[Biarsenical probes for specific and multifunctional modification of proteins]

Chemical modifications of proteins are crucial for studying their functions, biophysical properties and cellular localization. The most important is the protein fluorescent modification utilized in biochemistry, molecular biology and medicinal diagnostics. Precision of fluorophore attachment in certain part of the protein or amino acid sequence is very important for the labeling and subsequent characterization or protein applications. One of the most important type of probes used for fluorescent protein labeling are biarsenical probes. They are not only to localize proteins in cell but based on their chemical properties they are widely applied for studying protein folding, degradation, control of the activity, protein inactivation with singlet oxygen, oligomerization and protein purification. Here, author presents principles of protein labeling with biarsenical probes with special attention to factors affecting proper protein modification. Review includes the most interesting applications of biarsenical probes in molecular biology, molecular diagnostics and analytical biochemistry.

Vesicular zinc promotes presynaptic and inhibits postsynaptic long-term potentiation of mossy fiber-CA3 synapse

The presence of zinc in glutamatergic synaptic vesicles of excitatory neurons of mammalian cerebral cortex suggests that zinc might regulate plasticity of synapses formed by these neurons. Long-term potentiation (LTP) is a form of synaptic plasticity that may underlie learning and memory. We tested the hypothesis that zinc within vesicles of mossy fibers (mf) contributes to mf-LTP, a classical form of presynaptic LTP. We synthesized an extracellular zinc chelator with selectivity and kinetic properties suitable for study of the large transient of zinc in the synaptic cleft induced by mf stimulation. We found that vesicular zinc is required for presynaptic mf-LTP. Unexpectedly, vesicular zinc also inhibits a form of postsynaptic mf-LTP. Because the mf-CA3 synapse provides a major source of excitatory input to the hippocampus, regulating its efficacy by these dual actions, vesicular zinc is critical to proper function of hippocampal circuitry in health and disease.

Thionein/metallothionein control Zn(II) availability and the activity of enzymes

Fundamental issues in zinc biology are how proteins control the concentrations of free Zn(II) ions and how tightly they interact with them. Since, basically, the Zn(II) stability constants of only two cytosolic zinc enzymes, carbonic anhydrase and superoxide dismutase, have been reported, the affinity for Zn(II) of another zinc enzyme, sorbitol dehydrogenase (SDH), was determined. Its log K is 11.2 +/- 0.1, which is similar to the log K values of carbonic anhydrase and superoxide dismutase despite considerable differences in the coordination environments of Zn(II) in these enzymes. Protein tyrosine phosphatase 1B (PTP 1B), on the other hand, is not classified as a zinc enzyme but is strongly inhibited by Zn(II), with log K = 7.8 +/- 0.1. In order to test whether or not metallothionein (MT) can serve as a source for Zn(II) ions, it was used to control free Zn(II) ion concentrations. MT makes Zn(II) available for both PTP 1B and the apoform of SDH. However, whether or not Zn(II) ions are indeed available for interaction with these enzymes depends on the thionein (T) to MT ratio and the redox poise. At ratios [T/(MT + T) = 0.08-0.31] prevailing in tissues and cells, picomolar concentrations of free Zn(II) are available from MT for reconstituting apoenzymes with Zn(II). Under conditions of decreased ratios, nanomolar concentrations of free Zn(II) become available and affect enzymes that are not zinc metalloenzymes. The match between the Zn(II) buffering capacity of MT and the Zn(II) affinity of proteins suggests a function of MT in controlling cellular Zn(II) availability.

Cellular zinc and redox buffering capacity of metallothionein/thionein in health and disease

Zinc is involved in virtually all aspects of cellular and molecular biology as a catalytic, structural, and regulatory cofactor in over 1000 proteins. Zinc binding to proteins requires an adequate supply of zinc and intact molecular mechanisms for redistributing zinc ions to make them available at the right time and location. Several dozen gene products participate in this process, in which interactions between zinc and sulfur donors determine the mobility of zinc and establish coupling between cellular redox state and zinc availability. Specifically, the redox properties of metallothionein and its apoprotein thionein are critical for buffering zinc ions and for controlling fluctuations in the range of picomolar concentrations of "free" zinc ions in cellular signaling. Metallothionein and other proteins with sulfur coordination environments are sensitive to redox perturbations and can render cells susceptible to injury when oxidative stress compromises the cellular redox and zinc buffering capacity in chronic diseases. The implications of these fundamental principles for zinc metabolism in type 2 diabetes are briefly discussed.

