Chemical and bioassay techniques to authenticate quality of the anti-leishmanial drug miltefosine

Miltefosine, an effective oral treatment of visceral leishmaniasis (VL), was selected in May 2005, by the governments of India, Nepal, and Bangladesh for the elimination of VL. However, abnormally high treatment failure rates reported in patients in Bangladesh, given a miltefosine generic product (“Miltefos”, Popular Pharmaceuticals Ltd.) during 2008, led the World Health Organization (WHO) to procure this formulation for quality testing. Proton (¹H) and phosphorous (³¹P) nuclear magnetic resonance (NMR) analyses of the Miltefos™ capsules did not give the peaks defined for Impavido^®, the quality assured VL treatment product from Aeterna Zentaris. Contents of capsules of Impavido^® yielded expected peaks for miltefosine (m/z 408.33 for the protonated parent ion and m/z 183.99 plus m/z 124.8 the fragment ions) that were absent in the Miltefos™ capsules. Furthermore, testing using an in vitro Leishmania donovani intracellular amastigote—macrophage model, yielded EC₅₀ values of between 2.55 and 4.06 μg/mL and 3.02 to 5.92 μg/mL for extracts from the Impavido^® capsules and the miltefosine standard, respectively. Lack of significant anti-leishmanial activity of Miltefos™ capsules was identified in this assay even at concentrations up to 100 μg/mL. Capsules of Miltefos™ were classified as falsified (absence of stated active pharmaceutical ingredient) by three methods—NMR and mass spectrometry analysis and bioassay.

Polar compounds isolated from the leaves of Albertisia delagoensis (Menispermaceae)

Aqueous infusions of the leaves of the shrub Albertisia delagoensis (Menispermaceae) are used in South Africa in traditional Zulu medicine to alleviate a variety of symptoms, including fever, and intestinal problems. We report the analysis of such an aqueous extract using the HPLC-NMR technique. A number of polar compounds were identified, including proto-quercitol, nicotinic acid, allantoic acid, 3,4-dihydroxy-benzoic acid, phthalic acid and the aporphine alkaloid derivative roemrefidine. Allantoic acid and roemrefidine have been fully characterised by (1)H- and (13)C-NMR and mass spectrometry. Earlier reports of antiplasmodial activity of roemrefidine and of A. delagoensis extracts are correlated with this study and with the antipyretic properties of neutral aqueous extracts.

Nuclear Magnetic Resonance Shift Reagents: Abnormal C Shifts Produced by Complexation of Lanthanide Chelates with Saturated Amines and n-Butyl Isocyanide

Lanthanide-induced shfits of (13)C nuclear magnetic resonances are reported for several amines and n-butyl isocyanide. Contact contributions to such shifts, especially of beta carbons, are clearly important for the chelates of Eu(+3) and Pr(+3). The importance of contact terms is shown to change in a rather predictable manner with the structure of the amine.

Large-scale human metabolic phenotyping and molecular epidemiological studies via 1H NMR spectroscopy of urine: investigation of borate preservation

Borate is an antibacterial preservative widely used in clinical and large-scale epidemiological studies involving urine sample analysis. Since it readily forms covalent adducts and reversible complexes with hydroxyl and carboxylate groups, the effects of borate preservation in (1)H NMR-spectroscopy-based metabolic profiling of human urine samples have been assessed. Effects of various concentrations of borate (range 0-30 mM) on (1)H NMR spectra of urine were observed at sequential time points over a 12 month period. Consistent with known borate chemistry, the principal alterations in the (1)H resonance metabolite patterns were observed for compounds such as mannitol, citrate, and alpha-hydroxyisobutyrate and confirmed by ESI-MS analysis. These included line-broadening, T(1) and T(2) relaxation, and chemical shift changes consistent with complex formation and chemical exchange processes. To further investigate complexation behavior in the urinary metabolite profiles, a new tool for visualization of multicomponent relaxation variations in which the spectra were color-coded according to the T(1) and T(2) proton relaxation times respectively (T(1) or T(2) ordered projection spectroscopy, TOPSY) was also developed and applied. Addition of borate caused a general decrease in (1)H T(1) values, consistent with nonspecific effects such as solution viscosity changes. Minor changes in proton T(2) relaxation rates were observed for the most strongly complexing metabolites. From a molecular phenotyping and epidemiologic viewpoint, typical interpersonal biological variation was shown to be vastly greater than any variation introduced by the borate complexation, which had a negligible effect on the metabolic mapping and classification of samples. While caution is indicated in the assignment of biomarker signals where metabolites have diol groupings or where there are adjacent hydroxyl and carboxylate functions, it is concluded that borate preservation is "fit-for-purpose" for (1)H NMR-based epidemiological studies, since the essential biochemical classification features of the samples are robustly maintained.

