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PubMed1999

Synthesis of a Novel Class of Trigonal Bipyramidal Nitrido Tc(V) Complexes with Phosphino-Thiol Ligands. Crystal Structure of [(99g)Tc(N)(L(1))(2)] [L(1) = 2-(Diphenylphosphino)ethanethiolato] and [(99g)Tc(N)(L(5))(2)] [L(5) = 2-(Ditolylphosphino)propanethiolato].

Reactions of the precursor complexes [(99g)Tc(N)Cl(2)(PPh(3))(2)] and [(99g)Tc(N)Cl(4)](-) with phosphine-thiol ligands (HL(n)()) of the type R(2)PCH(2)CH(2)SH (R = phenyl, methoxypropyl), R(2)'PCH(2)CH(2)CH(2)SH (R' = phenyl, tolyl), and R(2)' 'P-o-C(6)H(4)SH (R' ' = phenyl) afforded the five-coordinated, disubstituted nitrido technetium(V) complexes [(99g)Tc(N)(L(n)())(2)]. The complexes were characterized by elemental analysis, (1)H and (31)P NMR spectroscopy, FT IR, and positive FAB MS spectra. Structural characterization of [(99g)Tc(N)(L(1))(2)] (1) [HL(1) = (C(6)H(5))(2)PCH(2)CH(2)SH] and [(99g)Tc(N)(L(5))(2)] (5) [HL(5) = (o-CH(3)C(6)H(4))(2)PCH(2)CH(2)CH(2)SH] showed that the bidentate phosphino-thiol ligands are coordinated to the technetium center through the neutral phosphorus atom and the deprotonated thiol sulfur atom. These complexes possess an uncommon trigonal bipyramidal geometry with the two phosphorus atoms occupying the two transaxial positions and the two sulfur atoms on the equatorial plane along with the nitrido nitrogen atom. Compound 1 crystallizes in the monoclinic space group C2/c, a = 24.84(2) Å, b = 7.327(6) Å, c = 31.52(2) Å, beta = 111.06(10) degrees, and Z = 8. Compound 5 crystallizes in the monoclinic space group P2(1)/n, a = 11.090(1) Å, b = 14.387(2) Å, c = 11.087(1) Å, beta = 113.62(1) degrees, and Z = 2.

Journal Article↗

PubMed2006

Characterization and crystal structure of cadmium(II) halide complexes with amino acids and their derivatives: VII. Crystal structures of aquadibromo(3-aminopropanoic acid)cadmium(II), dichloro(4-aminobutanoic acid)cadmium(II), diaquabis(aminohexanoic acid)cadmium(II) tetrachlorocadmium(II), and dibromo(azetidine-3-carboxylic acid)cadmium(II).

Seven cadmium complexes: [CdX2(Hapro)(H2O)n] (X: Cl(1), Br(2)), [CdX2(Hgaba)] (X: Cl(3), Br(4)), [Cd(Hahex)2(H2O)2][CdCl4] (5), and [CdX2(Haze-3)](H2O)n (X: Cl(6), Br(7)) have been prepared and investigated by means of IR and FT Raman spectra. The crystal and molecular structures of 2, 3, 5 and 7 were determined by a single-crystal X-ray diffraction method. In complex 2, the cadmium atom is in a distorted octahedral geometry, ligated by two carboxyl oxygen atoms of Hapro, a water molecule, and three bromine atoms; one is terminal and each of the other two is bridging two cadmium atoms to make a polymer. The structure of 3 consists of one-dimensional polymers bridged by two chlorine atoms and a carboxyl group. The carboxyl oxygen atoms of Hgaba coordinate forkedly to two cadmium atoms. The cadmium atom of [Cd(Hahex)2(H2O)2]2+ in complex 5 is in a distorted octahedral geometry, ligated by four carboxyl oxygen atoms of two molecules of Hahex and by two water molecules. [Cd(Hahex)2(H2O)2]2+ exists between two layers which are formed of infinite [CdCl4]2- chains. The carboxyl oxygen atoms of Hahex coordinate to the same cadmium atom. In complex 7, the cadmium atom is ligated by two carboxyl oxygen atoms and four bridging bromine atoms to make a polymer.

Amino Acids↗

PubMed2002

Geometric and chemical patterns of interaction in protein--ligand complexes and their application in docking.

