Carbon-13 Magnetic Resonance Spectroscopy Research Articles

Physico-chemical and structural characterization of hemicelluloses from wheat straw by alkaline peroxide extraction

The treatment of wheat straw with 2% H2O2 at 50°C and pH 11.5 for 4–30h or with 2% H2O2–0.05% anthraquinone at 50°C and pH 11.5 for 4.5h resulted in the release of 79–86% of the original lignin and 77–91% of the original hemicelluloses, which contained 3.8–6.5% of associated lignins. Xylose was the major sugar constituent in all the solubilized hemicellulosic fractions, and arabinose, glucose, and galactose were present in small amounts. The isolated seven hemicellulosic samples were further characterized by Fourier transform infrared, and carbon-13 magnetic resonance spectroscopy as well as gel permeation chromatography. Comparison of these hemicellulosic fractions with those obtained by alkali extraction from wheat straw in the absence of hydrogen peroxide provided evidence of similar chemical composition and structure. The treatment by alkaline peroxide under the conditions used did not result in any significant change in the macromolecular structure of hemicelluloses.

Comparative study of hemicelluloses from wheat straw by alkali and hydrogen peroxide extractions

As compared to traditional alkaline extractions, alkaline peroxide was used to isolate hemicelluloses from wheat straw. Yields of the solubilized hemicelluloses ranged from 18.9% (2% H2O2 extraction at 90°C for 2 h at pH 11.5) to 26.6% (2% H2O2 extraction at 50°C for 16 h at pH 12.5). The optimum hemicellulose yield (92% of the original hemicelluloses in water treated wheat straw) was obtained when the treatment was performed at 50°C for 16 h at pH 12.5 by use of 2% H2O2. All the hemicellulosic preparations were much lighter in color than those obtained using traditional alkaline extractions in the absence of bleaching. The results, obtained by the destructive method such as acid hydrolysis, showed that extraction of wheat straw with aqueous 10% KOH following alkaline bleaching released the hemicellulosic fractions, which were enriched in xylose, whereas extraction of the straw with aqueous 2% H2O2 under alkaline conditions (pH 11.5–12.5) solubilized the hemicellulosic fractions, which were relatively higher in arabinose and glucose. The nine isolated hemicellulosic samples were further characterized by non-destructive methods such as Fourier transform infrared (FT–IR), and carbon-13 magnetic resonance spectroscopy (13C-NMR) as well as gel permeation chromatography (GPC). It has been demonstrated that the alkaline peroxide treatments under the conditions used do not affect the overall structure of hemicelluloses. The only one major change of the hemicelluloses was found to be degradation during the beaching of alkali-soluble hemicelluloses with 2% H2O2 at 60°C for a period of 16 h at pH 11.5 or bleaching using a relatively higher concentration (5%) of hydrogen peroxide at 60°C for 8 h at pH 11.5.

Characterization of lignins from wheat straw by alkaline peroxide treatment

Treatment of wheat straw with 2% H2O2 at 50°C and pH 11.5 for 4–30 h or with 2% H2O2–0.05% anthraquinone (AQ) at 50°C and pH 11.5 for 4.5 h resulted in solubilization of 79–86% of the original lignin and 77–91% of the original hemicelluloses, respectively. The seven isolated pure lignin samples were characterized by both destructive methods such as alkaline nitrobenzene oxidation and non-destructive techniques such as ultraviolet (UV), Fourier transform infrared (FT–IR), and carbon-13 magnetic resonance spectroscopy (13C-NMR) as well as gel permeation chromatography (GPC). The results showed that addition of 0.05% AQ to the alkaline peroxide solution did not give any significant effect on the enhancement of delignification. Comparison of these pure lignin fractions with alkaline lignin preparation obtained from wheat straw in the absence of hydrogen peroxide provided evidence of similar chemical composition and structure. It was found that the alkaline peroxide treatment under the conditions used did not result in any significant change in the macromolecular structure of lignin. The only one minimal change of the lignin moiety was the formation of quinone structure.