Structures of the chain metaphosphates NaM(PO3)3 (M = Ca or Sr)

Solid-state (23)Na and (31)P magic-angle spinning nuclear magnetic resonance spectroscopy and X-ray crystallography have been used to study the structures of the chain metaphosphates NaCa(PO(3))(3) and NaSr(PO(3))(3). The compounds are isostructural and crystallise in space group P(-1) with the following parameters: NaCa(PO(3))(3), a = 6.711 A, b = 6.934 A, c = 7.619 A, alpha = 83.44 degrees , beta = 81.41 degrees , gamma = 82.80 degrees ; NaSr(PO(3))(3)a = 6.805 A, b = 7.133 A, c = 7.720 A and alpha = 83.71 degrees , beta = 80.48 degrees , gamma = 82.87 degrees . Both structures contain anionic metaphosphate chains of (PO(3))(n) (n) with ionic contacts to Na(+) ions in distorted octahedral sites and Ca(2+) (or Sr(2+)) in distorted dodecahedral sites. (31)P and (23)Na NMR are entirely consistent with the crystallographic data and an empirical method for assigning (31)P resonances to particular crystallographically unique P atoms is described.

The role of aluminium and silicon in the setting chemistry of glass ionomer cements

A model of the setting chemistry of glass-ionomer cements (GICs) is proposed based on 27Al and 29Si solid state nuclear magnetic resonance spectroscopy data on three GICs. All the precursor glasses are found to contain three aluminium species viz.: four, five and six-coordinate aluminium environments as well as four-bridging silicate tetrahedra. Upon cement formation, Al3+ ions in the glass are leached out from the surface layer of the glass. On entering the cement matrix, these ions adopt six-coordination and crosslink the polymer chains as part of the setting reaction. The remaining four-coordinate aluminium is distributed between two species: one in the inert core of the glass particles; and a second, less concentrated, in the surface layer of the glass particles, modified by the curing reactions. There is some evidence for residual five and six coordinate aluminium species in the final cement in some of the systems. In the case of the silicate tetrahedra, the curing reactions result in a decrease in the number of aluminium atoms in the second coordination sphere, with a subsequent recondensation of silicate network of the glass.

Comparison of rosmarinic acid content in commercial tinctures produced from fresh and dried lemon balm (Melissa officinalis)

PURPOSE

To measure the rosmarinic acid content of eight commercial tinctures derived from fresh (n= 5) and dried (n=3) Melissa officinalis herb.

METHODS

Rosmarinic acid and the internal standard (esculin) were purchased from Aldrich Chemical Co. The column used was a Luna C18, 5 um (150 x 4.6 mm I.D., Phenomenex) maintained at ambient room temperature. The HPLC system consisted of a Shimadzu SCL-6B controller, Shimadzu LC-6A pumps, Shimadzu SPD-6A UV single wavelength spectrophotometric detector set to 320 nm and Shimadzu SIL-6B autosampler. Gradient elution of the samples and standard were performed using ammonium formate (0.02 M; pH 6.25 at 27 oC; eluent A) and methanol (eluent B). The gradient elution initial conditions were 2% of eluent B with linear gradient to 60% at 30 min, followed by linear gradient to 90% of eluent B at 31 min, this proportion being maintained for 4 min. The column was then returned to the initial condition at 36 min and maintained until the end of the run at 43 min. The flow rate was 1 mL/min. The assay was validated for sensitivity, accuracy and reproducibility.

RESULTS

The content of rosmarinic acid in commercial tinctures was significantly higher in the tinctures made from dried plant material (2.96 - 22.18 mg/mL) compared to fresh plant tinctures (

CONCLUSION

These results have implications both for the manufacturers of commercial tinctures and also for herbal practitioners in the choice of tinctures for treating Herpes simplex infection.

