Thioether Sulfur Research Articles

From Zero‐dimensional to One‐dimensional: Use of Metal‐­Ligand Affinity in Supramolecular Assembly

AbstractThe self‐assembly of 4‐MTPP [4‐MTPP = 2‐(methylthio)‐4‐(pyridin‐4‐yl)pyrimidine] with Cu(NO3)2 and AgNO3 was structurally investigated. For Cu(NO3)2, a discrete mononuclear CuII coordination compound, [Cu(4‐MTPP)2(NO3)2] (1), resulted that is exclusively based on Cu–N coordination. For AgNO3, a unique one‐dimensional double‐chain structure (2) was obtained with the Ag–N distances varying from 2.181(9) to 2.223(9) Å, and the average Ag–S distance being 2.98 Å. Compared to zero‐dimensional 1, the extension to one‐dimensional 2 is considered to result from the specific affinity between Ag+ and the ligand 4‐MTPP that is attributed to the strong coordinating tendency of silver for aromatic nitrogen and thioether sulfur atoms.

Role of the Tyr-Cys Cross-link to the Active Site Properties of Galactose Oxidase

The catalytically relevant, oxidized state of the active site [Cu(II)-Y·-C] of galactose oxidase (GO) is composed of antiferromagnetically coupled Cu(II) and a post-translationally generated Tyr-Cys radical cofactor [Y·-C]. The thioether bond of the Tyr-Cys cross-link has been shown experimentally to affect the stability, the reduction potential, and the catalytic efficiency of the GO active site. However, the origin of these structural and energetic effects on the GO active site has not yet been investigated in detail. Here we present copper and sulfur K-edge X-ray absorption data and a systematic computational approach for evaluating the role of the Tyr-Cys cross-link in GO. The sulfur contribution of the Tyr-Cys cross-link to the redox active orbital is estimated from sulfur K-edge X-ray absorption spectra of oxidized GO to be about 24 ± 3%, compared to the values from computational models of apo-GO (15%) and holo-GO (22%). The results for the apo-GO computational models are in good agreement with the previously reported value for apo-GO (20 ± 3% from EPR). Surprisingly, the Tyr-Cys cross-link has only a minimal effect on the inner sphere, coordination geometry of the Cu site in the holo-protein. Its effect on the electronic structure is more striking as it facilitates the delocalization of the redox active orbital onto the thioether sulfur derived from Cys, thereby reducing the spin coupling between the [Y·-C] radical and the Cu(II) center (752 cm(-1)) relative to the unsubstituted [Y·] radical and the Cu(II) center (2210 cm(-1)). Energetically, the Tyr-Cys cross-link lowers the reduction potential by about 75 mV (calculated) allowing a more facile oxidation of the holo active site versus the site without the cross-link. Overall, the Tyr-Cys cross-link confers unique ground state properties on the GO active site that tunes its function in a remarkably nuanced fashion.

Plasticity in the copper–thioether bond: Manifestation in blue Cu proteins and in synthetic analogs

The binding of thioethers to transition metals in biological and biomimetic systems is reviewed with a focus on copper. Literature data show that copper(I) ions have a stronger tendency to bind thioethers, e.g. methionine-like ligands, than the isoelectronic Zn(II) ions. The plasticity in the Cu(II) coordination sphere, and the diffuseness of the lone pair electrons of a thioether sulfur, allow Cu(II)–S(thioether) bond distances to vary from 2.4 to 3.2Å, as shown by an in-depth analysis of protein structures (Protein Structure Database, PDB) and molecular structures of copper coordination compounds (Cambridge Structural Database, CSD).

Molecular Insight into the Ligand–IgG Interactions for 4-Mercaptoethyl-pyridine Based Hydrophobic Charge-Induction Chromatography