Dual nanomolar and picomolar Zn(II) binding properties of metallothionein

Each of the seven Zn(II) ions in the Zn(3)S(9) and Zn(4)S(11) clusters of human metallothionein is in a tetrathiolate coordination environment. Yet analysis of Zn(II) association with thionein, the apoprotein, and analysis of Zn(II) dissociation from metallothionein using the fluorescent chelating agents FluoZin-3 and RhodZin-3 reveal at least three classes of sites with affinities that differ by 4 orders of magnitude. Four Zn(II) ions are bound with an apparent average log K of 11.8, and with the methods employed, their binding is indistinguishable. This binding property makes thionein a strong chelating agent. One Zn(II) ion is relatively weakly bound, with a log K of 7.7, making metallothionein a zinc donor in the absence of thionein. The binding data demonstrate that Zn(II) binds with at least four species: Zn(4)T, Zn(5)T, Zn(6)T, and Zn(7)T. Zn(5)T and Zn(6)T bind Zn(II) with a log K of approximately 10 and are the predominant species at micromolar concentrations of metallothionein in cells. Central to the function of the protein is the reactivity of its cysteine side chains in the absence and presence of Zn(II). Chelating agents, such as physiological ligands with moderate affinities for Zn(II), cause dissociation of Zn(II) ions from metallothionein at pH 7.4 (Zn(7)T <==> Zn(7-n)T + nZn(2+)), thereby affecting the reactivity of its thiols. Thus, the rate of thiol oxidation increases in the presence of Zn(II) acceptors but decreases if more free Zn(II) becomes available. Thionein is such an acceptor. It regulates the reactivity and availability of free Zn(II) from metallothionein. At thionein/metallothionein ratios > 0.75, free Zn(II) ions are below a pZn (-log[Zn(2+)](free)) of 11.8, and at ratios < 0.75, relatively large fluctuations of free Zn(II) ions are possible (pZn between 7 and 11). These chemical characteristics match cellular requirements for Zn(II) and suggest how the molecular structures and redox chemistries of metallothionein and thionein determine Zn(II) availability for biological processes.

Cellular zinc and redox buffering capacity of metallothionein/thionein in health and disease

Zinc is involved in virtually all aspects of cellular and molecular biology as a catalytic, structural, and regulatory cofactor in over 1000 proteins. Zinc binding to proteins requires an adequate supply of zinc and intact molecular mechanisms for redistributing zinc ions to make them available at the right time and location. Several dozen gene products participate in this process, in which interactions between zinc and sulfur donors determine the mobility of zinc and establish coupling between cellular redox state and zinc availability. Specifically, the redox properties of metallothionein and its apoprotein thionein are critical for buffering zinc ions and for controlling fluctuations in the range of picomolar concentrations of "free" zinc ions in cellular signaling. Metallothionein and other proteins with sulfur coordination environments are sensitive to redox perturbations and can render cells susceptible to injury when oxidative stress compromises the cellular redox and zinc buffering capacity in chronic diseases. The implications of these fundamental principles for zinc metabolism in type 2 diabetes are briefly discussed.

The zinc/thiolate redox biochemistry of metallothionein and the control of zinc ion fluctuations in cell signaling

Free zinc ions are potent effectors of proteins. Their tightly controlled fluctuations ("zinc signals") in the picomolar range of concentrations modulate cellular signaling pathways. Sulfur (cysteine) donors generate redox-active coordination environments in proteins for the redox-inert zinc ion and make it possible for redox signals to induce zinc signals. Amplitudes of zinc signals are determined by the cellular zinc buffering capacity, which itself is redox-sensitive. In part by interfering with zinc and redox buffering, reactive species, drugs, toxins, and metal ions can elicit zinc signals that initiate physiological and pathobiochemical changes or lead to cellular injury when free zinc ions are sustained at higher concentrations. These interactions establish redox-inert zinc as an important factor in redox signaling. At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein. They can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Sequence-specific Ni(II)-dependent peptide bond hydrolysis in a peptide containing threonine and histidine residues