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MESH Headings

• Chromatography, High Pressure Liquid/methods
• Cinnamates/analysis
• Cinnamates/chemistry
• Depsides/analysis
• Depsides/chemistry
• Herbal Medicine/standards
• Melissa/chemistry
• Pharmaceutical Preparations/analysis
• Pharmaceutical Preparations/chemistry
• Plant Extracts/analysis
• Plant Extracts/chemistry
• Rosmarinic Acid

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Reaction of o-phenylenediamine with diacetyl monoxime: characterisation of the product by solid-state 13C and 15N MAS NMR

2,3-dimethylquinoxaline (DMQ) and dimethylglyoxime (DMGH2) form a 1:1 hydrogen-bonded complex in the solid state, which is completely dissociated in methanol solution. There are small differences in solid-state 13C shifts between the separated components DMQ and DMGH2 and the complex. The changes in 15N solid-state chemical shifts are more significant: the hydrogen bond imparting a low frequency shift of ca 19 ppm. The effect of direct protonation on the DMQ solid-state 15N shifts was measured, and the experimental 15N data correlated with those from GIAO molecular orbital (MO) calculations.

NMR investigation into the dynamic behaviour of a novel C2-symmetric macrocyclic ligand

The variable-temperature (1)H NMR spectra of a tosylated triazacyclononane are reported. The spectra show a dynamic process which is assigned to hindered rotation about the amide-like N--S bonds, and a value for the free energy of activation for the rotations is derived, DeltaG* = 50.2 kJ mol(-1) at 273 K. Steric and electronic factors which influence the barrier are discussed.

Stray-field imaging and multinuclear magnetic resonance spectroscopy studies on the setting of a commercial glass-ionomer cement

A commercial glass-ionomer (Fuji II) was studied using different NMR techniques. 1H and 19F stray-field imaging (STRAFI) were used to monitor the curing kinetics of the cement and two processes were distinguished: the gelation and maturation phases. Characterization of the aluminium and silicon species present in the glass component and cement was performed by conventional 27Al and 29Si MAS and two-dimensional 27Al triple quantum MAS NMR (MQMAS) techniques. Quantification of the aluminium in the glass component and in the cement indicates the leaching of about 32% of the 4-coordinate aluminium, about 100% of the 5-coordinate aluminium and about 41% of the 6-coordinate aluminium, during the setting reaction. It is also shown that the 5-coordinate aluminium is only present in the surface layer created by the acid attack during the cement formation process. In the cement, silicon maintains species with four bridging oxygens after the leaching of tetrahedral (4-coordinate) aluminium. The silicon analysis correlates well with the aluminium determinations.

Structure, modelling and dynamic behaviour of aza- and azaoxamacrocyclic ligands derived from (R,R)-1,2-diaminocyclohexane

Investigations into the conformational behaviour of macrocyclic ligands 5 and 6 derived from (R,R)-1,2-diaminocyclohexane have been undertaken using molecular modelling, single crystal X-ray diffraction and variable temperature 1H NMR spectroscopy. These have revealed that the lowest energy conformers in both cases do not possess the expected C2-element of symmetry, which can only be accessed at higher temperatures. Instead both molecules exist as C1-conformers at room temperature and in the solid state. In solution a range of dynamic exchange processes is observed which result, in part from the inherent strain in these fused bicyclic systems. An unexpected but characteristic feature of the C1-symmetric conformers is highlighted by the presence of a signal at unexpectedly low field in their 1H NMR spectra due to the interaction of two of the sulfonyl oxygen atoms with one of the bridgehead hydrogen atoms.