Hydrophobic charge-induction chromatography (HCIC) with 4-mercaptoethyl-pyridine (MEP) as the ligand is a novel technology for antibody purification. In the present work, the molecular simulation methods were used to investigate the interactions between MEP ligand and Fc fragment of IgG (Fc-A). Six ligands with different structures of spacer arm were studied with molecular docking and dynamics simulation at neutral and acidic pH. The binding modes and the interaction energies were analyzed. The results indicated that all ligands tested could bind into the selected pocket on the C(H2) domain of Fc-A at neutral pH. The pyridine ring on the top of MEP ligands acts as a major role to provide the hydrophobic association and hydrogen bond for the ligand-IgG binding; meanwhile, the sulfone group on the spacer arm might form the additional hydrogen bond and enhance the binding of ligand onto the surface of IgG. The replacements of thioether sulfur atom on the spacer arm with either nitrogen or oxygen atom seem to have little influence on the binding. The influences of pH on the ligand-IgG interactions were also studied with the molecular dynamics simulation. It was found that MEP ligands would departed from the surface of Fc-A at low pH due to the electrostatic repulsion. The ligands with a sulfone group on the spacer arm would weaken the electrostatic repulsion and need more acidic conditions for the departing of ligand. The molecular simulation results were in agreement with some experimental observations, which would be useful to elucidate the molecular mechanism of HCIC and design a novel ligand to improve the efficiency of antibody separation.

Unprecedented C-2 arylation of indole with diazonium salts: Syntheses of 2,3-disubstituted indoles and their antimicrobial activity.

A novel reaction of indole with aryldiazonium salts leading to the formation of 2-aryl-3-(arylazo)indoles was discovered. The products were found to possess potent anti-MRSA and anti-LLVRE activities. The SAR studies indicate that the potentially metabolically labile azo functionality can be replaced with ether oxygen and thioether sulfur atoms without any loss of activity.

A spectroscopic and electrochemical investigation of the oxidation pathway of glycyl-d,l-methionine and its N-acetyl derivative induced by gold(III)

The 1H nuclear magnetic resonance spectroscopy was applied to study the reaction of the dipeptide glycyl-d,l-methionine (H-Gly-d,l-Met-OH) and its N-acetylated derivative (Ac-Gly-d,l-Met-OH) with hydrogen tetrachloridoaurate(III) (H[AuCl4]). The corresponding peptide and [AuCl4]– were reacted in 1:1, 2:1, and 3:1 molar ratios, and all reactions were performed at pD 2.45 in 0.01 M DCl as solvent and at 25°C. It was found that the first step of these reactions is coordination of Au(III) to the thioether sulfur atom with formation of the gold(III)-peptide complex [AuCl3(R-Gly-Met-OH-S)] (R=H or Ac). This intermediate gold(III) complex further reacts with an additional methionine residue to generate the R-Gly-Met-OH chlorosulfonium cation as the second intermediate product, which readily undergoes hydrolysis to give the corresponding sulfoxide. The oxidation of the methionine residue in the reaction between H-Gly-d,l-Met-OH and [AuCl4]– was five times faster (k 2 = 0.363 ± 0.074 M-1s-1) in comparison to the same process with N-acetylated derivative of this peptide (k 2 = 0.074 ± 0.007 M-1s-1). The difference in the oxidation rates between these two peptides can be attributed to the free terminal amino group of H-Gly-d,l-Met-OH dipeptide. The mechanism of this redox process is discussed and, for its clarification, the reaction of the H-Gly-d,l-Met-OH dipeptide with [AuCl4]– was additionally investigated by UV-Vis and cyclic voltammetry techniques. From these measurements, it was shown that the [AuCl2]– complex under these experimental conditions has a strong tendency to disproportionate, forming [AuCl4]– and metallic gold. This study contributes to a better understanding of the mechanism of the Au(III)-induced oxidation of methionine and methionine-containing peptides in relation to the severe toxicity of anti-arthritic and anticancer gold-based drugs.

Copper(II) template synthesis of a new N2S2-donor macrocycle incorporating a pendent pyridyl substituent

The synthesis of the new potentially pentadentate ligand, 2,2′-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(sulfanediyl)diethanamine (L1), containing two thioether sulfurs, two –NH2 amines and a pyridyl nitrogen heteroatom is described. Reaction of L1 with copper(II) chloride and addition of hexafluorophosphate anion has led to isolation of the mixed anion complex Cu2(L1)2Cl(PF6)3. The synthesis and X-ray structure of cobalt(III) species, [Co(L1)Cl](PF6)2, is also reported. In situ reaction of L1 with copper(II) as a metal template in the presence of formaldehyde and the carbon acid nitroethane together with triethylamine (as base) led to macrocycle ring closure to yield [Cu(L2)Cl]PF6 (where L2 = 6,13-dimethyl-6-nitro-13-(pyridin-2-yl)-1,11-dithia-4,8-diazacyclotetradecane) whose X-ray structure shows that the copper centre has a distorted square pyramidal coordination geometry being bound by both –NH2 nitrogens and both sulfurs of L2 while the pyridyl nitrogen remains uncoordinated.

