature and anharmonicity as well as the i.r. data of model compounds, the absorption bands were assigned to the vibrations of structural groups. Evidence was obtained that 3-benzylidene-2,4-pentanediones in liquid or solid state and in organic solvents exist in two quasi planar conformations I and II.

Vapor pressures of some sulfur-containing, coal-related compounds

The need to find alternative energy sources has spawned intensive research and development of processes for coal IC quifaction and gasification. Almost all processes, including those involving coal, expend large amounts of capital on separation. Reliable thermodynamic data are needed for efficient design of coal-processing and separation equipment. Vapor pressure is an important physical property required to perform calculations for the design of separation equipment. I t is desireable to calculate vapor pressures of all components from general correlations rather than to measure new data for each new situation. To generate these correlations, however, we require experimental data for model compounds which resemble molecules that might be found in typical coals and coal products. Coal is of complex structure containing large aromatic rings, some of which contain heteroatoms such as N, 0, and S. Few data are available for these types of compounds, partly because vapor pressures of hlgh-molecular-weight compounds may be too low for measurement by conventional means. Sinke ( 7) and Macknick et al. (2) have shown that a gas-saturation method is suitable for measuring vapor pressures of high-molecular-weight hydrocarbons. Van de Rostyne et al. (3) extended the use of this method to highmolecular-weight compounds containing nitrogen. This work reports vapor pressures of high-molecular-weight, coakelated compounds containing sulfur.

The Conformation of Bilin Chromophores in Biliproteins: Ramachandran-Type Calculations

Ramachandran-type calculations are performed for conformations of bilin chromophores pres­ent in the biliproteins phycocyanin, phycoerythrin and phytochrome. The atomic coordinates are taken from x-ray data of crystalline model compounds, namely biliverdin for pyrrole rings B, C, D and substituted succinimides for the hydrogenated ring A including a thioether containing β-sub­stituent. Maxima and minima for steric hindrance are calculated for rotation of the thioether side chain, the rotation of pyrrole rings at single bonds (syn-anti-forms) and at double bonds (Z-E-isomers) of the methine bridges. Whereas quasi-planar structures are possible for all syn, Z-forms, only twisted structures are possible if anti, E-forms are considered. The relevance for the bilin con­formations of native biliproteins and of the Pr ⇌ Pfr phototransformation is discussed.

NMR Studies of Lignins. 3. 1H NMR Spectroscopic Data for Lignin Model Compounds.

NMR Studies of Lignins. 3. 1H NMR Spectroscopic Data for Lignin Model Compounds.

Improved Cocure of EPDM-Polydiene Blends by Conversion of EPDM into Macromolecular Cure Retarder

Abstract The 1,2-addition of certain N-chlorothioamides to the olefinic sites of EPDM produces modified polymers which exhibit improved covulcanization in blends with polydiene rubbers. A mechanism is postulated in which the modified EPDM retards the cure of the polydiene phase and at the same time is itself activated toward crosslinking. Model compound data are presented to support these views.

Urea and Guanidine Hydrochloride Denaturation of Ribonuclease, Lysozyme, α-Chymotrypsin, and b-Lactoglobulin

Abstract The unfolding of ribonuclease, lysozyme, α-chymotrypsin, and goat β-lactoglobulin by urea and guanidine hydrochloride (GmHCl) has been followed with the use of optical rotation measurements. Urea denaturation leads to a more negative rotation for each protein than does GmHCl denaturation, but the concentration dependence is such that the rotations are almost identical in the absence of denaturant. This indicates that both urea and GmHCl denaturation lead to a similar, randomly coiled conformation. By assuming a two-state mechanism, an apparent free energy of unfolding, ΔGapp, has been calculated as a function of denaturant concentration. ΔGapp varies linearly with denaturant concentration. The dependence of ΔGapp on concentration ranges from 1.1 to 2.1 Cal per mole per molar concentration for urea and from 1.9 to 4.1 Cal per mole per molar concentration for GmHCl for the four proteins. These values depend on the size and composition of the part of the polypeptide chain which is freshly exposed to denaturant by unfolding. By using model compound data and a procedure developed by Tanford (Advan. Protein Chem. (1970) 24, 1–95), an estimate of the amount of buried polypeptide chain needed to account for the experimental results has been obtained. The estimates based on urea and on GmHCl are in excellent agreement. This offers further support for the idea that the extent of unfolding is the same for the two denaturants and also suggests that the mechanism of unfolding is the same in the presence of the two denaturants. The relative effectiveness of the two denaturants depends on the protein. GmHCl is 2.8 times more effective than urea in unfolding ribonuclease but only 1.7 times more effective for lysozyme. This results because the buried polypeptide chain is more polar for ribonuclease than for lysozyme and the solubilizing ability of GmHCl relative to urea is greater for polar groups than for nonpolar groups. Estimates of ΔGapp in the absence of denaturant, ΔGh2oapp, were obtained from the analysis with the use of model compound data and also from a direct linear extrapolation of ΔGapp to zero concentration of denaturant. For urea, the two approaches lead to values of ΔGh2oapp in reasonably good agreement. For GmHCl, only the values of ΔGh2oapp from the direct extrapolation are in good agreement with the results from urea. The ΔGh2oapp values obtained are 9.7 ± 1.7 Cal per mole for ribonuclease at pH 6.6, 6.1 ± 0.4 for lysozyme at pH 2.9, 8.3 ± 0.4 for α-chymotrypsin at pH 4.3, and 11.7 ± 0.8 for β-lactoglobulin at pH 3.2. These results suggest that several recent estimates of ΔGh2oapp are too high by 3 to 10 Cal per mole.