Thermodynamic origin of cis/trans isomers of a proline-containing beta-turn model dipeptide in aqueous solution: a combined variable temperature 1H-NMR, two-dimensional 1H,1H gradient enhanced nuclear Overhauser effect spectroscopy (NOESY), one-dimensional steady-state intermolecular 13C,1H NOE, and molecular dynamics study

The cis/trans conformational equilibrium of the two Ac-Pro isomers of the beta-turn model dipeptide [13C]-Ac-L-Pro-D-Ala-NHMe, 98% 13C enriched at the acetyl carbonyl atom, was investigated by the use of variable temperature gradient enhanced 1H-nmr, two-dimensional (2D) 1H,1H nuclear Overhauser effect spectroscopy (NOESY), 13C,1H one-dimensional steady-state intermolecular NOE, and molecular dynamics calculations. The temperature dependence of the cis/trans Ala(NH) protons are in the region expected for random-coil peptides in H2O (delta delta/delta T = -9.0 and -8.9 ppb for the cis and trans isomers, respectively). The trans NH(CH3) proton indicates smaller temperature dependence (delta delta/delta T approximately -4.8 ppb) than that of the cis isomer (-7.5 ppb). 2D 1H,1H NOESY experiments at 273 K demonstrate significant NOEs between ProH alpha-AlaNH and AlaNH-NH(R) for the trans isomer. The experimental NOE data, coupled with computational analysis, can be interpreted by assuming that the trans isomer most likely adopts an ensemble of folded conformations. The C-CONH(CH3) fragment exhibits significant conformational flexibility; however, a low-energy conformer resembles closely the beta II-turn folded conformations of the x-ray structure of the related model peptide trans-BuCO-L-Pro-Me-D-Ala-NHMe. On the contrary, the cis isomer adopts open conformations. Steady-state intermolecular solute-solvent (H2O) 13C,1H NOE indicates that the water accessibility of the acetyl carbonyl carbons is nearly the same for both isomers. This is consistent with rapid fluctuations of the conformational ensemble and the absence of a highly shielded acetyl oxygen from the bulk solvent. Variable temperature 1H-nmr studies of the cis/trans conformational equilibrium indicate that the trans form is enthalpically favored (delta H degree = -5.14 kJ mole-1) and entropically (delta S degree = -5.47 J.K-1.mole-1) disfavored relative to the cis form. This demonstrates that, in the absence of strongly stabilizing sequence-specific interresidue interactions involving side chains and/or charged terminal groups, the thermodynamic difference of the cis/trans isomers is due to the combined effect of intramolecular and intermolecular (hydration) induced conformational changes.

Determination of the relative NH proton lifetimes of the peptide analogue viomycin in aqueous solution by NMR-based diffusion measurement

In aqueous solution, exchanging peptide NH protons experience two environments, that of the peptide itself with a relatively slow diffusion coefficient and that of the water solvent with a faster diffusion coefficient. Although in slow exchange on the NMR chemical shift timescale, the magnetic field gradient dependence of the NH peak intensities in an experiment used to measure diffusion coefficients reflects the relative time periods spent in the two environments and this allows the determination of the relative solvent accessibility of exchangeable protons in peptides or proteins. To test this approach, the magnetic field gradient dependent intensities of the chemically shifted amide and amine NH protons of the peptide antibiotic viomycin have been measured using the high resolution longitudinal-eddy-current-delay (LED) NMR method incorporating solvent water peak elimination by non-excitation. The NH resonances of viomycin have been assigned previously and their relative exchange rates determined. Here, the gradient dependence of each NH proton intensity is reported, and these, after a bi- exponential least squares fitting, yield the fractional lifetimes of the protons spent in the peptide and water environments during the diffusion period of the experiment.

13C- and 57Fe-NMR studies of the Fe-C-O unit of heme proteins and synthetic model compounds in solution: comparison with IR vibrational frequencies and X-ray structural data

13C- and 57Fe-NMR spectra of several carbon monoxide hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and solvent polarity are reported. The 13C shieldings of heme models cover a 4.0 ppm range that is extended to 7.0 ppm when several hemoglobin CO and myoglobin CO species at different pHs are included. Both heme models and heme proteins obey a similar excellent linear delta(13C) versus nu(C-O) relationship that is primarily due to modulation of pi backbonding from Fe d pi to the CO pi* orbital by the distal pocket polar interactions. There is no direct correlation between delta(13C) and Fe-C-O geometry. The poor monotonic relation between delta(13C) and nu(Fe-C) indicates that the iron-carbon pi bonding is not a primary factor influencing delta(13C) and delta(57Fe). The delta(57Fe) was found to be extremely sensitive to deformation of the porphyrin geometry, and increased shielding by more than 600 ppm with increased ruffling was observed for various heme models of known X-ray structures.