Previously we demonstrated that Ni(II) complexes of Ac-Thr-Glu-Ser-His-His-Lys-NH2 hexapeptide, representing residues 120-125 of human histone H2A, and some of its analogs undergo E-S peptide bond hydrolysis. In this work we demonstrate a similar coordination and reactivity pattern in Ni(II) complexes of Ac-Thr-Glu-Thr-His-His-Lys-NH2, its threonine analogue, studied using potentiometry, electronic absorption spectroscopy and HPLC. For the first time we present the detailed temperature and pH dependence of such Ni(II)-dependent hydrolysis reactions. The temperature dependence of the rate of hydrolysis yielded activation energy E(a) = 92.0 kJ mol(-1) and activation entropy DeltaS# = 208 J mol(-1) K(-1). The pH profile of the reaction rate coincided with the formation of the four-nitrogen square-planar Ni(II) complex of Ac-Thr-Glu-Thr-His-His-Lys-NH2. These results expand the range of protein sequences susceptible to Ni(II) dependent cleavage by those containing threonine residues and permit predictions of the course of this reaction at various temperatures and pH values.

Zinc-buffering capacity of a eukaryotic cell at physiological pZn

In spite of the paramount importance of zinc in biology, dynamic aspects of cellular zinc metabolism remain poorly defined at the molecular level. Investigations with human colon cancer (HT-29) cells establish a total cellular zinc concentration of 264 microM. Remarkably, about 10% of the potential high-affinity zinc-binding sites are not occupied by zinc, resulting in a surplus of 28 muM ligands (average Kd(c) = 83 pM) that ascertain cellular zinc-buffering capacity and maintain the "free" zinc concentration in proliferating cells at picomolar levels (784 pM, pZn = 9.1). This zinc-buffering capacity allows zinc to fluctuate only with relatively small amplitudes (DeltapZn = 0.3; below 1 nM) without significantly perturbing physiological pZn. Thus, the "free" zinc concentrations in resting and differentiated HT-29 cells are 614 pM and 1.25 nM, respectively. The calculation of these "free" zinc concentrations is based on measurements at different concentrations of the fluorogenic zinc-chelating agent and extrapolation to a zero concentration of the agent. It depends on the state of the cell, its buffering capacity, and the zinc dissociation constant of the chelating agent. Zinc induction of thionein (apometallothionein) ensures a surplus of unbound ligands, increases zinc-buffering capacity and the availability of zinc (DeltapZn = 0.8), but preserves the zinc-buffering capacity of the unoccupied high-affinity zinc-binding sites, perhaps for crucial physiological functions. Jointly, metallothionein and thionein function as the major zinc buffer under conditions of increased cellular zinc.

Concentrations of extracellular free zinc (pZn)e in the central nervous system during simple anesthetization, ischemia and reperfusion

"Free Zn2+" (rapidly exchangeable Zn2+) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn2+ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn2+ in the brain nor the presumed co-release of free Zn2+ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn2+ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 microM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn2+ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn2+ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn2+ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.

Synaptic release of zinc from brain slices: factors governing release, imaging, and accurate calculation of concentration

Cerebrocortical neurons that store and release zinc synaptically are widely recognized as critical in maintenance of cortical excitability and in certain forms of brain injury and disease. Through the last 20 years, this synaptic release has been observed directly or indirectly and reported in more than a score of publications from over a dozen laboratories in eight countries. However, the concentration of zinc released synaptically has not been established with final certainty. In the present work we have considered six aspects of the methods for studying release that can affect the magnitude of zinc release, the imaging of the release, and the calculated concentration of released zinc. We present original data on four of the issues and review published data on two others. We show that common errors can cause up to a 3000-fold underestimation of the concentration of released zinc. The results should help bring consistency to the study of synaptic release of zinc.

