trans-Cyano(6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine)cobalt(III) bis(perchlorate) hydrate and trans-hydroxo(6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine)cobalt(III) bis(perchlorate)

The crystal structures of a pair of closely related macrocyclic cyano- and hydroxopentaaminecobalt(III) complexes, as their perchlorate salts, are reported. Although the two complexes, [Co(CN)(C11H27N5)](ClO4)2.H2O and [Co(OH)(C11H27N5)](ClO4)(2), exhibit similar conformations, significant differences in the Co-N bond lengths arise from the influence of the sixth ligand (cyano as opposed to hydroxo). The ensuing hydrogen-bonding patterns are also distinctly different. Disorder in the perchlorate anions was clearly resolved and this was rationalized on the basis of distinct hydrogen-bonding motifs involving the anion O atoms and the N-H and O-H donors.

Geometric isomers of chloro(6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine)cobalt(III) tetrachlorozincate(II)

The crystal structures of a pair of cis and trans isomers of the macrocyclic chloropentaamine title complex, as their tetrachlorozincate(II) salts, [CoCl(C11H27N5)][ZnCl4], are reported. The two distinct isomeric forms lead to significant variations in the Co-N bond lengths and, furthermore, hydrogen bonding between the complex ions is influenced by the folded (cis) or planar (trans) conformations of the coordinated ligand.

Structural definition of the low-temperature phase transition of tris(ethane-1,2-diamine)zinc(II) dinitrate

The 93 K X-ray crystal structure of tris(ethane-1,2-diamine)zinc(II) dinitrate is reported. As predicted by the spectroscopic studies of other workers, there is a reversible phase transition of the structure at low temperature. We have determined this temperature to be 143 K. The structure at this temperature and below resembles that of the room temperature structure, except the crystallographic D-3 symmetry of the complex cation (296 K) is lowered to C-2 ( below 144 K) by subtle changes in cation-anion hydrogen bonding. No change in the conformation of the cation or its bond lengths and angles was found.

Disulfide bond mutagenesis and the structure and function of the head-to-tail macrocyclic trypsin inhibitor SFTI-1

SFTI-1 is a novel 14 amino acid peptide comprised of a circular backbone constrained by three proline residues, a hydrogen-bond network, and a single disulfide bond. It is the smallest and most potent known Bowman-Birk trypsin inhibitor and the only one with a cyclic peptidic backbone. The solution structure of [ABA(3,11)]SFTI-1, a disulfide-deficient analogue of SFTI-1, has been determined by H-1 NMR spectroscopy. The lowest energy structures of native SFTI-1 and [ABA(3,11)]SFTI-1 are similar and superimpose with a root-mean-square deviation over the backbone and heavy atoms of 0.26 +/- 0.09 and 1.10 +/- 0.22 Angstrom, respectively. The disulfide bridge in SFTI-1 was found to be a minor determinant for the overall structure, but its removal resulted in a slightly weakened hydrogen-bonding network. To further investigate the role of the disulfide bridge, NMR chemical shifts for the backbone H-alpha protons of two disulfide-deficient linear analogues of SFTI-1, [ABA(3,11)]SFTI-1[6,5] and [ABA(3,11)]SFTI-1[1,14] were measured. These correspond to analogues of the cleavage product of SFTI-1 and a putative biosynthetic precursor, respectively. In contrast with the cyclic peptide, it was found that the disulfide bridge is essential for maintaining the structure of these open-chain analogues. Overall, the hydrogen-bond network appears to be a crucial determinant of the structure of SFTI-1 analogues.