We present a new method for representing the binding site of a protein receptor that allows the use of the DOCK approach to screen large ensembles of receptor conformations for ligand binding. The site points are constructed from templates of what we called "attached points" (ATPTS). Each template (one for each type of amino acid) is composed of a set of representative points that are attached to side-chain and backbone atoms through internal coordinates, carry chemical information about their parent atoms and are intended to cover positions that might be occupied by ligand atoms when complexed to the protein. This method is completely automatic and proved to be extremely fast. With the aim of obtaining an experimental basis for this approach, the Protein Data Bank was searched for proteins in complex with small molecules, to study the geometry of the interactions between the different types of protein residues and the different types of ligand atoms. As a result, well-defined patterns of interaction were obtained for most amino acids. These patterns were then used for constructing a set of templates of attached points, which constitute the core of the ATPTS approach. The quality of the ATPTS representation was demonstrated by using this method, in combination with the DOCK matching and orientation algorithms, to generate correct ligand orientations for >1000 protein--ligand complexes.

Amino Acids↗

PubMed2005

trans-Diaquabis(6-hydroxypicolinato-kappa2N,O2)copper(II).

In the title compound, [Cu(C6H4NO3)2(H2O)2], the Cu(II) ion lies on an inversion centre and has an elongated octahedral environment, equatorially trans-coordinated by two N,O-bidentate picolinate ligands and axially coordinated by two water O atoms. The complex molecules form layers, which are linked by O-H...O hydrogen bonds between the aqua ligands and neighbouring carboxylate groups. An intramolecular hydrogen bond between the coordinated carboxylate O atom and the hydroxy H atom is also observed.

Copper↗

PubMed2003

AFM characterization of tilt and intrinsic flexibility of Rhodobacter sphaeroides light harvesting complex 2 (LH2).

Atomic force microscopy (AFM) has developed into a powerful tool to investigate membrane protein surfaces in a close-to-native environment. Here we report on the surface topography of Rhodobacter sphaeroides light harvesting complex 2 (LH2) reconstituted into two-dimensional crystals. These photosynthetic trans-membrane proteins formed cylindrical oligomeric complexes, which inserted tilted into the lipid membrane. This peculiar packing of an integral membrane protein allowed us to determine oligomerization and tilt of the LH2 complexes, but also protrusion height and intrinsic flexibility of their individual subunits. Furthermore the surface contouring reliability and limits of the atomic force microscopy could be studied. The two-dimensional crystals examined had sizes of up to 5 microm and, as revealed by a 10 A cryo electron microscopy projection map, p22(1)2(1) crystal symmetry. The unit cell had dimensions of a = b = 150 A and gamma = 90 degrees, and housed four nonameric complexes, two pointing up and two pointing down. AFM topographs of these 2D crystals had a lateral resolution of 10 A. Further, the high vertical resolution of approximately 1 A, allowed the protrusion height of the cylindrical LH2 complexes over the membrane to be determined. This was maximally 13.1 A on one side and 3.8 A on the other. Interestingly, the protrusion height varied across the LH2 complexes, showing the complexes to be inserted with a 6.2 degree tilt with respect to the membrane plane. A detailed analysis of the individual subunits showed the intrinsic flexibility of the membrane protruding peptide stretches to be equal and independent of their protrusion height. Furthermore, our analysis of membrane proteins within this peculiar packing confirmed the high vertical resolution of the atomic force microscopy on biological samples, and led us to conclude that the image acquisition function was equally accurate for contouring protrusions with heights up to approximately 15 A.

Amino Acid Sequence↗

PubMed2006

Scoring a diverse set of high-quality docked conformations: a metascore based on electrostatic and desolvation interactions.

Predicting protein-protein interactions involves sampling and scoring docked conformations. Barring some large structural rearrangement, rapidly sampling the space of docked conformations is now a real possibility, and the limiting step for the successful prediction of protein interactions is the scoring function used to reduce the space of conformations from billions to a few, and eventually one high affinity complex. An atomic level free-energy scoring function that estimates in units of kcal/mol both electrostatic and desolvation interactions (plus van der Waals if appropriate) of protein-protein docked conformations is used to rerank the blind predictions (860 in total) submitted for six targets to the community-wide Critical Assessment of PRediction of Interactions (CAPRI; http://capri.ebi.ac.uk). We found that native-like models often have varying intermolecular contacts and atom clashes, making unlikely that one can construct a universal function that would rank all these models as native-like. Nevertheless, our scoring function is able to consistently identify the native-like complexes as those with the lowest free energy for the individual models of 16 (out of 17) human predictors for five of the targets, while at the same time the modelers failed to do so in more than half of the cases. The scoring of high-quality models developed by a wide variety of methods and force fields confirms that electrostatic and desolvation forces are the dominant interactions determining the bound structure. The CAPRI experiment has shown that modelers can predict valuable models of protein-protein complexes, and improvements in scoring functions should soon solve the docking problem for complexes whose backbones do not change much upon binding. A scoring server and programs are available at http://structure.pitt.edu.

