Ligand Side Research Articles

PARACEST agents: modulating MRI contrast via water proton exchange.

Scientific interest in optimizing the properties of gadolinium (III) complexes as MRI contrast agents has led to many new insights into lanthanide ion coordination chemistry in the last two decades. Among these was the surprising observation that water exchange in lanthanide (III) derivatives of DOTA can be modulated dramatically by judicious choice of ligand side chain and Ln(3+) ionic radii. This resulted in the discovery of paramagnetic CEST agents for altering MRI image contrast based upon the chemical exchange saturation transfer mechanism. The goal of this article is to review the factors that govern water molecule and water proton exchange in these complexes and to compare the potential sensitivity of PARACEST agents versus Gd(3+)-based T(1) relaxation agents for altering tissue contrast.

Determination of Pb complexation in oxic and sulfidic waters using pseudovoltammetry.

Pseudovoltammetry was used to evaluate the actual Pb complexation occurring in natural water samples of varying oxygen and sulfide concentration. In pseudovoltammetry, the potential at which metal-ligand complexes are broken up to form the metal amalgam is used to determine the complexes' thermodynamic stability constants (KTHERM; corrected for metal and ligand side reaction coefficients) via the Nernst expression. This methodology removes the need for any metal additions and for subsequent modeling using fitting criteria, which provide only conditional stability constant data (KCOND). Using known organic ligands, a chelate scale ranging from log KTHERM = 4 to log KTHERM = 20 was developed as a template for comparison with samples collected from two stations of different salinities and at several depths in the Chesapeake Bay. These samples were observed to contain up to five different ligand compounds of unknown structural composition (log KTHERM > 8) with the strongest ligand fraction exceeding log KTHERM > 39 (the maximum observable thermodynamic stability constant due to the reduction of Na+). One possible explanation for the observed complexation is the existence of lead sulfide clusters. This was supported by laboratory experiments using electrochemistry and ICR-FTMS, which confirmed the formation of electrochemically inert multinuclear clusters with high stability constants (e.g., M3S3, log KTHERM = 62.9). However, in all field samples, (sub)nanomolar levels of acid-leachable sulfide were recovered at pH 5.0-6.2, which could be attributed to dissociation of lead sulfide complexes with moderate acidity. Recovery of sulfide increased from < 10% of the total dissolved Pb concentration (Pbdiss) in surface waters to 100% of the Pbdiss in the sulfide-rich bottom waters at the higher salinity location.

Solution studies of N′, N″, N″′-tris(3-aminopropyl)amine-based zinc(II) complexes and X-ray crystal structures of [Zn(trpn)](ClO 4) 2 and [Zn(trpn)(DETP)]ClO 4, DETP −= O, O-diethyl thiophospate. Catalytic activity of the complexes in the hydrolysis of the phosphotriester 2,4-dinitrophenyl diethyl phosphate

Solution studies (pH and 1H NMR titrations) on Zn 2+ complexes 1 and 2 derived from N′ ,N″ ,N″′-tris(3-aminopropyl)amine L1 and N′ ,N″ ,N″′-tris(3-dimethyl-aminopropyl)amine L2 revealed that the presence of the hydrophobic (methyl) groups attached to the tripodal ligand side arms decreased the p K a of Zn-bound water molecule from 9.99 for 1 to 8.01 for 2, respectively. The X-ray diffraction studies have established the structures [L1Zn](ClO 4) 2 1 and [L1Zn(DETP)](ClO 4) 3, DETP −= O, O-diethyl thiophospate. Compound 1 consists of a monomeric cation and ClO 4 − counter ions. The coordination geometry of the Zn(II) centers may be described as distorted tetrahedral. Whereas in compound 3 the zinc atom adopts the slightly distorted trigonal-bipyramidal coordination geometry with the three primary nitrogen's on the basal plane and the tertiary nitrogen and the oxygen of DETP − at the apex. The hydroxo complexes [ZnL(OH)] + species showed slight catalytic activity in the hydrolysis of the phosphotriester 2,4-dinitrophenyl diethyl phosphate.

Probing the role of axial methionine in the blue copper center of azurin with unnatural amino acids.