Crystal structures of reduced and oxidized DsbA: investigation of domain motion and thiolate stabilization

Background: The redox proteins that incorporate a thioredoxin fold have diverse properties and functions. The bacterial protein-folding factor DsbA is the most oxidizing of the thioredoxin family. DsbA catalyzes disulfide-bond formation during the folding of secreted proteins, The extremely oxidizing nature of DsbA has been proposed to result from either domain motion or stabilizing active-site interactions in the reduced form. In the domain motion model, hinge bending between the two domains of DsbA occurs as a result of redox-related conformational changes. Results: We have determined the crystal structures of reduced and oxidized DsbA in the same crystal form and at the same pH (5.6). The crystal structure of a lower pH form of oxidized DsbA has also been determined (pH 5.0). These new crystal structures of DsbA, and the previously determined structure of oxidized DsbA at pH 6.5, provide the foundation for analysis of structural changes that occur upon reduction of the active-site disulfide bond. Conclusions: The structures of reduced and oxidized DsbA reveal that hinge bending motions do occur between the two domains. These motions are independent of redox state, however, and therefore do not contribute to the energetic differences between the two redox states, instead, the observed domain motion is proposed to be a consequence of substrate binding. Furthermore, DsbA's highly oxidizing nature is a result of hydrogen bond, electrostatic and helix-dipole interactions that favour the thiolate over the disulfide at the active site.

Small molecular probes for G-protein-coupled C5a receptors: Conformationally constrained antagonists derived from the C terminus of the human plasma protein C5a

Activation of the human complement system of plasma proteins in response to infection or injury produces a 4-helix bundle glycoprotein (74 amino acids) known as C5a. C5a binds to G-protein-coupled receptors on cell surfaces triggering receptor-ligand internalization, signal transduction, and powerful inflammatory responses. Since excessive levels of C5a are associated with autoimmune and chronic inflammatory disorders, inhibitors of receptor activation may have therapeutic potential. We now report solution structures and receptor-binding and antagonist activities for some of the first small molecule antagonists of C5a derived from its hexapeptide C terminus. The antagonist NMe-Phe-Lys-Pro-D-Cha-Trp-D-Arg-CO2H (1) surprisingly shows an unusually well-defined solution structure as determined by H-1 NMR spectroscopy. This is one of the smallest acyclic peptides found to possess a defined solution conformation, which can be explained by the constraining role of intramolecular hydrogen bonding. NOE and coupling constant data, slow deuterium exchange, and a low dependence on temperature for the chemical shift of the D-Cha-NH strongly indicate an inverse gamma turn stabilized by a D-Cha-NH ... OC-Lys hydrogen bond. Smaller conformational populations are associated with a hydrogen bond between Trp-NH ... OC-Lys, defining a type II beta turn distorted by the inverse gamma turn incorporated within it. An excellent correlation between receptor-affinity and antagonist activity is indicated for a limited set of synthetic peptides. Conversion of the C-terminal carboxylate of 1 to an amide decreases antagonist potency 5-fold, but potency is increased up to 10-fold over 1 if the amide bond is made between the C-terminal carboxylate and a Lys/Orn side chain to form a cyclic analogue. The solution structure of cycle 6 also shows gamma and beta turns; however, the latter occurs in a different position, and there are clear conformational changes in 6 vs 1 that result in enhanced activity. These results indicate that potent C5a antagonists can be developed by targeting site 2 alone of the C5a receptor and define a novel pharmacophore for developing powerful receptor probes or drug candidates.

Complexes of 1-thia-4,7,10-triazacyclododecane ([12]aneN(3)S) - Synthesis, structure and stability constants