Models, Biological↗

PubMedDate not supplied

Mcg in 2030: new techniques for atomic position determination of immune complexes.

The lambda-type light chain dimer from a patient (Mcg) with multiple myeloma and amyloidosis was a pioneer protein for determining the three-dimensional structures of immunoglobulins, understanding the effects of ligand binding, and exploring the use of combinatorial methods to identify novel peptides complementary to protein active sites. Despite 30 years of intense study, there are still unanswered questions about the structure of the Mcg dimer, especially with respect to positions of hydrogen atoms and solvent molecules. In the present report, we describe two techniques that will help define the roles of solvent in ligand interactions and complex formation with this immunoglobulin fragment: (1) introduction of helium as a cryogenic agent during X-ray data collection; and (2) addition of neutron diffraction analyses. These techniques should provide improved resolution, and a more accurate structure of the Mcg dimer. Resolution enhancements of 0.5 A have been achieved in preliminary experiments with cryogenic helium, as compared with the best X-ray diffraction data obtained previously. In the near future, neutron diffraction studies should produce the first hydrogen structure for the Mcg dimer and help elucidate the ligand preferences and amyloidogenic properties of this eminently useful protein.

Amyloidosis↗

PubMed2002

Metal complexes of tetrapodal ligands: synthesis, spectroscopic and thermal studies, and X-ray crystal structure studies of Na(I), Ca(II), Sr(II), and Ba(II) complexes of tetrapodal ligands N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine and N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine.

Twelve complexes 1-12 of general category [M(ligand)(anion)(x)(water)(y)], where ligand = N,N,N',N'-tetrakis(2-hydroxypropyl/ethyl)ethylenediamine (HPEN/HEEN), anion = anions of picric acid (PIC), 3,5-dinitrobenzoic acid (DNB), 2,4-dinitrophenol (DNP), and o-nitrobenzoic acid (ONB), M = Ca(2+), Sr(2+), Ba(2+), or Na(+), x = 1 and 2, and y = 0-4, were synthesized. All of these complexes were characterized by elemental analysis, IR, (1)H and (13)C NMR, and thermal studies. X-ray crystal studies of these complexes 1-12, [Ca(HPEN)(H(2)O)(2)](PIC)(2).H(2)O (1), [Ca(HEEN)(PIC)](PIC) (2), Ba(HPEN)(PIC)(2) (3), [Na(HPEN)(PIC)](2) (4), Ca(HPEN)(H(2)O)(2)](DNB)(2).H(2)O (5),Ca(HEEN)(H(2)O)](DNB)(2).H(2)O (6), [Sr(HPEN)(H(2)O)(3)](DNB)(2) (7), [Ba(HPEN)(H(2)O)(2)](DNB)(2).H(2)O](2) (8), [[Ba(HEEN)(H(2)O)(2)](ONB)(2)](2) (9), [[Sr(HPEN)(H(2)O)(2)](DNP)(2)](2) (10), [[Ba(HPEN)(H(2)O)(2)](DNP)(2)](2) (11), and [Ca(HEEN)(DNP)](DNP) (H(2)O) (12), have been carried out at room temperature. Factors which influence the stability and the type of complex formed have been recognized as H-bonding interactions, presence/absence of solvent, nature of the anion, and nature of the cation. Both the ligands coordinate the metal ion through all the six available donor atoms. The complexes 1 and 5-11 have water molecules in the coordination sphere, and their crystal structures show that water is playing a dual character. It coordinates to the metal ion on one hand and strongly hydrogen bonds to the anion on the other. These strong hydrogen bonds stabilize the anion and decrease the cation-anion interactions by many times to an extent that the anions are completely excluded out of the coordination sphere and produce totally charge-separated complexes. In the absence of water molecules as in 2 and 3 the number of hydrogen bonds is reduced considerably. In both the complexes the anions case interact more strongly with the metal ion to give rise to a partially charge-separated 2 or tightly ion-paired 3 complex. High charge density Ca(2+) forms only monomeric complexes. It has more affinity toward stronger nucleophiles such as DNP and PIC with which it gives partially charge-separated eight-coordinated complexes. But with relatively weaker nucleophile like DNB, water replaces the anion and produces a seven coordinated totally charge-separated complex. Sr(2+) with lesser charge/radius ratio forms only charge-separated monomeric as well as dimeric complexes. Higher coordination number of Sr(2+) is achieved with coordinated water molecules which may be bridging or nonbridging in nature. All charge-separated complexes of the largest Ba(2+) are dimeric with bridging water molecules. Only one monomeric ion-paired complex was obtained with Ba(PIC)(2). Na(+) forms a unique dinuclear cryptand-like complex with HPEN behaving as a heptadentate chelating-cum-bridging ligand.