Expressed protein ligation was used to replace the axial methionine of the blue copper protein azurin from Pseudomonas aeruginosa with unnatural amino acids. The highly conserved methionine121 residue was replaced with the isostructural amino acids norleucine (Nle) and selenomethionine (SeM). The UV-visible absorption, X- and Q-band EPR, and Cu EXAFS spectra of the variants are slightly perturbed from WT. All variants have a predominant S(Cys) to Cu(II) charge transfer band around 625 nm and narrow EPR hyperfine splittings. The Se EXAFS of the M121SeM variant is also reported. In contrast to the small spectral changes, the reduction potentials of M121SeM, M121Leu, and M121Nle are 25, 135, and 140 mV, respectively, higher than that of WT azurin. The use of unnatural amino acids allowed deconvolution of different factors affecting the reduction potentials of the blue copper center. A careful analysis of the WT azurin and its variants obtained in this work showed the large reduction potential variation was linearly correlated with the hydrophobicity of the axial ligand side chains. Therefore, hydrophobicity is the dominant factor in tuning the reduction potentials of blue copper centers by axial ligands.

Modeling and molecular dynamics of glutamine transaminase K/cysteine conjugate β-lyase

The homodimeric, pyridoxal 5′-phosphate (PLP)-dependent enzyme glutamine transaminase K/cysteine conjugate β-lyase (GTK/β-lyase) has been implicated in the bioactivation of chemopreventive compounds. This paper describes the first homology model of rat renal GTK/β-lyase and its active site residues, deduced from molecular dynamics (MD) simulations of the binding mode of 13 structurally diverse cysteine S-conjugates and amino acids after Amber-parametrization of PLP. Comparison with Thermus thermophilus aspartate aminotransferase (tAAT) and Trypanosoma cruzi tyrosine aminotransferase (tTAT), used as templates for modeling GTK/β-lyase, showed that the PLP-binding site of GTK/β-lyase is highly conserved. Binding of the ligand α-carboxylate-group occurred via the conserved residues Arg 432 and Asn 219, and Asn 50 and Gly 70. Two pockets accommodated the various ligand side chains. A small pocket, located directly above PLP, was of a highly hydrophobic and aromatic character. A larger pocket, formed partly by the substrate access channel, was more hydrophilic and notably involved the salt bridge partners Glu 54 and Arg 99∗ (∗ denotes the other subunit). Ligand-binding residues included Leu 51, Phe 71, Tyr 135, Phe 373 and Phe 312∗, and π-stacking interactions were often observed. Tyr 135 and Asn 50 were prominent in hydrogen bonding with the sulfur-atom of cysteine S-conjugates. The observed binding mode of the ligands corresponded well with their experimentally determined inhibitory potency toward GTK/β-lyase. The current homology model thus provides a starting point for further validation of the role of active site residues in ligand-binding by means of mutagenesis studies. Ultimately, insight in the binding of ligands to GTK/β-lyase may result in the rational design of new ligands and selective inhibitors.

The Influence of Ligand Side Chain on the Enantioselectivity of Lewis Acid Catalyzed Diels—Alder Reactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Use of selective Trp side chain labeling to characterize protein-protein and protein-ligand interactions by NMR spectroscopy.

Recent studies on amino acid occurrence in protein binding sites suggest that only a reduced number of residues are responsible for most interaction energy in protein-protein and protein-ligand interactions. Above all, tryptophan (Trp) seems to be the most frequent residue in protein's hot spots. Here we report a novel, efficient, and cost-effective method to selectively incorporate specific isotope labels into the side chains of Trp residues in recombinant proteins. We show that the method proposed allows selective NMR observation of Trp side chains that enables studies of ligand binding, protein-protein interactions, hydrogen binding, protein folding, and side chain dynamics. Examples with the protein BIR3 will be given.

Origins of PDZ Domain Ligand Specificity: STRUCTURE DETERMINATION AND MUTAGENESIS OF THE ERBIN PDZ DOMAIN

The LAP (leucine-rich repeat and PDZ-containing) family of proteins play a role in maintaining epithelial and neuronal cell size, and mutation of these proteins can have oncogenic consequences. The LAP protein Erbin has been implicated previously in a number of cellular activities by virtue of its PDZ domain-dependent association with the C termini of both ERB-B2 and the p120-catenins. The present work describes the NMR structure of Erbin PDZ in complex with a high affinity peptide ligand and includes a comprehensive energetic analysis of both the ligand and PDZ domain side chains responsible for binding. C-terminal phage display has been used to identify preferred ligands, whereas binding affinity measurements provide precise details of the energetic importance of each ligand side chain to binding. Alanine and homolog scanning mutagenesis (in a combinatorial phage display format) identifies Erbin side chains that make energetically important contacts with the ligand. The structure of a phage-optimized peptide (Ac-TGW(-4)ETW(-1)V; IC(50) = approximately 0.15 microm) in complex with Erbin PDZ provides a structural context to understand the binding energetics. In particular, the very favorable interactions with Trp(-1) are not Erbin side chain-mediated (and therefore may be generally applicable to many PDZ domains), whereas the beta2-beta3 loop provides a binding site for the Trp(-4) side chain (specific to Erbin because it has an unusually long loop). These results contribute to a growing appreciation for the importance of at least five ligand C-terminal side chains in determining PDZ domain binding energy and highlight the mechanisms of ligand discrimination among the several hundred PDZ domains present in the human genome.