The 12-membered macrocyclic ligand 1-thia-4,7, 10-triazacyclododecane ([12]aneN(3)S) has been synthesised, although upon crystallization from acetonitrile a product in which carbon dioxide had added to one secondary amine in the macrocyclic ring (H[12]aneN(3)SCO(2). H2O) was isolated and subsequently characterised by X-ray crystallography. The protonation constants for [12]aneN(3)S and stability constants with Zn(II), Pb(II), Cd(II) and Cu(II) have been determined either potentiometrically or spectrophotometrically in aqueous solution, and compared with those measured or reported for the ligands 1-oxa-4,7,10-triazacyclododecane ([12]aneN(3)O) and 1,4,7,10-tetraazacyclododecane ([12]aneN(4)). The magnitudes of the stability constants are consistent with trends observed previously for macrocyclic ligands as secondary amine donors are replaced with oxygen and thioether donors although the stability constant for the [Hg([12]aneN(4))](2+) complex has been estimated from an NMR experiment to be at least three orders of magnitude larger than reported previously. Zinc(II), mercury(II), lead(II), copper(II) and nickel(II) complexes of [12]aneN(3)S have been isolated and characterised by X-ray crystallography. In the case of copper(II), two complexes [Cu([12]aneN(3)S)(H2O)](ClO4)(2) and [Cu-2([12]aneN(3)S)(2)(OH)(2)](ClO4)(2) were isolated, depending on the conditions employed. Molecular mechanics calculations have been employed to investigate the relative metal ion size preferences of the [3333], asym-[2424] and sym-[2424] conformation isomers. The calculations predict that the asym-[2424] conformer is most stable for M-N bond lengths in the range 2.00-2.25 Angstrom whilst for the larger metal ions the [3333] conformer is dominant. The disorder seen in the structure of the [Zn([12]aneN(3)S)(NO3)](+) complex is also explained by the calculations. (C) 1999 Elsevier Science Ltd. All rights reserved.

Mesoionic pyridopyrimidinylium and pyridooxazinylium olates and non-mesoionic pyridopyrimidinones. Structures in the solid state, solution, and matrices

X-Ray crystal structures, C-13 NMR spectra and theoretical calculations (B3LYP/6-31G*) are reported for the mesoionic (zwitterionic) pyridopyrimidinylium- and pyridooxazinyliumolates 2a, 3a and 5a,b as well as the enol ether 11b and the enamine 11c. The 1-NH compounds like 1a, 2a and 3a exist in the mesoionic form in the crystal and in solution, but the OH tautomers such as 1b and 2b dominate in the gas phase as revealed by the Ar matrix IR spectra in conjunction with DFT calculations. All data indicate that the mesoionic compounds can be regarded as intramolecular pyridine-ketene zwitterions (cf. 16 --> 17) with a high degree of positive charge on the pyridinium nitrogen, a long pyridinium N-CO bond (ca. 1.44-1.49 Angstrom), and normal C=O double bonds (ca. 1.22 Angstrom). All mesoionic compounds exhibit a pronounced tilting of the olate C=O groups (the C=O groups formally derived from a ketene) towards the pyridinium nitrogen, giving NCO angles of 110-118 degrees. Calculations reveal a hydrogen bond with 6-CH, analogous to what is found in ketene-pyridine zwitterions and the C3O2-pyridine complex. The 2-OH tautomers of type 1b, 2b, and 11 also show a high degree of zwitterionic character as indicated by the canonical structures 11 12.

Determination of the stability constants of mercury(II) complexes of mixed donor macrobicyclic encapsulating ligands

The reactions of mercury(II) with the mixed donor encapsulating ligands 3,6,16-trithia-6,11,19-triazabicyclo[6.6.6]icosane (AMN(3)S(3)sar) and 1-amino-8-methyl-6,19-dithia-3,10,13,16-tetraazabicyclo[6.6.6]icosane (AMN(4)S(2)sar) have been studied. NMR ligand-ligand competition experiments with the ligands 1,4,8,11-tetraazaeyclotetradecane ([14]aneN(4)), 1-thia-4,7,10-triazacyclododecane ([12]aneN(3)S) and ethylenediaminetetraacetic acid (EDTA) with AMN(3)S(3)sar and Hg(II) indicated that [14]aneN(4) would be an appropriate competing ligand for the, determination of the Hg(II) stability constant. Calculations indicated the ratio of concentrations of AMN3S3sar, [14]aneN(4) and Hg(II) required for the determination of the stability constant ranged from 1:1:1 to 1:5:1. Refinement of the titration curves yielded log(10)K[Hg(AMN(3)S(3)sar)](2+) = 17.7. A similar competition titration resulted in the determination of the stability constant for the AMN(4)S(2)sar system as log(10)K[Hg(AMN(4)S(2)sar)](2+) = 19.5. The observed binding constants for the mixed N/S donor systems and the hexaaza analogues sar (3,6,10,13,16,19-hexaazabicyclo [6.6.6]icosane) and diamsar (1,8-diamino-3,6,10,13,16,19 -hexazabicyclo [6.6.6] icosane (log(10)K-[Hg(diamsar)](2+) = 26.4; log(10)K[Hg(sar)](2+) = 28.1) differ by approximately ten orders of magnitude. The difference is ascribed not to a cryptate effect but to a mismatch in the Hg-N and Hg-S bond lengths in the N/S systems.