Changes in adipose tissue composition in malnourished patients before and after liver transplantation: A carbon-13 magnetic resonance spectroscopy and gas-liquid chromatography study

We investigated adipose tissue fatty acid composition in 22 moderately to severely malnourished patients with cirrhosis and in 22 healthy volunteers by in vivo carbon-13 magnetic resonance spectroscopy (MRS). Gas-liquid chromatography (GLC) of adipose tissue samples was also performed in 11 of the patients and in 4 volunteers. In vivo 13C magnetic resonance spectra were obtained from the subcutaneous adipose tissue before and after eight weeks following orthotopic liver transplantation (OLT). Adipose tissue biopsy samples were obtained for GLC analysis at the time of transplantation in the patients and at inguinal hernia repair in the 4 volunteers. No significant differences were found in the subcutaneous adipose tissue total-saturated, - polyunsaturated or -monounsaturated fatty acid composition between patients and healthy volunteers by in vivo 13C MRS. GLC analysis of adipose tissue samples confirmed that total levels of saturated, poly-, and monounsaturated fatty acids remained the same but revealed significant differences in levels of individual fatty acids, particularly n-3 fatty acids (total n-3, cirrhotics: .84% +/- .07% vs. controls: 1.36% +/- .13%, P < .01). Eight weeks following transplantation, recipients showed a considerable increase in body mass (pretransplantation: 59.3 +/- 3.2 vs. posttransplantation: 63.2 +/- 3 kg, P < .01). 13C MRS revealed a significant increase in saturated fatty acids (pretransplantation: 21.6 +/- 2.8 vs. posttransplantation: 25.5% +/- 1.2%, P < .05) and a significant decrease in unsaturated fatty acids. The application of noninvasive MRS techniques may be important to identify the differential uptake of fats, examining both specific fatty acids and different body fat compartments. In the future, this may be useful in optimizing the dietary management of severely malnourished patients with chronic liver disease before liver transplantation.(Hepatology 1997 Jan;25(1):178-83)

Changes in adipose tissue composition in malnourished patients before and after liver transplantation: a carbon-13 magnetic resonance spectroscopy and gas-liquid chromatography study.

We investigated adipose tissue fatty acid composition in 22 moderately to severely malnourished patients with cirrhosis and in 22 healthy volunteers by in vivo carbon-13 magnetic resonance spectroscopy (MRS). Gas-liquid chromatography (GLC) of adipose tissue samples was also performed in 11 of the patients and in 4 volunteers. In vivo 13C magnetic resonance spectra were obtained from the subcutaneous adipose tissue before and after eight weeks following orthotopic liver transplantation (OLT). Adipose tissue biopsy samples were obtained for GLC analysis at the time of transplantation in the patients and at inguinal hernia repair in the 4 volunteers. No significant differences were found in the subcutaneous adipose tissue total-saturated, -polyunsaturated or -monounsaturated fatty acid composition between patients and healthy volunteers by in vivo 13C MRS. GLC analysis of adipose tissue samples confirmed that total levels of saturated, poly-, and monounsaturated fatty acids remained the same but revealed significant differences in levels of individual fatty acids, particularly n-3 fatty acids (total n-3, cirrhotics: .84% +/- .07% vs. controls: 1.36% +/- .13%, P < .01). Eight weeks following transplantation, recipients showed a considerable increase in body mass (pretransplantation: 59.3 +/- 3.2 vs. posttransplantation: 63.2 +/- 3 kg, P < .01). 13C MRS revealed a significant increase in saturated fatty acids (pretransplantation: 21.6 +/- 2.8 vs. posttransplantation: 25.5% +/- 1.2%, P < .05) and a significant decrease in unsaturated fatty acids. The application of noninvasive MRS techniques may be important to identify the differential uptake of fats, examining both specific fatty acids and different body fat compartments. In the future, this may be useful in optimizing the dietary management of severely malnourished patients with chronic liver disease before liver transplantation.

Substrate selection early after reperfusion of ischemic regions in the working rabbit heart.

Several substrates are available in vivo for oxidation by the myocardium. Although substrate selection has been studied extensively in normoxic myocardium, relatively little is known about substrate preference very early during reperfusion after ischemia. Carbon-13 isotopomer analysis was used to study substrate usage by nonischemic and reperfused-ischemic myocardium in a working heart that was subjected to 15 min or regional ischemia and reperfused for 5 min. Compared with nonischemic myocardium, the contribution of acetoacetate to acetyl coenzyme A was increased in the reperfused-ischemic region, and the contribution of exogenous lactate was decreased. Free fatty acid oxidation, however, was not different in the two regions. These results indicate that (1) early during reperfusion, ketone body oxidation may be more significant than has been emphasized, (2) the relative contribution of fatty acids to acetyl coenzyme A is not sensitive to ischemia followed by reperfusion, and (3) Carbon-13 magnetic resonance spectroscopy methods may be used for analysis of spatial heterogeneity of metabolism in the heart.