Characterization of metal ion-exchange on modified surfaces of porous carbon

Two biologically based ligands, cysteine (Cys) and poly-l-cysteine (PLCys), were coupled with Carbopack-X, a commercially available porous carbon surface after acid activation, through simple cross-linking techniques. The resulting Cys-X and PLCys-X systems are effective for metal binding with capacities in the range of 1–25μmol metal/g column, depending on the metal tested. Both columns demonstrate similar selectivity for the soft and moderately soft acid metals such as Cd2+, Cu+, and Pb2+, probably as a result of the soft basic character of the thiol ligand side chains, and exhibit minimal binding of the harder groups IA and IIA metals. Even though PLCys-X and Cys-X displayed similar strong and weak binding characteristics, the PLCys-X system contained stronger binding sites than those possessed by Cys-X. The strongest site displayed by Cys-X has a binding constant of 1×109, with the ability to compete for metal with moderately strong ligands such as ethylenediamine. However, the strongest PLCys-X site possessed a binding constant of about 1013, with the ability to compete for metals bound by very strong ligands such as EDTA. Both systems gave quantitative release of bound metals upon exposure to acid. Exposing the thiol-rich ligands to chemical reducing or oxidizing environments altered the metal binding characteristics of both columns, with at least a 50% decrease in metal binding on the oxidized columns due to thiol binding sites because of disulfide bond formation.

Isotope Effects of Copper in Cu(II) Ligand-Exchange Systems by Ion Exchange Chromatography.

The isotope effects of copper in ligand exchange processes were studied by means of chelating ion exchange resin system and EDTA complex/cation-exchange resin system. Two cases of different ionic interactions systems were investigated using chelating ion exchange resin system; Cu-H and Cu-Co systems. Both cases were investigated using the breakthrough technique, while the reverse breakthrough technique was applied in the case of EDTA. The chromatograms obtained from two different chelating resin systems were not ideal but they can be improved in future work. The heavier isotope 65Cu was found to be enriched in the ligand side in all cases, the resin phase in the case of chelating resin and the solution phase in the case of EDTA complex system. The separation coefficients in the two chelating systems were found to be constant and equal to 1.1×10-5. Concerning the EDTA complex system, the chromatogram obtained was ideal one and the separation coefficient was found to be 1.3×10-5.

Agonist alkyl tail interaction with cannabinoid CB1 receptor V6.43/I6.46 groove induces a helix 6 active conformation