The effects of atropisomerism and porphyrin deformation on 57Fe shieldings in superstructured hemoprotein models

57Fe NMR chemical shifts of superstructured heme model compounds have been found to be extremely sensitive to atropisomerism and deformation (ruffling) of the porphyrin geometry.

Solid State Dynamics of Tricarbonyl(eta-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(eta-1,5-cycloheptadienylium)iron Tetrafluoroborate

The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). (57)Fe Mössbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) Å, c = 21.761(3) Å, V = 2449.7(5) Å(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.

Detection of aldehydes and their conjugated hydroperoxydiene precursors in thermally-stressed culinary oils and fats: investigations using high resolution proton NMR spectroscopy

High field (400 and 600 MHz) proton NMR spectroscopy has been employed to investigate the thermally-induced autoxidation of glycerol-bound polyunsaturated fatty acids present in intact culinary frying oils and fats. Heating of these materials at 180 degrees C for periods of 30, 60 and 90 min. generated a variety of peroxidation products, notably aldehydes (alkanals, trans-2-alkenals and alka-2,4-dienals) and their conjugated hydroperoxydiene precursors. Since such aldehydes appear to be absorbed into the systemic circulation from the gut in vivo, the toxicological significance of their production during standard frying practices is discussed.

Generation of lipid peroxidation products in culinary oils and fats during episodes of thermal stressing: a high field 1H NMR study

The oxidative deterioration of glycerol-bound polyunsaturated fatty acids (PUFAs) in culinary oils and fats during episodes of heating associated with normal usage (30-90 min at 180 degrees C) has been monitored by high field 1H NMR spectroscopy. Thermal stressing of PUFA-rich culinary oils generated high levels of n-alkanals, trans-2-alkenals, alka-2,4-dienals and 4-hydroxy-trans-2-alkenals via decomposition of their conjugated hydroperoxydiene precursors, whereas only low concentrations of selected aldehydes were produced in oils with a low PUFA content, lard and dripping when subjected to the above heating episodes. Samples of repeatedly used, PUFA-rich culinary oils obtained from restaurants also contained high levels of each class of aldehyde. The dietary, physiological and toxicological ramifications of the results obtained are discussed.

A comparative evaluation of the metabolic profiles of normal and inflammatory knee-joint synovial fluids by high resolution proton NMR spectroscopy

High resolution 1H NMR spectroscopy has been employed to investigate the metabolic profile of healthy human knee-joint synovial fluid (SF) and the biochemical data acquired have been compared with those of matched serum, and inflammatory knee-joint SF samples. Results obtained indicate that the healthy human knee-joint has a hypoxic status (high lactate level when expressed relative to that of paired serum) that is milder than that of the inflamed human knee-joint. Moreover, normal SF differs from that of inflammatory SF in that it contains little or no NMR-detectable lipoprotein-associated fatty acids and 'acute-phase' glycoproteins, an observation reflecting the limited passage of these macromolecules from plasma into the synovial space in healthy subjects.

Generation of substance P carbamate in neutral aqueous solution. Relevance to inflammatory joint diseases

High-field proton (1H) nuclear magnetic resonance (NMR) spectroscopy has been employed to evaluate the formation of substance P carbamate in aqueous solution. Equilibration of substance P with physiologically relevant concentrations of bicarbonate (2.50 x 10(-2) mol.dm-3) at pH 7.00 generated a new multiplet signal centred at 4.13 ppm in its NMR spectrum, characteristic of the alpha-proton of peptide carbamate species. High-field 1H NMR spectroscopy also demonstrated that the model dipeptide, Arg-Gly, formed a carbamate in neutral aqueous solutions containing 2.50 x 10(-2) mol.dm-3 HCO3-. The physiological significance of these results is discussed in view of the central roles of vasoactive neuropeptides in human joint diseases and the hypercapnic environment of the inflamed rheumatoid joint.