917-93 Myocardial Fatty Acid Oxidation: Saturated Versus Unsaturated

Fatty acid (FA) metabolism is an integral part of myocardial cellular metabolism and has been studied in normal, ischemic, post-ischemic, and hibernating myocardium with Positron Emission Tomography and carbon-14 methods. These tracer studies generally follow only the saturated (S) FA, palmitate. Human plasma contains a mixture of unsaturated (unS) and SFAs in a 1.3: 1 ratio. It remains unclear if the oxidation of palmitate is representative of a biological mixture of FAs. The purpose of this study was to determine if oxidation of SFAs or unSFAs is similar to a biological mixture. Working rat hearts were supplied with carbon-13 enriched lactate (8 mM) in addition to FA (0.27 mM) as either SFA, unSFA or a biological mixture. The FAs studied (palmitic[16:0], palmitoleic[16:1]. stearic[18:0], OIeic[18:1]. Iinoleic[18:2]) constitute &gt; 85% of the FAs found in plasma. The mixture consisted of all 5 FAs in their relative proportions in human plasma. Carbon-13 Magnetic Resonance Spectroscopy and isotopomer analysis were used to determine the contribution of each substrate to the acetylCoA pool. The results were analyzed using ANOVA and Student-NewmanKeuls tests. The type of FA used did not significantly alter CO or RPP. The oxidation of unSFAs was significantly less than the oxidation of SFAs or the biological mixture of FAs.Fatty acidnFA utilization(%)CO (ml/min)Rpp (mmHg × beats/min)Mixture464 ± 445 ± 1029 ± 5Saturated855 ± 742 ± 1.328 ± 5Unsaturated1235 ± 10*46 ± 1228 ± 5*p &lt; 0.05 p &lt; 0.05 Under these conditions saturated FAs are preferred for oxidation compared to unsaturated FAs. The oxidation of palmitate or stearate appears representative of a biological mixture.

In vivo hepatic glucose disposal during pregnancy

Pregnancy places many demands on the mother both physiologically and metabolically. The mother must sustain nument balance to both herself and the developing fetus(es). As a consequence, late pregnancy is associated with insulin resistance [ 13 and glucose intolerance together with increased lipid and ketone body circulation and utilization by a wide range of tissue types [2]. It is thought that these and other metabolic changes ensure channeling of the blood glucose to the uterine system, suggesting that glucose is a preferred fuel substrate for the conceptus, possibly due to the ease of mobilization and transport of glucose, or glucose oxidation processes being the better developed. The liver controls glucose homeostasis, therefore for increased knowledge of the pathogenesis of insulin resistant states, studies of the kinetics of hepatic glucose disposal are necessary. In this study proton-decoupled carbon13 magnetic resonance spectroscopy (MRS) has been employed as a tool for in vivo hepatic measurements to elucidate the liver's glucose utilizatioddisposal capacity during pregnancy. Two groups of age-matched female albino Wistar rats were time-mated. One group was allowed food ad libitum [Group I] whereas Group I1 (routinely meal fed-RMF [3]) were transferred to a reverse lighvdark mom [light off 10:00/light on 22:00] and allowed access to chow for 2hr/day from day 5 of pregnancy. Group ID were age-matched virgin controls fed ad libitum. Immediately after the last meal on day 20 of gestation (term=21), the rat was anaesthetized with sodium pentobarbital. The liver was exposed and the hepatic portal vein cannulated to a line containing D-glucose-l-'3C(99atom % I3C) [2OOmg/ml]. A dual-tuned protodcarbon coil was placed directly on top of the liver and the animal placed into the bore of the magnet. Using a 4.7 Tesla, 33cm horizontal bore imaging spectrometer, proton-decoupled carbon spectra were acquired at 50.309MHz with an acquisition time of 0.205s, relaxation time of O.ls, a pulse width of 70ps and spectral width of 12kHz. Spectra were accumulated every 2.5min over a 65min period. After accumulation of two base-line spectra, D-glucose1'3C[75mg;30mg.min-'] was infused. Spectra were referenced to C-1, I3 glucose at 96.60ppm. The C-1 glycogen resonance at 100.48ppm was evaluated. Glucose disposal and glycogen synthesis rates were assessed from the relative peak areas normalized to the -CH, integrated area which, from lipid extraction studies, remains constant in the fed groups. From Fig. I , very little incorporation of C-glucose into glycogen was detected in Groups I and 111, whereas Group I1 had a much enhanced incorporation rate, in an approximately linear fashion. Glucose profiles of the three groups indicate relatively slow removal of glucose from the splanchnic region in Groups 11 and 111 in contrast to Group I. Late pregnancy is shown to diminish intracellular glucose utilization by the liver, reflected in the lack of glucose assimilation and utilization compared with the same fed virgin condition whereas by routine meal feeding, enhanced glucose uptake and incorporation into glycogen can be seen in gestationally the same condition. This study suggests an important change in glucose homeostasis by the liver during pregnancy and the importance of MRS for investigating in vivo glucose kinetics. C 1 C 2C 30 40 50 60