Our modeling studies have suggested that β branched amino acids Val, lle, or Thr located (i,i+3) or (i,i+4) apart on an alpha helix can form a groove into which a ligand alkyl chain can fit. Experimental support for this idea comes from the crystal structure of adipocyte lipid binding protein complexed with stearic acid (Xu et al. J Biol Chem 1993, 268, 7874). We hypothesized that the transmembrane helix 6 (TMH 6) βXXβ motif of the CB1/CB2 receptors (V6.43/I6.46), which immediately precedes the conserved TMH 6 CWXP motif, serves as an interaction site for the alkyl tail of cannabinoid (CB) ligands and that interaction with this motif may trigger receptor activation. Conformational memories (CM) calculations on TMH 6 of CB1 and CB2 were used to explore the conformation of TMH 6 in unbound and complexed states. The conserved Pro 6.50 generated a kink in the α-helical structure that behaved as a flexible hinge. In the context of a three-dimensional model of the CB1 receptor, a helix from the more kinked family of CB1 TMH 6 conformers calculated by CM brought the intracellular portion of TMH 6 in proximity to TMH 3, analogous to the inactive state TMH 3/6 conformation seen in the X-ray crystal structure of rhodopsin (Palczewski et al. Science 2000, 289, 739). A CM study of CB1 TMH 6 in which a pentane molecule (as a model system for the CB ligand side chain) interacts with the V6.43/I6.46 groove was also conducted. The results of this calculation showed that alkyl chain interaction with the V6.43/I6.46 groove directly modulates the overall conformation of TMH 6, biasing the population of TMH 6 conformers toward the family of less kinked CB1 TMH 6 conformations calculated by CM. In the context of the CB1 bundle, this conformational change would cause the intracellular end of TMH 6 to move away from TMH 3. Such a movement is consistent with recent experimental results for agonist induced conformational changes at the intracellular side of TMH 6 in the β2-adrenergic receptor (Jensen et al. J Biol Chem 2001, 276, 9279). In contrast to results for CB1, CB2 TMH 6 showed a smaller range of kink angles possible for unbound TMH 6, with no significant shift in the populations of TMH 6 when the V6.43/I6.46 groove was occupied by pentane. We hypothesized that the profound flexibility differences between wild-type (WT) CB1 vs. WT CB2 TMH 6 revealed by CM calculations may be due to the size of residue 6.49 which immediately precedes P6.50 of the CWXP motif (G6.49 in WT CB1 and F6.49 in WT CB2). To test this hypothesis, using CM, we compared the flexibilities of WT CB1 and CB2 TMH 6 with those of the switch mutants, CB1 G6.49F and CB2 F6.49G. Consistent with results reported above, the degree of kinking (average of 100 conformers) was distinctly different between CB1 (40.9°; std. dev. ± 16.9°) and CB2 (24.6°; std. dev. ± 4.3°), with CB1 TMH 6 exhibiting a noticeably wider range of kink angles than CB2. These flexibilities were essentially switched in the mutants [CB1 G6.49F mutant (25.3°; std. dev. ± 5.7°) and a CB2 F6.49G mutant (44.3°; std. dev. ± 21.4°]. Taken together, these results suggest that TMH 6 in CB1, but not in CB2, is sensitive to conformational modulation by an alkyl chain bound in the V6.43/I6.46 groove. Furthermore, results suggest that the small size of residue 6.49 in CB1 facilitates the P6.50 flexible hinge motion of CB1 TMH 6. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 88: 76–86, 2002

The influence of ligand side chain on the enantioselectivity of Lewis acid catalyzed Diels-Alder reactions.

The enantioselective Lewis acid-catalyzed Diels-Alder reaction of 3-(2-propenoyl)-1,3-oxazolidin-2-one 8 with cyclopentadiene was examined using a series of chiral mox ligands 2-6, deferring in the side chain at 2-position of the chiral oxazoline and in the nature of the substituent at the chiral center (4-position) of the oxazoline ring, and a combination of N-[(1R)-2-chloro-1-phenylethyl]-2-[(4R)-4-phenyl-4,5-dihydrooxazol-2-yl]butyramide 2-MgI(2)-I(2) was the most efficient catalyst.

Tuning the activity of catechol oxidase model complexes by geometric changes of the dicopper core.

Dicopper(II) complexes of a series of different pyrazolate-based dinucleating ligands [L1](-)-[L4](-) have been synthesized and characterized structurally and spectroscopically. A major difference between the four complexes is the individual metal-metal separation that is enforced by the chelating side arms of the pyrazolate ligand scaffold: it varies from 3.45 A in 2 x (BF4)4 to 4.53 A in 4 x (ClO4)2. All complexes have been evaluated as model systems for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechole (DTBC) as the test substrate. They were shown to exhibit very different catecholase activities ranging from very efficient to poor catalysts (k(obs) between 2430+/-202 and 22.8+/-1.2 h(-1)), with an order of decreasing activity 2 x (ClO4)4 > 1 x (ClO4)2 > 3 x (ClO4)2 >> 4 x (ClO4)2. A correlation of the catecholase activities with the variation in Cu...Cu distances, as well as other effects resulting from the distinct redox potentials, neighboring groups, and the individual coordination spheres are discussed. Saturation behavior for the rate dependence on substrate concentration was observed in only two cases, that is, for the most active 2 x (ClO4)4 and for the least active 4 x (ClO4)2, whereas a catalytic rate that is almost independent of substrate concentration (within the range studied) was observed for 1 x (ClO4)2 and 3 x (ClO4)2. H2O2 was detected as the product of O2 reduction in the catecholase reaction of the three most active systems. The structures of the adducts of "L3Cu2" and "L4Cu2" with a substrate analogue (tetrachlorocatecholate, TCC) suggest a bidentate substrate coordination to only one of the copper ions for those catalysts that feature short ligand side arms and correspondingly exhibit larger metal-metal separations; this possibly contributes to the lower activity of these systems. TCC binding is supported by several H-bonding interactions to water molecules at the adjacent copper or to ligand-side-arm N-donors; this emphasizes the importance of functional groups in proximity to the bimetallic active site.