Journal Article↗

PubMed2001

Structure solution of the basic decagonal Al-Co-Ni phase by the atomic surfaces modelling method.

The atomic surfaces modelling technique has been used to solve the structure of the basic Ni-rich Al-Co-Ni decagonal phase. Formula Al70.6Co6.7Ni22.7, space group P10, five-dimensional unit-cell parameters: d1 = d4 = 4.752 (3) A, d2 = d3 = 3.360 (2) A, d5 = 8.1710 (2) A; alpha12 = alpha34 = 69.295 degrees, alpha13 = alpha24 = 45 degrees, alpha14 = 41.410 degrees, alpha23 = alphai5 = 90 degrees (i = 1-4), V = 291.2 (7) A5; D(x) = 3.887 Mg x m(-3). Refinement based on /F/; 2767 unique reflections (/F/ > 0), 749 parameters, R = 0.17, wR = 0.06. Describing the structure of quasicrystals embedded in n-dimensional superspace in principle takes advantage of n-dimensional periodicity to select the minimal set of degrees of freedom for the structure. The method of modelling of the atomic surfaces yielded the first fully detailed structure solution of this phase. Comparison with numerous former, less accurate models confirms several features already derived, but adds a new essential insight of the structure and its complexity. The atoms fill the space forming recurrent structure motifs, which we will (generically) refer to as clusters. However, no unique cluster exists, although differences are small. Each cluster shows a high degree of structural disorder. This gives rise to a large configurational entropy, as much as expected in a phase which is stable at high temperature. On the other side, the cluster spatial arrangement is perfectly quasiperiodic. These considerations, corroborated by analysis of the structural relationship with neighbouring periodic phases, strongly suggest the existence of a non-local, long-range interaction term in the total energy which may be essential to the stability.

Journal Article↗

PubMed1995

Atomic force microscopy of nucleoprotein complexes.

Recent data on the AFM studies of nucleoprotein complexes of different types are reviewed in this paper. The first section describes the progress in the sample preparation methods for AFM studies of nucleic acids and nucleoprotein complexes. The second part of this paper reviews AFM data on studies of complexes of DNA with regulatory proteins. These studies include two different types of DNA distortion induced by proteins binding: local bending of DNA at sites of protein binding and formation of large loops due to protein-protein interactions between molecules bound to distant sites along the DNA molecules (DNA looping). The prospects for use of AFM for physical mapping of genomes are discussed in this section as well. The third part of the paper reviews data on studies of complexes of DNA with non-sequence specific binding proteins. Special emphasis is given to studies of chromatin which have resulted in progress in the understanding of structure of native chromatin fiber. In this section, novel data on AFM studies of RecA-DNA filaments and complexes of dsRNA with the dsRNA-specific protein p25 are also presented. Discussion of the substrate preparation procedures in relation to the AFM studies of nucleoprotein complexes is given in the final section.

Animals↗

PubMed2006

Selective recruitment of membrane protein complexes onto gold substrates patterned by dip-pen nanolithography.

Dip-pen nanolithography (DPN) is employed to develop a generic array platform for the selective recruitment of membrane protein complexes. An atomic force microscope tip inked with HS(CH2)16NH2 is used to generate amino-terminated domains on gold. These domains can be arranged into microscopic and submicroscopic patterns, and the untreated gold substrate is subsequently blocked with HS(CH2)2CONH(CH2CH2O)15CH3, a compound known to resist the unspecific binding of proteins and cells. The patterned gold substrate is exposed to an enriched membrane fraction from mutant Rhodobacter sphaeroides, which contains photosynthetic core complexes consisting of the reaction center and the light-harvesting complex LH1. The selective recruitment to the patterned domains, governed primarily by electrostatic interactions, is confirmed by contact mode atomic force microscopy.