Synthesis, electrochemistry and metal binding properties of monosubstituted ferrocenoyl peptides with thioether-containing sidechains

Ferrocenoyl peptides incorporating thioether functionality respond more strongly to mercury(II) than to other heavy metal ions in solution. Compounds reported previously in this context are all 1,1′-disubstituted, and all include two or more sulfur-containing amino acids. To test whether two thioether groups are required for effective mercury binding by these systems, we have prepared a series of singly-substituted ferrocenoyl peptides from ferrocenecarboxylic acid and l-methionine, S-methyl- l-cysteine or S-trityl- l-cysteine, and tested them as electrochemical probes for mercury(II). Nine ferrocenoyl peptides have been synthesised using a Boc-protecting group strategy and HBTU-mediated peptide coupling. The electrochemical properties of these compounds have been determined using cyclic voltammetry, and all show fully reversible one electron oxidation steps. Forward sweep half wave peaks (E F), reverse sweep half wave peaks (E R), peak separations (ΔE P) and half wave potentials (E 1/2) are reported. Changes in the potential of the iron(II)/iron(III) redox couple of the ferrocene core have been used to quantify heavy metal–peptide interactions, revealing that these monotopic systems also respond more strongly to mercury(II) than to zinc(II), cadmium(II), silver(I) and lead(II). NMR experiments to characterise the peptide–mercury interaction implicate the thioether sulfur as the site of mercury binding and indicate 1:1 stoichiometry. The crystal structure of ferrocenoyl- S-methyl- l-cysteine methyl ester is also reported. The greater responsiveness of these systems to mercury(II) makes them interesting leads for the development of biologically inspired sensors for this toxic heavy metal.

SET7/9 Catalytic Mutants Reveal the Role of Active Site Water Molecules in Lysine Multiple Methylation

SET domain lysine methyltransferases (KMTs) methylate specific lysine residues in histone and non-histone substrates. These enzymes also display product specificity by catalyzing distinct degrees of methylation of the lysine ε-amino group. To elucidate the molecular mechanism underlying this specificity, we have characterized the Y245A and Y305F mutants of the human KMT SET7/9 (also known as KMT7) that alter its product specificity from a monomethyltransferase to a di- and a trimethyltransferase, respectively. Crystal structures of these mutants in complex with peptides bearing unmodified, mono-, di-, and trimethylated lysines illustrate the roles of active site water molecules in aligning the lysine ε-amino group for methyl transfer with S-adenosylmethionine. Displacement or dissociation of these solvent molecules enlarges the diameter of the active site, accommodating the increasing size of the methylated ε-amino group during successive methyl transfer reactions. Together, these results furnish new insights into the roles of active site water molecules in modulating lysine multiple methylation by SET domain KMTs and provide the first molecular snapshots of the mono-, di-, and trimethyl transfer reactions catalyzed by these enzymes.

Association of biotin with silver (I) in solution: a circular dichroism study

A strong polymeric helical complex of Ag(I) with biotin, a water-soluble vitamin also called vitamin H or B 7, has been described in the crystal phase, yet with missing spatial characteristic of its solution structure. In this work, the complexation of d-(+)-biotin with Ag + in aqueous solution has been studied by means of circular dichroism spectroscopy and mass spectrometry with electrospray ionization in positive mode (MS-ESI+). For comparison, the spectra of the Nujol suspension of studied systems in the solid state were also measured. The results indicate that the Ag(I)–biotin complex has a helical arrangement of the biotin molecules in solution. Our results show that the binding process involves the thioether sulfur atom and ureido carbonyl oxygen atom, while the carboxylate side group is not coordinated with the silver. The MS-ESI experiments have revealed the composite structures of Ag + with biotin of various constitutions, which represent the products of the polymeric helical Ag(I)–biotin complex.