Formation, characterization, and reactivity of bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands.

Bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands have been successfully generated by treating the corresponding bis(mu-hydroxo)dinickel(II) complexes or bis(mu-methoxo)dinickel(II) complex with an equimolar amount of H(2)O(2) in acetone at low temperature. The bis(mu-oxo)dinickel(III) complexes exhibit a characteristic UV-vis absorption band at approximately 410 nm and a resonance Raman band at 600-610 cm(-1) that shifted to 570-580 cm(-1) upon (18)O-substitution. Kinetic studies and isotope labeling experiments using (18)O(2) imply the existence of intermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(mu-oxo)dinickel(III) complex. The bis(mu-oxo)dinickel(III) complexes supported by the mononucleating ligands (L1(X) = para-substituted N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X = OMe, Me, H, Cl) gradually decompose, leading to benzylic hydroxylation of the ligand side arm (phenethyl group). The kinetics of the ligand hydroxylation process including kinetic deuterium isotope effects (KIE), p-substituent effects (Hammett plot), and activation parameters (Delta H(H)(*) and Delta S(H)(*)) indicate that the bis(muxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(mu-oxo)dicopper(III) complex. Such a reactivity of bis(mu-oxo)dinickel(III) complexes has also been suggested by the observed reactivity toward external substrates such as phenol derivatives and 1,4-cyclohexadiene. The thermal stability of the bis(mu-oxo)dinickel(III) complex is significantly enhanced when the dinucleating ligand with a longer alkyl strap is adopted instead of the mononucleating ligand. In the m-xylyl ligand system, no aromatic ligand hydroxylation occurred, showing a sharp contrast with the reactivity of the (mu-eta(2):eta(2)-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic aromatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems are discussed on the basis of the detailed comparison of these two systems with the same ligand.

Crystal structure of the oxidized cytochrome c2 from Blastochloris viridis

The crystal structure of the oxidized cytochrome c 2 from Blastochloris (formerly Rhodopseudomonas) viridis was determined at 1.9 Å resolution. Structural comparison with the reduced form revealed significant structural changes according to the oxidation state of the heme iron. Slight perturbation of the polypeptide chain backbone was observed, and the secondary structure and the hydrogen patterns between main-chain atoms were retained. The oxidation state-dependent conformational shifts were localized in the vicinity of the methionine ligand side and the propionate group of the heme. The conserved segment of the polypeptide chain in cytochrome c and cytochrome c 2 exhibited some degree of mobility, interacting with the heme iron atom by the hydrogen bond network. These results indicate that the movement of the internal water molecule conserved in various c-type cytochromes drives the adjustments of side-chain atoms of nearby residue, and the segmental temperature factor changes along the polypeptide chain.

Structural Insights into the Mode of Action of a Pure Antiestrogen

Background: Estrogens exert their effects on target tissues by binding to a nuclear transcription factor termed the estrogen receptor (ER). Previous structural studies have demonstrated that each class of ER ligand (agonist, partial agonist, and SERM antagonist) induces distinctive orientations in the receptor's carboxy-terminal transactivation helix. The conformation of this portion of the receptor determines whether ER can recruit and interact with the components of the transcriptional machinery, thereby facilitating target gene expression. Results: We have determined the structure of rat ERβ ligand binding domain (LBD) in complex with the pure antiestrogen ICI 164,384 at 2.3 Å resolution. The binding of this compound to the receptor completely abolishes the association between the transactivation helix (H12) and the rest of the LBD. The structure reveals that the terminal portion of ICI's bulky side chain substituent protrudes from the hormone binding pocket, binds along the coactivator recruitment site, and physically prevents H12 from adopting either its characteristic agonist or AF2 antagonist orientation. Conclusions: The binding mode adopted by the pure antiestrogen is similar to that seen for other ER antagonists. However, the size and resultant positioning of the ligand's side chain substituent produces a receptor conformation that is distinct from that adopted in the presence of other classes of ER ligands. The novel observation that binding of ICI results in the complete destabilization of H12 provides some indications as to a possible mechanism for pure receptor antagonism.