Cyanocobalt(III) complexes of penta- and tetradentate-coordinated macrocyclic hexaamines

The pendent-arm macrocyclic hexaamine trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L) may coordinate in tetra-, penta- or hexadentate modes, depending on the metal ion and the synthetic procedure. We report here the crystal structures of two pseudo-octahedral cobalt(III) complexes of L, namely sodium trans-cyano(trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine)cobalt(III) triperchlorate, Na[Co(CN)(C13H30N6)](ClO4)(3) or Na{trans-[CoL(CN)]}(ClO4)(3), (I), where L is coordinated as a pentadentate ligand, and trans-dicyano(trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) cobalt (III) trans-dicyano (trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diaminium)cobalt(III) tetraperchlorate tetrahydrate, [Co(CN)(2)(Cl4H32N6)][Co(CN)(2)(Cl4H30N6)](ClO4)(4)•-4H(2)O or trans-[CoL(CN)(2)]trans-[Co(H2L)(CN)(2)] (ClO4)(4)•-4H(2)O, (II), where the ligand binds in a tetradentate mode, with the remaining coordination sites being filled by C-bound cyano ligands. In (I), the secondary amine Co-N bond lengths lie within the range 1.944 (3)-1.969 (3) &ANGS;, while the trans influence of the cyano ligand lengthens the Co-N bond length of the coordinated primary amine [Co-N = 1.986 (3) &ANGS;]. The Co-CN bond length is 1.899 (3) &ANGS;. The complex cations in (11) are each located on centres of symmetry. The Co-N bond lengths in both cations are somewhat longer than in (I) and span a narrow range [1.972 (3)-1.982 (3) &ANGS;]. The two independent Co-CN bond lengths are similar [1.918 (4) and 1.926 (4) &ANGS;] but significantly longer than in the structure of (1), again a consequence of the trans influence of each cyano ligand.

On the structure, electrochemistry, and spectroscopy of the (N,N'-bis(2'-(dimethylamino)ethyl)-N,N'-dimethylpropane-1,3-diamine)copper(II) ion

The synthesis, spectroscopy, and electrochemistry of the acyclic tertiary tetraamine copper(II) complex [CuL(1)](ClO4)(2) (L(1) = N,N-bis(2'-(dimethylamino)ethyl)-N,N'-dimethylpropane-1,3-diamine) is reported. The X-ray crystal structure of [CuL(1)(OClO3)(2)] reveals a tetragonally elongated CuN4O2 coordination sphere, exhibiting relatively long Cu-N bond lengths for a Cu-II tetraamine, and a small tetrahedral distortion of the CuN4 plane. The [CuL(1)](2+) ion displays a single, reversible, one-electron reduction at -0.06 V vs Ag/AgCl. The results presented herein illustrate the inherent difficulties associated with the separation and characterization of Cu-II complexes of tertiary tetraamines, and some previously incorrect assertions and unexplained observations of other workers are discussed.

Structural and electron self-exchange rate variations in isomeric (hexaamine)cobalt(III/II) complexes

The syntheses and characterisation of the new macrocyclic hexaamine trans-(5(S),7(S),12(R),14(R)-tetramethyl)-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L-6) and its Co-III complex are reported. The X-ray crystal structural analyses of [CoL6]Cl-2(ClO4) [monoclinic, space group C2/c, a = 16.468(3) Angstrom, b = 9.7156(7) Angstrom, c = 15.070(3) Angstrom, beta = 119.431(8)degrees, Z = 4] and the closely related cis-diamino-substituted macrocyclic complex [CoL2](ClO4)(3) . 2H(2)O (L-2 = cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) [orthorhombic, space group Pna2(1), a = 16.8220(8) Angstrom, b = 10.416(2) Angstrom, c = 14.219(3) Angstrom, Z = 4] reveal significant variations in the observed Co-N bond lengths and coordination geometries, which may be attributed to the trans or cis disposition of the pendent primary amines. The Co-III/II self-exchange electron transfer rate constants for these and other closely related hexaamines have been determined, and variations of some 2 orders of magnitude are found between pairs of trans and cis isomeric Co-III complexes.