Sequence-specific Ni(II)-dependent peptide bond hydrolysis in a peptide containing threonine and histidine residues

Previously we demonstrated that Ni(II) complexes of Ac-Thr-Glu-Ser-His-His-Lys-NH2 hexapeptide, representing residues 120-125 of human histone H2A, and some of its analogs undergo E-S peptide bond hydrolysis. In this work we demonstrate a similar coordination and reactivity pattern in Ni(II) complexes of Ac-Thr-Glu-Thr-His-His-Lys-NH2, its threonine analogue, studied using potentiometry, electronic absorption spectroscopy and HPLC. For the first time we present the detailed temperature and pH dependence of such Ni(II)-dependent hydrolysis reactions. The temperature dependence of the rate of hydrolysis yielded activation energy E(a) = 92.0 kJ mol(-1) and activation entropy DeltaS# = 208 J mol(-1) K(-1). The pH profile of the reaction rate coincided with the formation of the four-nitrogen square-planar Ni(II) complex of Ac-Thr-Glu-Thr-His-His-Lys-NH2. These results expand the range of protein sequences susceptible to Ni(II) dependent cleavage by those containing threonine residues and permit predictions of the course of this reaction at various temperatures and pH values.

Structure-function relationships in glutathione and its analogues

The results are presented of measurements of protonation constants (potentiometry and NMR), UV spectroscopic properties and redox potentials of GSH and its five analogues, which are modified at the C-terminal glycine residue (gammaGlu-Cys-X, X = Gly, Gly-NH2, Gly-OEt, Ala, Glu, Ser). Strong linear correlations were found between various properties of the thiol and other functions of these peptides. These results allow discussion of the relationships between the structures and properties in glutathione and its analogues, and provide a novel chemical background for the issue of control of GSH reactivity.

Acid–base versus structural properties of an aminoglycoside antibiotic––sisomicin: NMR and potentiometric approach

Aminoglycoside antibiotics constitute a class of the drugs of high interest, whose therapeutic action is based upon the electrostatic interaction with the variety of RNA molecules. The positive charge of these drugs molecules, located at their amino functions, has a prevailing influence on this process. The potentiometry and (1)H NMR spectroscopy are applied hereby to achieve the characteristics of the acid-base properties of particular protonating groups. We found that the pK values of deprotonation processes cover a wide values range 6-9.8. The correlation spectra of sisomicin, both COSY and TOCSY, allowed attributing unambiguously individual signals to the corresponding protons. These spectra involve a lot of the cross-peaks originating from the B and C rings protons, while the analogous signals originating from A rings protons are less numerous. Molecular modeling provided that the methylated amino group of A ring is located too far from the protonated functions of the remaining rings to affect their pK values. The phenomena observed herein are discussed in line of strength of the analogous processes observed for other aminoglycosides. As the result, four types of amino groups consisted within these antibiotics are distinguished.

A formula for correlating pKa values determined in D2O and H2O

A linear correlation between pH-meter readings in equivalent D2O and H2O solutions, determined experimentally, leads to a novel equation, which allows for a direct recalculation of pKa values measured in D2O into a H2O equivalent: pKH=0.929pKH*+0.42. The comparison of this equation with the previously used approach is discussed.

Contrasting Effects of Metal Ions on S-Nitrosoglutathione, Related to Coordination Equilibria: GSNO Decomposition Assisted by Ni(II) vs Stability Increase in the Presence of Zn(II) and Cd(II)

Complex formation between nitrosoglutathione (GSNO) and Zn(II), Cd(II), and Ni(II) ions was studied by potentiometry and spectroscopic techniques. GSNO forms simple ML and ML2 type complexes (L = GSNO) with these ions. The stability of GSNO in HEPES buffer solution, pH 7.4, increased in the presence of both Zn(II) and Cd(II), due to an indirect mechanism. A concentration-dependent destabilization of GSNO by Ni(II) ions was found to be linearly dependent on the NiL complex concentration. NiL forms ternary complexes readily. The NiLA- stoichiometry was found for l-His, and NiHLB3- and NiLB4- complexes were detected for GSSG as the second ligand. The formation of these complexes was found to inhibit GSNO decay, by limiting the concentration of the NiL complex. The mechanism of Ni(II)-assisted GSNO decomposition contains several steps, with a hypothetical ternary complex with GSH as a likely active form. These results provide experimental evidence for the stabilization of GSNO in solution by metal ions, which may provide an additional level of control and/or impairment of cellular redox signaling. The Ni(II)-dependent destabilization of GSNO may constitute a novel epigenetic mechanism in nickel carcinogenesis.