Conformational analysis and docking study of potent factor XIIIa inhibitors having a cyclopropenone ring

A conformational analysis and docking study of potent factor XIIIa inhibitors having a cyclopropenone ring were carried out in an attempt to obtain structural insight into the inhibition mechanism. First, stable conformers of the inhibitors alone were obtained from the conformational analysis by systematic search and molecular dynamics. Next, a binding form model of factor XIIIa was built based on an X-ray crystal structure of the enzyme. Finally, the docking study of the inhibitors into the model’s binding site was performed. From the resulting stable complex structures, it was found that the cyclopropenone ring fits the active site located at the base of the binding cavity with high complementarity. The carbonyl oxygen of the cyclopropenone ring formed a hydrogen bond to the indole NH group of Trp279 and the terminal carbon atom of the reactive CC double bond was in close proximity to the sulfur atom of the catalytic residue, Cys314. This binding mode suggests a possible inhibition mechanism, whereby the cysteine residue reacts with the cyclopropenone ring of the inhibitor, forming an enzyme-ligand adduct. In addition, the higher interaction energies between factor XIIIa and the inhibitors alluded to the probable binding sites of the ligand side chain.

Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH

As part of our program to develop more stable cyano (CN) high-performance liquid chromatography (HPLC) column packings, we have evaluated hydrolytic stability as a function of ligand connectivity, chain length, and side group steric protection and the pH of the mobile phase. Three accelerated tests were used to evaluate stability: (1) A non-HPLC screening test measuring carbon loss in refluxing MeOH-100 m M KH 2PO 4 pH 4.5 (1:1, v/v) solution; (2) a continuous flow HPLC test measuring capacity factor maintenance in 1% trifluoroacetic acid in water (pH 1.02) at 80°C; and (3) a continuous flow HPLC test measuring column efficiency maintenance in 50 m M triethylamine in water (pH 10.00) at 50°C. The stability of the CN phases was found to be dependent on both ligand chemical structure and the pH of the test conditions. The starting screen test of intermediate pH was least able to differentiate the CN phases based on structure, because two different degradation mechanisms appear to offset each other (acid induced siloxane bond cleavage vs. base induced silica dissolution). A trifunctional and a sterically protected CN phase were notably stable under the acidic test conditions, but had poor stability under basic conditions. Conversely, chain extension afforded poor stability under acidic conditions, but did afford improved stability at higher pH. In total, the data indicate that good CN column stability can be achieved by using a trifunctional or a sterically protected phase in acidic mobile phases. However, as mobile phases of intermediate or higher pH are employed, shorter column lifetimes can be expected due to an accelerated dissolution of the underlying silica substrate. Materials were also compared chromatographically using a mixture of non-polar, polar, and basic analytes under reversed-phase conditions.

Organic Syntheses via Transition Metal Complexes. 104. Cyclopentadienyl Thiolacylates from (1-Alkynyl)carbene Complexes (M = Cr, W): Sulfur-Centered Fragmentation of Ligand Side Chains.

Organic Syntheses via Transition Metal Complexes. 104. Cyclopentadienyl Thiolacylates from (1-Alkynyl)carbene Complexes (M = Cr, W): Sulfur-Centered Fragmentation of Ligand Side Chains.

The receptor tyrosine phosphatase CRYP? affects growth cone morphology

During development of the nervous system receptor tyrosine kinases and receptor protein tyrosine phosphatases act in a coordinate way during axon growth and guidance. In the developing avian retinotectal system, many different receptor protein tyrosine phosphatases are expressed. Most of them have unknown functions. Retinal ganglion cells express at least three different members of this receptor family on their axons and growth cones: CRYPalpha, CRYP-2 and PTPmu. CRYPalpha interacts heterophilically with at least two different ligands found in the basal membranes of the retina and the optic tectum. To analyze the role of the CRYPalpha-ligand interaction, retinal ganglion cell axons were grown on retinal basal membranes (inner limiting membrane) and the receptor-ligand interaction was blocked from both the receptor side (by receptor specific antibodies) and from the ligand side by using a receptor-alkaline phosphatase fusion protein. Both of these treatments reduced average retinal axon length and induced a dramatic change in morphology of retinal ganglion cell growth cones on basal membranes, but not on other substrates like laminin, N-cadherin, matrigel- and detergent-treated basal membranes. These results suggest that CRYPalpha and its ligand act as growth-promoting molecules during intraretinal axon growth.