Carbon-13 Magnetic Resonance Spectroscopy

Carbon-13 magnetic resonance spectroscopy (13C MRS) is being used to investigate metabolism in the brain, liver, heart, and skeletal muscle of animals. The tricarboxylic acid cycle, glycolysis, gluconeogenesis, hepatic ketogenesis, and metabolism of ethanol are some of the metabolic processes amenable to 13C MRS. Clinical investigation using 13C in humans will become widespread as improvements in sensitivity, localization, enrichment, and decoupling are realized. Carbon-13 MRS offers potential as a new investigative probe into organ physiology.

Radiolabelling with organomercury compounds—III. High specific activity synthesis of [ 131I]6-iodo-androsten-5-enes and [ 131I]6-iodo-pregnen-5-enes

The synthesis of [ 131I]6-iodoandrost-5-en-3β,17β-diol, [ 131I]6-iodoandrost-5-en-3β-ol,17-one, [ 131I]6-iodopregn-5-en-3β-ol-20-one and [ 131I]6-iodopregn-5-en-3β,20-diol has been carried out. The synthetic route was based on the corresponding 6-chloromercury derivatives prepared by direct mercuration of the parent steroids. The chloromercury compounds provided a simple high yield method for labelling these steroids with radioiodines. In addition the method gives products with “no-carrier-added” specific activity. The “cold chemistry” of the 6-chloromercury steroids was also examined and they were readily transformed into the corresponding 6-iodo and 6-bromo-steroids. All compounds were extensively characterized by proton and carbon magnetic resonance spectroscopy. The stability of these 6-halo-steroids is very good and is similar to that found for 6-halo-cholesterols. A preliminary tissue distribution study of these “no-carrier-added” [ 131I]6-iodo-steroids shows very rapid adrenal uptake with peak values occurring at less than 15 min.

Sterols from the sponge Haliclona chilensis

1. 1. The sterol composition of the sponge Haliclona chilensis (Thiele) has been analysed by argentic column- and thin-layer chromatography and reverse-phase high-performance liquid chromatography detecting twenty-eight components. 2. 2. These were studied by gas chromatography-mass spectrometry and in some cases by proton and carbon-13 magnetic resonance spectroscopy. 3. 3. It was established that the components were C 26-, C 27-, C 28-, C 29- and C 30- Δ 5 sterols and in minor amounts Δ 0 and Δ 7 analogs with saturated and unsaturated side-chains. 4. 4. 24-Keto- Δ 5 and - Δ 0 sterols with saturated and unsaturated side-chains were also found among the components of the mixture. 5. 5. This is the first report on the presence of 24-keto-5α-cholestan-3β-ol and 24-keto-5α-cholest-25-en-3β-ol in sponges.

Formation of thioether conjugates of the bladder carcinogen ANFT catalyzed by prostaglandin H synthase.

Certain 2-aminothiazole-substituted 5-nitrofuran carcinogens specifically and potently induce tumors of the urinary tract. Cooxidation by the hydroperoxidase activity of prostaglandin H synthase (PHS) has been implicated in the initiation of this process in vivo. The molecular mechanism of this process was examined utilizing 2-amino-4-(5-nitro-2-furyl)thiazole (ANFT) as the organic cosubstrate. The ability of ANFT to release electrons was determined by cyclic voltammetry, which established that ANFT is oxidized to yield an extremely reactive intermediate by an apparent one-electron mechanism. In conventional incubations containing solubilized microsomal PHS at pH 7.8, glutathione was found to trap approximately 50% of the ANFT present as a soluble metabolite. Comparative u.v. spectrophotometry, cyclic voltammetry, proton and carbon magnetic resonance spectroscopy, and chemical synthesis established that conjugate formation occurs by S-substitution of the thiazole ring to yield 2-amino-4-(5-nitro-2-furyl)-5-(glutathion-S-yl)thiazole. The 5-nitrofuran ring is not altered by this pathway. This suggests that PHS may oxidize the aminothiazole ring of ANFT by direct electron withdrawal to generate a reactive electrophilic intermediate which may react with critical cellular nucleophiles. This conjugate, or a further metabolite thereof, may be a biologic marker of PHS activity and may be useful in assessing the effects of potential chemoprotective agents on hydroperoxidase activity in vivo.