Correlations between complexation modes and redox activities of Ni(II)-GSH complexes

The formation of Ni(II) complexes of GSH in conditions of 4-fold GSH excess over Ni(II) was studied by potentiometric titrations, UV-vis and CD spectroscopies, and magnetic susceptibility measurements. The following set of complexes was obtained in the pH range of 6-12: NiHL, Ni(2)L(2)(2)(-), NiHL(2)(3)(-), NiL(2)(4)(-), and NiH(-)(1)L(2)(5)(-). The first of these is an octahedral species, coordinated through the donors of the Glu moiety of GSH, while the remaining ones are largely square-planar, with participation of the thiol in Ni(II) coordination. Magnetic moments indicate the presence of a spin equilibrium for Ni(2)L(2)(2)(-), NiHL(2)(3)(-), and NiL(2)(4)(-) complexes. Phosphate ions apparently decompose the Ni(2)L(2)(2)(-) complex, converting it into a monomeric, high spin, ternary species. Among the molecular forms of GSH, HL(2)(-) is the one most susceptible to air oxidation, due to a presence of ionic interactions between its protonated amine and deprotonated thiol moieties. The complexation of Ni(II) accelerates air oxidation of GSH in alkaline solutions by a factor of 4, but this effect is absent at neutral pH. The damage to plasmid DNA by H(2)O(2) is facilitated by Ni(II) ions and inhibited by excess of GSH. However, the analysis of the concentration profile of this process indicates that octahedral Ni(II) complexes with GSH are involved in the formation of double strand breaks. Finally, numerical simulations of intracellular Ni(II) distribution, made possible by the determination of stability constants of Ni(II) complexes of GSH, indicate that histidine and ATP, rather than GSH, may act as ligands for Ni(II) in vivo. Altogether, our results suggest that the direct impact of GSH on Ni(II) toxicity may be of a limited character.

Coordination properties of tris(2-carboxyethyl)phosphine, a newly introduced thiol reductant, and its oxide

Acid-base properties and metal-binding abilities of tris(2-carboxyethyl)phosphine (TCEP), a newly introduced thiol group protectant, were studied in solution, using potentiometry, (1)H and (31)P NMR, and UV-vis spectroscopy, and also in the solid state by X-ray diffraction. Stability constants of complexes of the P-oxide of TCEP (TCEPO) were established by potentiometry. The list of metal ions studied included Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II). Cu(II) catalyzed oxidation of TCEP to TCEPO. For all other systems ML complexes were found as major species at neutral pH with TCEP and TCEPO. Monoprotonated MHL species were also detected in weakly acidic conditions for all TCEP complexes and for the Pb(II) complex of TCEPO, while hydrolytic MH(-1)L complexes were found for TCEP at the weakly alkaline pH range. The NiL(4) complex was found to form at excess of TCEP. Overall, the complexes were found to be rather weak, with log beta(ML) values around 3-5 for TCEP and 1.5-2.5 for TCEPO. The phosphorus pK(a) value for TCEP, 7.68, suggests that it can be a good buffer for studies at physiological pH.

May GSH and L-His contribute to intracellular binding of zinc? Thermodynamic and solution structural study of a ternary complex

GSH and L-His are abundant biomolecules and likely biological ligands for Zn(II) under certain conditions. Potentiometric titrations provide evidence of formation of ternary Zn(II) complexes with GSH and L-His or D-His with slight stereoselectivity in favour of L-His (ca. 1 log unit of stability constant). The solution structure of the ZnH(GSH)(L-His)(H2O) complex at pH 6.8, determined by NMR, includes tridentate L-His, monodentate (sulfur) GSH, and weak interligand interactions. Calculations of competitivity of this complex for Zn(II) binding at pH 7.4 indicate that it is likely to be formed in vivo under conditions of GSH depletion. Otherwise, GSH alone emerges as a likely Zn(II) carrier.

Short peptides are not reliable models of thermodynamic and kinetic properties of the N-terminal metal binding site in serum albumin

A comparative study of thermodynamic and kinetic aspects of Cu(II) and Ni(II) binding at the N-terminal binding site of human and bovine serum albumins (HSA and BSA, respectively) and short peptide analogues was performed using potentiometry and spectroscopic techniques. It was found that while qualitative aspects of interaction (spectra and structures of complexes, order of reactions) could be reproduced, the quantitative parameters (stability and rate constants) could not. The N-terminal site in HSA is much more similar to BSA than to short peptides reproducing the HSA sequence. A very strong influence of phosphate ions on the kinetics of Ni(II) interaction was found. This study demonstrates the limitations of short peptide modelling of Cu(II) and Ni(II) transport by albumins.