Oxidation of Ru-bound thiolate thioether and its NS4-ligand containing thiolate and thioether sulfur donors: synthesis, characterization, and X-ray structures

A ruthenium complex [Ru(H2O){Et2NpyS4(O3)2}] · (4) containing sulfinates, sulfenates S-donors, and water as co-ligand has been synthesized from oxidation of hydrazine complex [Ru(N2H4)(Et2NpyS4)] (3) and completely characterized with X-ray structural analysis [Et2Npy= 4-(diethylamino)-2,6-bis[(2-mercaptophenyl)thiomethyl]pyridine(2−)]. Complex 4 exhibits distorted octahedral coordination of the ruthenium center and the d [Ru–Ssulfinates] and d [Ru–Ssulfenates] are almost equivalent. The average d[S–Osulfinate] at 148.0 pm is ca 10 pm longer than the average d [S–Osulfenate] at 138.0 pm. The sulfinates IR(KBr) bands are located in the range 1132–1113 cm−1 as two band sets, ν(SO)asym and ν(SO)sym, whereas the sulfenates show a single absorbance at approximately 882 cm−1. The lower frequency of the ν(SO) stretches of the sulfenates as compared to that of their sulfinate rivals indicates a weaker S–O bond in the sulfenates and is consistent with X-ray crystal structure data (vide infra). Oxidation of the dithiol ligand Et2NpyS4–H2 (1) under the same condition afforded the disulfide, in this case forming a macrocyclic ligand S,S-Et2NpyS4 (2) as is evidenced by an X-ray crystal structure determination. Thus, the oxidation of 3 involves activation of either the oxygen or thiolate by the ruthenium center.

<b>Synthesis, characterization and anti-tumour activity of iron(III) Schiff base complexes with unsymmetric tetradentate ligands</b>

The synthesis and characterization of two new iron(III) complexes, [Fe(pythsalI)]Cl2 and [Fe(pythsalBr)]Cl2 with the NSNO-donor tetradentate Schiff base ligands pythsalHX [(5–X-N-(2pyridylethylsulfanylethyl) salicylideneimine] (X = I, Br)) obtained from the inserted condensation of 1-(2-pyridyl)-3-thia-5-aminopentane with the respective derivative salicylaldehyde in a 1:1 molar ratio is reported. The iron(III) complexes were characterized by several techniques using elemental analysis (C, H, N), FT-IR, electronic spectra and molar conductance measurements. The elemental analysis data suggest the stoichiometry to be 1:2 [M:L] ratio formation. The molar conductance measurements reveal the presence of 1:2 electrolytic nature complexes. Infrared spectral data agreed with the coordination to the central metal ion through deprotonated phenolic oxygen, imine and pyridine type nitrogens and the thioether sulfur atoms. From ligand field spectral data an octahedral geometry is assigned to the iron(III) ion in all these complexes. These new compounds have showed anti-tumour activity against two kinds of cancer cells that are K562 (human chronic myeloid leukemia) and Jurkat (human T lymphocyte carcinoma). KEY WORDS: Iron(III) complexes, Tetradentate ligands, Unsymmetric Schiff base, NSNO-donor, Anti-tumour, K562, Jurkat Bull. Chem. Soc. Ethiop. 2010, 24(2), 193-199.

Heme-coordinating inhibitors of neuronal nitric oxide synthase. Iron-thioether coordination is stabilized by hydrophobic contacts without increased inhibitor potency.

The heme-thioether ligand interaction often occurs between heme iron and native methionine ligands, but thioether-based heme-coordinating (type II) inhibitors are uncommon due to the difficulty in stabilizing the Fe-S bond. Here, a thioether-based inhibitor (3) of neuronal nitric oxide synthase (nNOS) was designed, and its binding was characterized by spectrophotometry and crystallography. A crystal structure of inhibitor 3 coordinated to heme iron was obtained, representing, to our knowledge, the first crystal structure of a thioether inhibitor complexed to any heme enzyme. A series of related potential inhibitors (4-8) also were evaluated. Compounds 4-8 were all found to be type I (non-heme-coordinating) inhibitors of ferric nNOS, but 4 and 6-8 were found to switch to type II upon heme reduction to the ferrous state, reflecting the higher affinity of thioethers for ferrous heme than for ferric heme. Contrary to what has been widely thought, thioether-heme ligation was found not to increase inhibitor potency, illustrating the intrinsic weakness of the thioether-ferric heme linkage. Subtle changes in the alkyl groups attached to the thioether sulfur caused drastic changes in the binding conformation, indicating that hydrophobic contacts play a crucial role in stabilizing the thioether-heme coordination.