Adsorption↗

PubMed2000

Translocation-independent dimerization of the EcoKI endonuclease visualized by atomic force microscopy.

Bacterial type I restriction/modification systems are capable of performing multiple actions in response to the methylation pattern on their DNA recognition sequences. The enzymes making up these systems serve to protect the bacterial cells against viral infection by binding to their recognition sequences on the invading DNA and degrading it after extensive ATP-driven translocation. DNA cleavage has been thought to occur as the result of a collision between two translocating enzyme complexes. Using atomic force microscopy (AFM), we show here that EcoKI dimerizes rapidly when bound to a plasmid containing two recognition sites for the enzyme. Dimerization proceeds in the absence of ATP and is also seen with an EcoKI mutant (K477R) that is unable to translocate DNA. Only monomers are seen when the enzyme complex binds to a plasmid containing a single recognition site. Based on our results, we propose that the binding of EcoKI to specific DNA target sequences is accompanied by a conformational change that leads rapidly to dimerization. This event is followed by ATP-dependent translocation and cleavage of the DNA.

DNA Restriction Enzymes↗

PubMed2003

Ultrafast terahertz probes of transient conducting and insulating phases in an electron-hole gas.

Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. For example, the long-range Coulomb interaction between electrical charges has a simple form, yet is responsible for a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of electronic quasiparticles among themselves and with lattice vibrations and light. Oppositely charged electron and hole quasiparticles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons. The pathways between such states, however, remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing centre-of-mass momenta. In contrast, transitions between internal exciton levels, which occur in the far-infrared at terahertz (1012 s(-1)) frequencies, are independent of this restriction, suggesting their use as a probe of electron-hole pair dynamics. Here we employ an ultrafast terahertz probe to investigate directly the dynamical interplay of optically-generated excitons and unbound electron-hole pairs in GaAs quantum wells. Our observations reveal an unexpected quasi-instantaneous excitonic enhancement, the formation of insulating excitons on a 100-ps timescale, and the conditions under which excitonic populations prevail.

Journal Article↗

PubMed1997

Vibration Spectroscopy Study of Phenylphosphonate at the Water-Aluminum (Hydr)Oxide Interface

Two previously published surface complexation models of phenylphosphonate on aged gamma-Al2 O3 and boehmite (gamma-AlOOH), respectively, have been examined by means of FTIR and FT-Raman spectroscopy. The spectral features of adsorbed phenylphosphonate were studied as a function of pH and total phenylphosphonate concentration. No evidence for a phase transformation into a three-dimensional aluminum phenylphosphonate phase was found. This suggested that phenylphosphonate is sorbed as surface complexes under the conditions used in this study. Both the infrared and the Raman data showed that the surface-bound phenylphosphonate ions undergo protonation reactions as pH is varied. These results together with the fact that the ligand has only two donor atoms for complexation and protonation led to the conclusion that a monodentate coordination to the surface is most likely in both systems. Overall, the spectroscopic results were in good qualitative agreement with the thermodynamic surface complexation models.

Journal Article↗

PubMed2006

High-resolution protein-protein docking.

The high-resolution prediction of protein-protein docking can now create structures with atomic-level accuracy. This progress arises from both improvements in the rapid sampling of conformations and increased accuracy of binding free energy calculations. Consequently, the quality of models submitted to the blind prediction challenge CAPRI (Critical Assessment of PRedicted Interactions) has steadily increased, including complexes predicted from homology structures of one binding partner and complexes with atomic accuracy at the interface. By exploiting experimental information, docking has created model structures for real applications, even when confronted with challenges such as moving backbones and uncertain monomer structures. Work remains to be done in docking large or flexible proteins, ranking models consistently, and producing models accurate enough to allow computational design of higher affinities or specificities.

Animals↗

PubMed1998

Dithiocarbamate Ligands as Building-Blocks in the Coordination Chemistry of Gold.