Structural Dynamics in (ND4)2[Cu(D2O)6](SO4)2

The extended X-ray absorption fine structure spectroscopy (EXAFS) of (ND4)(2)[CU(D2O)(6)](SO4)(2) at 5, 14,100, 200, and 298 K is reported. This indicates that the Cu-O bond lengths of the Cu(D2O)(6)(2+) ion do not change significantly within this temperature range, which contrasts with EPR results and X-ray and neutron diffraction experiments, which imply that two of the Cu-(D2O) bonds converge in length as the temperature is raised. The EXAFS measurements thus confirm that the bond distances yielded by the diffraction experiments refer to the average positions of ligands involved in a dynamic equilibrium in which the directions of the long and intermediate bonds of the Jahn-Teller distorted Cu(D2O)(6)(2+) ion are interchanged in the crystal lattice. Analysis of the displacement parameters is consistent with this interpretation, as are the wave functions calculated using a model involving Jahn-Teller vibronic coupling and the influence of lattice strain interactions.

Characterization of [Fe(AMN3S3sarH)]3+ - a rigorously low-spin iron(II) complex

The iron(II) complex [Fe(AMN(3)S(3)sarH)](ClO4)(3).3H(2)O (AMN(3)S(3)sarH = 8-ammonio-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]icosane) has been synthesized and characterized by single crystal structure and spectroscopic methods. The Fe(II)-S(thiaether) bond lengths are short, indicative of a large degree of metal-ligand orbital mixing (pi-acceptor character) of the thiaether ligand. The complex is stable to metal centred oxidation. (C) 2002 Elsevier Science Ltd. All rights reserved.

Crystal structures of free, IMP-, and GMP-bound Escherichia coli hypoxanthine phosphoribosyltransferase

Crystal structures have been determined for free Escherichia coli hypoxanthine phosphoribosyltransferase (HPRT) (2.9 Angstrom resolution) and for the enzyme in complex with the reaction products, inosine 5'-monophosphate (IMP) and guanosine 5-monophosphate (GMP) (2.8 Angstrom resolution). Of the known 6-oxopurine phosphoribosyltransferase (PRTase) structures, E. coli HPRT is most similar in structure to that of Tritrichomonas foetus HGXPRT, with a rmsd for 150 Calpha atoms of 1.0 Angstrom. Comparison of the free and product bound structures shows that the side chain of Phe156 and the polypeptide backbone in this vicinity move to bind IMP or GMP. A nonproline cis peptide bond, also found in some other 6-oxopurine PRTases, is observed between Leu46 and Arg47 in both the free and complexed structures. For catalysis to occur, the 6-oxopurine PRTases have a requirement for divalent metal ion, Usually Mg2+ in vivo. In the free structure, a Mg2+, is coordinated to the side chains of Glu103 and Asp104. This interaction may be important for stabilization of the enzyme before catalysis. E. coli HPRT is unique among the known 6-oxopurine PRTases in that it exhibits a marked preference for hypoxanthine as substrate over both xanthine and guanine. The structures suggest that its substrate specificity is due to the modes of binding of the bases. In E. coli HPRT, the carbonyl oxygen of Asp 163 would likely form a hydrogen bond with the 2-exocyclic nitrogen of guanine (in the HPRT-guanine-PRib-PP-Mg2+ complex). However, hypoxanthine does not have a 2-exocyclic atom and the HPRT-IMP structure suggests that hypoxanthine is likely to occupy a different position in the purine-binding pocket.