Iron(III) Schiff base complexes with asymmetric tetradentate ligands: synthesis, spectroscopy, and antimicrobial properties

The synthesis and characterization is reported of four iron(III) complexes of general formula [Fe(pythsalX)(H2O)2]Cl2, derived from the NSNO-donor tetradentate Schiff base ligands pythsalHX ([5-X-N-(2-pyridylethylsulfanylethyl)salicylideneimine] (X = OMe, N2Ph, I, NO2). The complexes were characterized by physico-chemical and spectroscopic methods. The thermal stabilities of both the free Schiff bases and their complexes were studied by differential scanning calorimetry and thermogravimetric analyses. The spectroscopic data suggest that the Schiff base ligands coordinate through deprotonated phenolic oxygen, imine, and pyridine-type nitrogens and the thioether sulfur atoms to give an octahedral geometry around the iron(III) atom in all these complexes. The free Schiff bases and their complexes have been screened for antimicrobial activities and the results show that the free Schiff bases are more potent antibacterials than the complexes.

Cisplatin Interaction with Cysteine and Methionine in Aqueous Solution: Computational DFT/PCM Study

In this paper we explore cisplatin interactions with sulfur-containing amino acids in a polarizable continuum model. Two cisplatin hydrated complexes were considered as reactants (chloro complex, cis-[Pt(NH3)2Cl(H2O)]+; hydroxo complex, cis-[Pt(NH3)2(OH)(H2O)]+). We considered the following reaction mechanism: first step, substitution of the aqua ligand by amino acid; second step, dissociative chelate formation. For the optimized complex (at the B3LYP/6-31+G(d)/COSMO level), the energy profile was determined using the B3LYP/6-311++G(2df,2pd) level and two different PCM models-COSMO and UAKS/DPCM methods which were adapted for use on transition metal complexes. The results show thermodynamic preference for bonding by cysteine sulfur followed by the amino group nitrogen, methionine thioether sulfur, and carboxyl-group oxygen. Methionine slightly prefers the Pt-N(Met) coordination in the chloro complex, but in the hydroxo complex it prefers the Pt-S(Met) coordination. A similar trend follows from the bonding energies: BE(Pt-S(Cys)) = 80.8 kcal/mol and BE(Pt-N(Met)) = 76 kcal/mol. According to the experimental observations, the most stable structures found are kappa2(S,N) chelates. In the case of methionine, the same thermodynamic stability is predicted also for the kappa2(N,O) chelate. This differs from the gas-phase results, where kappa2(S,N) and even kappa2(S,O) were found to be more stable than kappa2(N,O) complex.

Nickel-alkyl bond formation in the active site of methyl-coenzyme m reductase.

Methyl-coenzyme M reductase (MCR) catalyzes the methane-forming step in methanogenic archaea and most probably also the methane-oxidizing step in methanotrophic archaea. The enzyme contains coenzyme F(430) as prosthetic group. F(430) is a nickel porphinoid that has to be in the reduced Ni(I) state for the enzyme to be active. The presently discussed catalytic mechanisms of MCR can in principle be divided into two basic models. In one model the key intermediate features a methyl-Ni(III) species being either formed in a nucleophilic substitution reaction or in an oxidative addition reaction. In the other model first the thioether sulfur of methyl-coenzyme M binds to the Ni(I), which subsequently results in the release of the methyl group as methyl radical leaving behind a Ni(II)-sulfur bond. The experimental evidence for and against a methyl-nickel intermediate is reviewed.

Ion-pair binding by mixed N,S-donor 2-ureidopyridine ligands

The synthesis of a simple ambidentate ligand 1-(3-methylsulfanyl-phenyl)-3-pyridin-2-yl-urea (L) capable of binding metal ions via a pyridyl nitrogen atom or thioether sulfur donor is reported. The pyridyl functionality is located adjacent to an anion binding urea group allowing this region of the ligand to bind contact ion-pairs by simultaneous coordination and hydrogen bonding interactions. This intimate ion-pair binding coordination mode is demonstrated by the X-ray crystal structures of [Ag(L)]X (X = CH(3)CO(2)(-), 1; NO(3)(-), 2). The non-coordinating PF(6)(-) anion is not bound as an ion-pair in [Ag(L)](PF(6)) (3) and the urea NH groups exhibit short contacts to carbonyl oxygen and pi-systems. The X-ray crystal structure of [Ag(L)(4)]BF(4) (4) is also reported, showing the ligand to be exclusively S-bound. Anion binding by L and its silver(I) complex is also explored by solution (1)H NMR spectroscopic methods.