The monodentate dithiocarbamate complexes [Au(2)(S(2)CNR(2))(2){&mgr;-(PPh(2))(2)C=CH(2)}] (R = Me (1), Et (2), Bz (3)) are obtained by reaction of [Au(2)Cl(2){&mgr;-(PPh(2))(2)C=CH(2)}] with NaS(2)CNR(2). The free sulfur atoms in complex 2 can subsequently be used to coordinate a third metal center, giving the trinuclear derivatives [Au(2)M(&mgr;-S(2)CNEt(2))(2){&mgr;-(PPh(2))(2)C=CH(2)}]X (X = ClO(4), M = Au (5), Ag (6); X = PF(6), M = Cu (7)). The dinuclear tricoordinate gold(I) complexes [Au(2)(&mgr;-S(2)CNEt(2)){&mgr;-(PPh(2))(2)C=CH(2)}(2)]ClO(4) (8) and [Au(2)(&mgr;-S(2)CNEt(2)){&mgr;-(PPh(2))(2)C=CH(2)}(PPh(3))(2)]ClO(4) (9) are obtained by reaction of [Au(2)(&mgr;-S(2)CNEt(2)){&mgr;-(PPh(2))(2)C=CH(2)}]ClO(4) (4) with (PPh(2))(2)C=CH(2) or PPh(3). The former is also obtained by reaction of [Au(2){&mgr;-(PPh(2))(2)C=CH(2)}(2)](ClO(4))(2) with sodium N,N-diethyldithiocarbamate (1:1). The crystal structures of complexes 2 and 8 are established by X-ray crystallography.

Journal Article↗

PubMed1988

Drug-protein interactions. A study of levamisole-serum albumin complex by 1H-NMR spectroscopy.

The chemical interaction of levamisole with human serum albumin (HSA) has been investigated using the technique of nuclear magnetic resonance spectroscopy. Binding to HSA occurs primarily with the imidazolidine and thiazolidine groups of levamisole as it has been demonstrated by selective changes in the relaxation times and the chemical shifts of the protons attached to the carbon atoms. The complex appears as the result of a weak linkage and may play a primordial role in the protein-protein interaction.

Humans↗

PubMed2002

Variable-temperature nuclear magnetic resonance spectroscopy allows direct observation of carboxylate shift in zinc carboxylate complexes.

Tetranuclear complexes [Zn(4)(bdmap)(2)(OOCR)(6)] 1 (R = Me) and 2 (R = Et), where Hbdmap = 1,3-bis(dimethylamino)-2-propanol, were prepared from zinc carboxylates and Hbdmap in tetrahydrofuran (THF). The solid-state structures of isomers 1a and 2a consist of two pairs of zinc atoms, each bridged by two mu-1,2 and one mu-1,1 carboxylate ligands. Two pairs are connected by two tridentate bdmap ligands with oxygen acting as a bridging donating atom. The complexes retain the tetranuclear structure in solution and two dynamic processes are observed from variable-temperature (1)H and (13)C NMR spectra. A low-temperature process (LT dynamics) observed already below 200 K is a coalescence of the mu-1,2 and the mu-1,1 resonances to a single resonance. An additional dynamic process (HT dynamics) is observed above 247 K (1) and 263 K (2), leading to a coalescence of two dimethylamino resonances. Both dynamic processes are rationalized by a mechanism involving changes in the carboxylate coordination mode termed as carboxylate shift. The LT dynamics is ascribed to interconversions of a single mu-1,2 and a single mu-1,1 carboxylate ligation by rotations of 60 degrees. The interconversions involve all carboxylate ligands in 1 and 2. The HT dynamics is ascribed to the exchange of the coordinating geometries of two carboxylate-bridged zinc atoms. We propose a mechanism that starts with a cleavage of the Zn-N coordination bond. The resulting coordinatively unsaturated zinc atom acquires an additional oxygen donor atom by carboxylate shift of mu-1,2 carboxylate to mu-1,1 mode. The activation parameters (DeltaH values in kilocalories per mole, DeltaS values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra: for 1 in THF-d(8), DeltaH(LT) = 8.1(3), DeltaS(LT) = -12(2), DeltaH(HT) = 17.9(2), DeltaS(HT) = 14(1); for 1 in CDCl(3), DeltaH(HT) = 13.6(5), DeltaS(HT) = 3(3); for 1 in CD(2)Cl(2), DeltaH(HT) = 9.9(3), DeltaS(HT) = -8(2); for 2 in THF-d(8), DeltaH(LT) = 11(1), DeltaS(LT) = -5(3), DeltaH(HT) = 19.6(5), DeltaS(HT) = 18(3). Polymeric [Zn(4)(bdmap)(2)(OOCMe)(6)](n) 1-catena crystallizes from a dichloromethane solution of 1. In 1-catena, the zinc atoms are linked into a chain through mu-1,2 and mu-1,1 acetate alternated by mu-1,2 acetate and bdmap.

Carboxylic Acids↗