Stray Cu(II) may cause oxidative damage when coordinated to the -TESHHK- sequence derived from the C-terminal tail of histone H2A

CH(3)CO-Thr-Glu-Ser-His-His-Lys-NH(2), a hexapeptide representing the 120-125 sequence of histone H2A, coordinates Cu(II) ions efficiently. Monomeric complexes are formed. In the major complex at physiological pH, CuH(-1)L, Cu(II) is coordinated equatorially through the imidazole nitrogen of the His-4 residue and the amide nitrogens of the Ser-3 and His-4 residues, and axially through the imidazole nitrogen of the His-5 residue. This complex reacts with H(2)O(2) and the resulting reactive oxygen intermediate efficiently oxidizes 2'-deoxyguanosine. The underlying mechanism involves the formation of Cu(III) and a metal-bound hydroxyl radical species.

Coordination chemistry of glutathione

The metal ion coordination abilities of reduced and oxidized glutathione are reviewed. Reduced glutathione (GSH) is a very versatile ligand, forming stable complexes with both hard and soft metal ions. Several general binding modes of GSH are described. Soft metal ions coordinate exclusively or primarily through thiol sulfur. Hard ones prefer the amino acid-like moiety of the glutamic acid residue. Several transition metal ions can additionally coordinate to the peptide nitrogen of the gamma-Glu-Cys bond. Oxidized glutathione lacks the thiol function. Nevertheless, it proves to be a surprisingly efficient ligand for a range of metal ions, coordinating them primarily through the donors of the glutamic acid residue.

Coordination chemistry of glutathione

The metal ion coordination abilities of reduced and oxidized glutathione are reviewed. Reduced glutathione (GSH) is a very versatile ligand, forming stable complexes with both hard and soft metal ions. Several general binding modes of GSH are described. Soft metal ions coordinate exclusively or primarily through thiol sulfur. Hard ones prefer the amino acid-like moiety of the glutamic acid residue. Several transition metal ions can additionally coordinate to the peptide nitrogen of the gamma-Glu-Cys bond. Oxidized glutathione lacks the thiol function. Nevertheless, it proves to be a surprisingly efficient ligand for a range of metal ions, coordinating them primarily through the donors of the glutamic acid residue.

An evaluation of least-squares fits to COSY spectra as a means of estimating proton-proton coupling constants. II. Applications to polypeptides

A new computational method for simultaneously estimating all the proton-proton coupling constants in a molecule from COSY spectra [Yang, J.-X. and Havel, T.F. (1994) J. Biomol. NMR, 4, 807-826] is applied to experimental data from two polypeptides. The first of these is a cyclic hexapeptide denoted as VDA (-D-Ala1-Phe2-Trp3-Lys(Z)4-Val5-Phe6-), in deuterated DMSO, while the second is a 39-residue protein, called decorsin, in aqueous solution. The effect of different data processing strategies and different initial parameter values on the accuracy of the coupling constants was explored. In the case of VDA, most of the coupling constants did not depend strongly on the initial values chosen for the optimization or on how the data were processed. This, together with our previous experience using simulated data, implies strongly that these values are accurate estimates of the coupling constants. They also differ by an average of only 0.36 Hz from the values of the 14 coupling constants that could be measured independently by established methods. In the case of decorsin, many of the coupling constants exhibited a moderate dependence on their initial values and a strong dependence on how the data were processed. With the most successful data processing strategy, the amide-alpha coupling constants differed by an average of 1.11 Hz from the 21 values that could be measured by established methods, while two thirds of the three-bond coupling constants fell within 1.0 Hz of the ranges obtained by applying the Karplus relation to an independently computed ensemble of distance geometry structures. The averages of the coupling constants over multiple optimizations using random initial values were computed in order to obtain the best possible estimates of the coupling constants. Most clearly incorrect averages can be identified by large standard deviations in the coupling constants or the associated line widths and chemical shifts, and can be explained by strong coupling and/or overlap with the water signal, the diagonal peaks or other cross peaks.