Elimination of solvent signals in carbon magnetic resonance spectroscopy

The problem of a strong or obscuring solvent signal in proton resonance spectroscopy was alleviated by the use of perdeuterated solvents. The practice of stabilizing the magnetic field-frequency ratio for the spectrometer via the deuterium resonance of the solvent followed from the almost routine use of deuterated solvents and was continued for carbon magnetic resonance. Perdeuterated solvents depleted in 13C were developed to remove interfering solvent signals in those cases where the reactivity of the reagents restricted the choice of solvent. However, these are expensive and only available for a few solvents. It has recently become possible to distinguish the number of hydrogen atoms directly bonded to a carbon atom by the modulation of the signal of that carbon atom in response solely to an appropriately gated proton decoupling sequence (I4). The inclusion of 180 and 90” proton pulses in the overall pulse sequence, although more complicated and demanding upon instrumentation, has been familiar longer as INEPT type experiments (5,6). With both types of pulsing sequences the spectrum can be displayed with lines arising from carbon atoms with an even number of attached hydrogens pointing down and those with an odd number pointing up. If such a spectrum is added with one obtained in the normal manner there should be complete cancellation of signals from quaternary and secondary carbons and reinforcement of signals for tertiary and primary carbon atoms. For most organic molecules the cancellation of signals from secondary and quaternary carbons is not quite complete because of long-range carbon-hydrogen coupling and because of the range of values found for one-bond carbon-hydrogen coupling constants. Therefore very weak signals are often found for them. In perdeuterated molecules the cancellation of the carbon resonance signals should be complete since there is no carbon-hydrogen spin coupling. This is illustrated in Fig. 1 for spectra acquired by the common one-pulse sequence and by the solvent signal removing sequence of Fig. 2. It can be seen that the signal of the quatemary carbon in 2-methylbutene-2 (indicated by an arrow) is not completely canceled. The deuterated tetrahydrofuran (Merck) was claimed to contain 99% deuterium. Therefore 8% of the molecules will contain one hydrogen. These are responsible for the residual solvent signals. NMR studies of model anion-pairs valuable in the investigation of

Carbon-13 Magnetic Resonance Spectroscopy of Salicylic Acid and Derivatives

Abstract 13C-NMR spectra of salicylic acid and several of its derivatives have been studied. Carbon atoms assignments were based on the additivity rules and the off-resonance spectra. However in the diortho substituted derivatives C-1 and C-2 were different from predicted values. This difference is attributed to intramolecular hydrogen bonding. Additivity rules were derived and found to be reliable. These rules could be applied for carbon chemical shift assignments of compounds having similar structures.

ChemInform Abstract: THE EXPERIMENTAL DETERMINATION OF THE CONFORMATIONAL FREE ENERGY, ENTHALPY, AND ENTROPY DIFFERENCES FOR ALKYL GROUPS IN ALKYLCYCLOHEXANES BY LOW TEMPERATURE CARBON‐13 MAGNETIC RESONANCE SPECTROSCOPY

Chemischer InformationsdienstVolume 11, Issue 18 Article ChemInform Abstract: THE EXPERIMENTAL DETERMINATION OF THE CONFORMATIONAL FREE ENERGY, ENTHALPY, AND ENTROPY DIFFERENCES FOR ALKYL GROUPS IN ALKYLCYCLOHEXANES BY LOW TEMPERATURE CARBON-13 MAGNETIC RESONANCE SPECTROSCOPY H. BOOTH, H. BOOTHSearch for more papers by this authorJ. R. EVERETT, J. R. EVERETTSearch for more papers by this author H. BOOTH, H. BOOTHSearch for more papers by this authorJ. R. EVERETT, J. R. EVERETTSearch for more papers by this author First published: May 6, 1980 https://doi.org/10.1002/chin.198018069AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume11, Issue18May 6, 1980 RelatedInformation

The experimental determination of the conformational free energy, enthalpy, and entropy differences for alkyl groups in alkylcyclohexanes by low temperature carbon-13 magnetic resonance spectroscopy