Versatility and dynamics of the copper(i) coordination sphere in sterically hindering tris(pyrazolyl)methane-incorporating macrobicycles

Two arene-capped macrobicycles (1 and 2) incorporating the tris(pyrazolyl)methane (Tpm) chelate have been prepared from a benzylthiol-functionalized Tpm precursor (3). Reaction of either macrobicycle with Cu(CH3CN)4+ leads to tetrahedral or trigonal-planar, fluxional complexes incorporating the Cu(CH3CN)+ subunit ([Cu(1)(CH3CN)]+ and [Cu(2)(CH3CN)]+). The acetonitrile ancillary ligand does not fit inside the macrobicycle cavity and can be removed by heating under vacuum, which produces the [Cu(1)]+ and [Cu(2)]+ species probably involving intramolecular thioether coordination. The [Cu(1)(CH3CN)]+ complex was shown to convert slowly in wet acetone into a helical coordination polymer, which is formulated as [Cu(1)(H2O)]nn+. In the crystal, the metal cation in tetrahedral geometry is bound to two pyrazole nitrogens and bridges two macrobicyclic subunits by intermolecular thioether sulfur coordination. Its coordination sphere is remarkably completed by a water molecule that is located inside the macrobicycle and is bound to the uncoordinated pyrazole fragment and to the mesitylene cap by OH⋯N and OH⋯π hydrogen bonding interactions.

Synthesis of an N<sub>3</sub>S<sub>2</sub>-type Ni(II) Complex Directed to NiSOD Active Site and Its Structural, Electrochemical, and Spectroscopic Properties

An N3S2-type Ni(II) complex (8) has been synthesized as a model compound of NiSOD active site, which is coordinated with an amide, an amine, and a pyridyl nitrogens and a thioether sulfur atom in the equatorial positions and with the remaining sulfur weakly bound at the axial position. The UV-vis spectroscopic and electrochemical characterizations of complex 8 showed that the square pyramidal structure was kept in the solution in both the Ni(II) and Ni(III) states. In Ni(III) state, EPR spectrum exhibited a rhombic signal pattern without any hyperfine splittings, indicating no structural change depending on oxidation states. If a protonation to the coordinated thiolate in Cys2 or Cys6 of NiSOD is caused during the oxidative disproportionation process, the thiolate may be kicked up to the axial position because the donor ability of the sulfur has been weakened.

The Proton Complex of a Diaza-macropentacycle: Structure, Slow Formation, and Chirality Induction by Ion Pairing with the Optically Active 1,1′-Binaphthyl-2,2′-diyl Phosphate Anion

The protonation of a sterically crowded [N2S6] macropentacycle (1) with 1 equiv of CF3SO3H in CDCl3 is slow and gives the singly (oo(+) [1 x H](+)) and doubly (o(+)o(+) [1 x 2H](2+)) protonated forms as kinetic products, the i(+)o form of [1 x H](+) being the thermodynamic product. i(+)o [1 x H](+) is C3 helically chiral in the solid state and in solution. The barrier to racemization (DeltaG(double dagger)) of the [1 x H](+) propeller is >71 kJ mol(-1). The ammonium proton is encapsulated in the tetrahedral coordination sphere provided by the endo (i) nitrogen bridgehead atom and the three proximal thioether sulfurs, which makes [1 x H](+) a proton complex. Use of the optically active acid (R)-(-)- or (S)-(+)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNPH) in chloroform allowed us to induce a significant diastereomeric excess (24% de), which produced a detectable ICD. The de was decreased in acetone-d6 (10%), suggesting that the sense of chirality of [1 x H](+) is controlled by ion-pair interactions. Detailed NMR studies allowed us to locate the chiral anion on the endo side of [1 x H](+), in the cavity lined by endo t-Bu groups, and to establish that the rate of anion exchange in [1 x H][(S,R)-(+/-)-BNP] was higher than the rate of propeller inversion of [1 x H](+).