NMR studies of structure and conformation in some highly substituted cyclohexenes. II. 13C NMR studies

13C NMR spectra of the phenyl and α-naphthyl analogs of 3-aryl-1,3,5,5-tetramethylcyclohex-l-ene and -6-ene were obtained and analyzed. Values of all the carbon chemical shifts as well as all the one-bond CH couplings are given. The stereochemically dependent substituent effects on 13C shifts are discussed on the basis of previously deduced conformations of these olefins. Model compound data are presented for both the aliphatic and aromatic parts of these arylcyclohexenes.

Solvent perturbation studies and analysis of protein and model compound data in denaturing organic solvents

For the study of proteins in the denaturing organic alcohols, methanol, ethanol, 1-propanol, trifluoroethanol, and 2-chloroethanol, and the assessment of the number or fraction of exposed tyrosyl and tryptophyl residues in these solvents by the solvent perturbation method of difference spectroscopy, the molar absorbance difference values, Δ ϵλ of the N-acetyl ethyl esters of tyrosine and tryptophan have been obtained with 20% dimethylsulfoxide as a perturbant, and are presented in the 350-260 nm region. Solvent perturbation data obtained on apomyoglobin, serum albumin, and ovalbumin in several of these solvents are presented and analyzed for the purpose of illustration of the computation and curve-fitting techniques used, employing the tabulated model compound data of this paper.

Synthesis of new optically active α-alkoxypropionanilides

Two promising routes towards synthesis of the hitherto unknown α-alkoxypropionanilides were explored with success. As expected, facile displacement of the bromide in l(−)α-bromopropionanilide by alkoxide lead to inversion of configuration with considerable racemization, whereas substitution of the chloride in l(−) α-alkoxypropionyl chloride by arylamine proceeded with little or no racemization. α-Ethoxypropionanilide and the higher alkoxy homologues appear as low-melting noncrystallizable gum. Optical properties and other physical data for few model compounds of this new anilide series are reported.

Interaction of Heavy Meromyosin with Substrate<subtitle>IV. Studies on the Tyrosine and Tryptophan Residues by Solvent Perturbation Method<xref ref-type="fn" rid="fn1"><sup>*</sup></xref></subtitle>

1. Solvent perturbation difference spectra of N-acetyltyrosine ethyl ester and N-acetyltryptophan-amide due to 20% ethylene glycol, 20% glycerol, 10% polyethylene glycol, and 20% sucrose were obtained in the presence of 100 mM Tris-HC1 (pH 8), 0.6M KC1, and 10 mM MgCl2 at 25°C. 2. Under the same conditions solvent perturbation difference spectra of heavy meromyosin and subfragment-1 were obtained. 3. Fraction of accessible tyrosyl and tryptophanyl residues in each protein was determined using the model compound data. For example, 69% tyrosine and 40% tryptophan in heavy meromyosin, and 66% tyrosine and 47% tryptophan in subfragment-1 were accessible to glycerol in native state. 4. No detectable change in the solvent perturbation difference spectra between the free protein and the complexed protein with ATP was observed both with heavy meromyosin and with subfragment-1. Considering the accuracy and the number of accessible residues in proteins, it is estimated that 4 tyrosyl and 1 tryptophanyl residues are buried into the interior of the protein molecule per one site of ES-complex accompanying the UV spectral change.

Physical-Chemical Studies on the Pyridoxal Phosphate Binding Site in Sodium Borohydride-reduced and Native Phosphorylase