13 C N.m.r. studies at low temperatures of solutions in CFCl3–CDCl3 of [Me-13C]methylcyclohexane, [α-13C]-isopropylcyclohexane, cis-1-ethyl-4-methylcyclohexane, cis-1-isopropyl-4-methylcyclohexane, and cis-1-ethyl-4-isopropylcyclohexane have yielded conformational enthalpy differences and conformational entropy differences for methyl, ethyl, and isopropyl groups in alkylcyclohexanes. For –ΔH°, the averaged values obtained, in kcal mol–1, were 1.75 (Me), 1.60 (Et), and 1.52 (Pri); for ΔS°, the averaged values, in cal mol–1 K–1, were –0.03 (Me), 0.64 (Et), and 2.31 (Pri). The determined values are in good agreement with those calculated by the Allinger group for the gas phase, using a method based on classical mechanics. As a result of the opposing trends for –ΔH° and ΔS° values, the sequence of –ΔG° values at 300 K, namely Pri > Et > Me, is reversed at temperatures below ca. 40 K.

Poly(2,3-epoxybutanes) and Poly(1,4-dichloro-2,3-epoxybutanes): Microstructure and Mechanism of Polymerization Studied by Carbon-13 Magnetic Resonance Spectroscopy

Poly(2,3-epoxybutanes) and Poly(1,4-dichloro-2,3-epoxybutanes): Microstructure and Mechanism of Polymerization Studied by Carbon-13 Magnetic Resonance Spectroscopy

The use of carbon-13 NMR spectroscopy to differentiate between cis- and trans- isomers of Rh(en) 2XY +

Carbon-13 magnetic resonance spectroscopy is shown to be a viable tool in differentiating geometric isomers of the type, cis- and trans-Rh(en) 2XY n*. The Rh(en) 3+ 3 ion and the trans-Rh(en ) 2XC1 n+ ions (where X = Cl, OH 2 and NH 3) display only one proton-decoupled C-13 signal. The cis complex ions, cis-Rh(en) 2XCl n+, display between two (X = Cl) and four (X = OH 2, NH 3) 13Cc peaks. The configurational changes of the individual chelate rings (λ ⇌ δ) appear to be fast on the nmr time scale. The splitting pattern and chemical shifts have enabled us to assign a cis configuration to the Rh(en) 2(enH)Cl 3+ product formed from photolysis of Rh(en) 3+ 3 in 1M HCl. The results reported herein will be compared and contrasted to earlier synthetic and 13C nmr studies.

Nuclear Magnetic Resonance Studies of the Complex Formation of Mercuric Chloride with L-Cysteine and with Glutathione

The binding of mercuric chloride, HgCl2, with an amino acid, L-cysteine, and with a tripeptide, glutathione, as a model for sulfhydryl proteins, has been investigated by proton and carbon-13 magnetic resonance spectroscopy. Binding to the active coordination sites was monitored, while observing the chemical shifts of protons and carbon nuclei of the ligand in various conditions of the molar ratio (HgCl2/ligand). The results indicate that mercuric chloride binds exclusively to the sulfhydryl group both in L-cysteine and glutathione. A weak secondary binding may also take place with the carboxylic acid group to some extent in the former and with the cysteinyl carbonyl group in the latter, respectively, up to equimolar point. It is also concluded that the stability of the binding of mercuric chloride depends on the pD of the solution. The results are discussed in terms of possible complex structure.

Tris(ethylenediamine)chromium(III) chloride as a relaxation reagent in carbon-13 NMR and an aid for structure determination in aqueous solution

The carbon-13 relaxation times of a number of amino acids and small peptides have been studied in the presence and absence of the paramagnetic, substitutionally inert complex tris(ethylenediamine)chromium(III) chloride. A metal-complex induced dipolar relaxation pathway is observed, uncomplicated by contact effects. The relaxation time ( T 1e) for this process can be readily measured. In neutral or basic solution the complex interacts preferentially with the carboxylate or carbonyl moiety. Relaxation times of other carbon nuclei reflect a greater average distance from the complex, providing a basis of distinguishing conformations in aqueous solution. Studies of T 1e of the d or l isomers of a given amino acid with either the d or the l isomer of the chromium complex were also carried out and identical T 1e values were observed in the cases studied. The use of tris(ethylenediamine)chromium(III) chloride as a relaxation reagent for carbon-13 magnetic resonance spectroscopy in aqueous solution is illustrated and discussed.