High resolution proton NMR investigations of rat blood plasma. Assignment of resonances for the molecularly mobile carbohydrate side-chains of 'acute-phase' glycoproteins

An intense broad resonance at 2.14 ppm present in high field (400, 500 and 600 MHz) Hahn spin-echo 1H-NMR spectra of rat blood plasma, but absent from those of human blood plasma is attributable to the presence of terminal O-acetylsialate sugars in the molecularly mobile carbohydrate side-chains of 'acute-phase' glycoproteins (predominantly alpha 1-acid glycoprotein). The presence of such alternative acetylsugars in the carbohydrate side-chains of rat plasma glycoproteins are of much physiological and experimental significance in view of the regular use of these animals in model systems of human inflammatory conditions.

Structure of vancomycin and a vancomycin/D-Ala-D-Ala complex in solution

Restrained molecular dynamics simulations were used to study the interactions between the glycopeptide antibiotic vancomycin and the dipeptide Ac-D-Ala-D-Ala. Restraints were obtained from a combination of homonuclear and heteronuclear two-dimensional NMR experiments (NOESY, ROESY, 1H-15N inverse correlation). The comparison between the structures obtained for vancomycin alone and for the complex suggests a new hypothesis on the binding mode of this system. The numerical simulations were not straightforward because vancomycin is made of building blocks for which standard force-fields are not available. The representation of unusual chemical environments is also mandatory. We believe that our extension of the force-field parameters to our system could be of more general interest. Furthermore, we consider vancomycin and its complex a good example for exploring the more general problem of molecular recognition, a challenge that has been widely approached in the past few years but for which no unique and general methodology has, so far, been recognized.

The conformation of gramicidin A in dimethylsulphoxide solution. A full analysis of the one- and two-dimensional 1H, 13C, and 15N nuclear-magnetic-resonance spectra

The combined application of one- and two-dimensional high-field NMR techniques has led to the first assignment of the 1H, 13C, and 15N spectra of the pentadecapeptide gramicidin A in dimethylsulphoxide solution. The 62.9-MHz and 100.6-MHz 13C spin-lattice relaxation times and 13C-[1H] NOE factors for the backbone alpha carbons have been analysed in the 'model-free' approach to give a single correlation time (tau m) for isotropic overall molecular motion and an order parameter and internal correlation time for each C alpha H group in the backbone. The relatively high and constant values for the order parameter along the backbone indicate a degree of ordering of the structure, while the internal correlation times show that internal motions are progressively more rapid towards the N terminus. The average values of the vicinal HNC alpha H couplings are 7.4 Hz and 8.4 Hz respectively for the alternate L- and D-amino acid residues. The values are not consistent with either a ribbon conformation for the backbone or a right-handed beta 6.3 helix; they are consistent with the model proposed by Glickson et al. [Glickson, J. D., Mayers, D. F., Settine, J. M. & Urry, D. W. (1972) Biochemistry 11, 477-486] in which there is a rapid conformational order in equilibrium disorder equilibrium, the ordered structure being the left-handed beta 6.3 helix and the disordered state having local random-coil character.

113Cd NMR studies of reconstituted seven-cadmium metallothionein: evidence for structural flexibility

A reproducible method for the reconstitution of rabbit liver metallothionein (MT) containing seven cadmium atoms per mole of protein is described. This protein was studied in detail by 113Cd NMR at 88-, 55-, and 44-MHz frequencies, including the effects of pH, temperature, and ionic strength on the spectra. Our results differ significantly from previous reports of 113Cd NMR on similar samples. Thus, the spectra of both chromatographically distinguishable isoforms MT1 and MT2 were not identical, and neither could be interpreted in terms of a unique static model with the seven Cd ions forming two independent clusters of four and three Cd ions. Large differential shifts of 113Cd resonances were observed with changes in temperature over the range 277-320 K and ionic strength (0.02-0.5 M). At low temperature a slow structural change (half-life of several minutes) was detected. The structure was more rigid at high ionic strength. The frequency dependence and two-dimensional J-resolved spectra revealed that 113Cd resonances were composed of several overlapping peaks, complicating the interpretation of fine structure in one-dimensional spectra. A new flexible model of the Cd cluster in metallothionein is proposed. This model incorporates dynamic thiolate exchange reactions and involves several configurational substates of the protein. The possible relationship of such flexibility to the function of metallothionein is discussed.