Abstract NaBH4-reduced phosphorylase b is a catalytically active species which retains many of the properties of the native enzyme. One property that is strikingly different, however, is the spectral behavior of bound pyridoxal phosphate. At pH 7.0, the pyridoxal phosphate shows essentially no absorption at 333 nm, where pyridoxal phsophate in the native enzyme absorbs; absorption at 333 nm returns upon lowering the pH. The 333 nm band at low pH in the reduced enzyme was found to be optically active, (ΔA/A) 333 nm ≅ 0.7 x 10-3. Circular dichroism of the reduced phosphorylase b at pH 7.0 indicated that no new band was formed above 300 nm; the ultraviolet spectrum also provided no evidence for a band above 300 nm. Guanidine·HCl (4.25 m) and sodium dodecyl sulfate (0.3%) caused the appearance of a band near 330 nm at pH 6.8. Difference spectra of reduced phosphorylase b (±sodium dodecyl sulfate; pH 5 versus pH 7) indicated that the hidden band must be below 300 nm. AMP (0.01 m) was shown to cause partial reversal of the hidden band to 333 nm conversion at pH 6.0; ATP was also effective. Substrates and AMP had no effect on the spectrum of the reduced phosphorylase b at pH 6.8. Ultracentrifugal studies showed that native and NaBH4-reduced enzyme have different conformations. The data were interpreted to mean that the 333 nm and hidden band forms of NaBH4-reduced phosphorylase represent different enzyme conformations. The hidden band form is thought to represent a form of pyridoxal phosphate in which the pyridinium nitrogen is unprotonated and the 3-hydroxyl group undissociated or strongly hydrogen bonded; the 333 nm species is believed to be a dipolar form of pyridoxal phosphate which is exposed to solvent. Spectral properties of enzyme-bound pyridoxal phosphate in native phosphorylase were compared to NaBH4-reduced enzyme, and it was suggested that pyridoxal phosphate exists in these two forms at pH 7.0 in the same microscopic environment. Circular dichroism of native phosphorylase b at 422 nm was observed. The presence of phosphate (0.01 m) and AMP (0.001 m) did not perturb the observed long wave length circular dichroism. Solvent perturbation studies showed that enzyme-bound pyridoxal phosphate in native phosphorylase is not available to ethylene glycol. From our results, evaluation of model compound data, and fluorescence results, a new structure was proposed for the 333 nm form for native phosphorylase. This species is thought to be a neutral tautomeric imine of pyridoxal phosphate.

220 MHz spectra of heparin, chondroitins, and other mucopolysaccharides

Characteristics of proton magnetic resonance spectra of heparins at 220 MHz, coupled with spectral data for model compounds and with information obtained chemically, indicate that heparins are composed principally of (1 → 4)-linked α-L-idopyranosyluronic acid residues (3) and 2-amino-2-deoxy-α-D-glucopyranosyl residues (1); also, that sulfate groups are located at position -2 and -6 of 1, and -2 of 3. D-Glucopyranosyluronic acid residues (2) appear to be minor constituents, which is at variance with all structures that have been proposed previously for heparin.Spectra of chondroitins A, B, and C, which are readily distinguishable, are in close accord with formulae that have been proposed for these mucopolysaccharides; on the same basis, the spectrum of keratan sulfate is less consistent. Hyaluronic acid affords a poorly resolved though, nonetheless, distinctive spectrum. The general view that mucopolysaccharides are constituted basically of residues of hexosamine and uronic acid in equimolar proportion receives support from the current findings. However, heparitin appears to possess a more heterogeneous type of structure than do these other polymers.

Spectroscopic Studies of the Structures of the Dimer of Diethylaluminum 2-Aminoethoxide (AlEt2OCH2CH2NH2)2 and Related Compounds

Abstract The infrared and proton nuclear magnetic resonance spectra of the solutions of diethylaluminum 2-aminoethoxide (AlEt2OCH2CH2NH2)2 and related compounds were measured. The infrared frequencies and intensities of the NH2 stretching, scissors, and rocking vibration bands, and the NMR internal chemical shifts of the ethyl groups attached to the aluminum atom were compared with the corresponding data of suitable model compounds. It was concluded by taking the results of these experiments into consideration that (AlEt2OCH2CH2NH2)2 was represented by the structural formula I with two Al←NH2 coordination bonds. Similarly, most of the related compounds studied were reasonably represented by the formulas with two Al←NRR′ coordination bonds, while (AlEt2OCH2CH2N(C6H11)2)2 was assumed to be represented by formula III with two Al←O coordination bonds, exceptionally.

Studies of the location of tyrosyl and tryptophanyl residues in proteins. I. Solvent perturbation data of model compounds

Studies of the location of tyrosyl and tryptophanyl residues in proteins. I. Solvent perturbation data of model compounds

Extractives of Mammea americana L. Part II. The 4-phenylcoumarins. Isolation and structure of Mammea A/AA, A/A cyclo D, A/BA, A/AB, and A/BB

In addition to the four 4-n-alkyl coumarins considered in Part I, five 4-phenyl-5,7-dioxygenated coumarins have been isolated from Mammea americana L. seeds. Two of them, A/AA and A/A cyclo D, are shown to be identical with mammeisin and mammeigin, respectively. The three new isomeric coumarins A/BA, A/AB, and A/BB are shown, by a combination of spectral data (mass, n.m.r., infrared, and ultraviolet), to possess 3-methylbut-2-enyl groups at position 6 (A/BA and A/BB) or 8 (A/AB) and to have a 3-methylbutyryl (A/BA) or a 2-methylbutyryl (A/AB and A/BB) group in the remaining phloroglucinol position. Spectral data for model compounds, synthesised to assist solution of the orientation problem, are described, and evidence is given supporting the structural assignment for mesuol. 2-Methoxyxanthone and the triterpene friedelin were also isolated.