Intramolecular Activation Research Articles

The Hydrolysis/Condensation Behaviour of Methacryloyloxyalkylfunctional Alkoxysilanes: Structure-Reactivity Relations

Various methacryloyloxymethylalkoxysilanes have been synthesized by nucleophilic substitution reactions starting from chloromethylalkoxysilanes under phase transfer catalysis conditions. The compounds thus obtained show an exceptionally high degree of reactivity with regard to hydrolysis and condensation both under acidic as well as under basic conditions compared to the established 3-methacryloyloxypropyltrimethoxysilane. A mechanistic model for this high reactivity by intramolecular activation is discussed.

Synthesis of mono- and dihalogenated derivatives of (Me2S)2B12H10 and palladium-catalyzed boron-carbon cross-coupling reactions of the iodides with grignard reagents.

Two series of compounds, 9-X-1,7-(Me(2)S)(2)B(12)H(9) and 9,10-X(2)-1,7-(Me(2)S)(2)B(12)H(8) (X = Cl, Br, I), have been synthesized from reactions of 1,7-(Me(2)S)(2)B(12)H(10) with various halogenating reagents. In addition, reactions of 1,7-(Me(2)S)(2)B(12)H(10) with 2,4-(NO(2))(2)C(6)H(3)SCl and PhSeBr resulted in 9-(2',4'-(NO(2))(2)C(6)H(3)S)-1,7-(Me(2)S)(2)B(12)H(9) and 9,10-(PhSe)(2)-1,7-(Me(2)S)(2)B(12)H(8), respectively. X-ray studies of the dibromo, monoiodo, and aryl thioether derivatives show that electrophilic substitution in 1,7-(Me(2)S)(2)B(12)H(10) takes place at positions 9 and 10, as in the case of the meta-carborane 1,7-C(2)B(10)H(12). From 1,12-(Me(2)S)(2)B(12)H(10) the halides 2-X-1,12-(Me(2)S)(2)B(12)H(9) (X = Br, I) were prepared. For both 1,7- and 1,12-(Me(2)S)(2)B(12)H(10) the best iodination results were obtained using iodine monochloride in refluxing acetonitrile. In the presence of 5 mol % (PPh(3))(2)PdCl(2) the iodides 9-I-1,7-(Me(2)S)(2)B(12)H(9), 2-I-1,12-(Me(2)S)(2)B(12)H(9), and 9,10-I(2)-1,7-(Me(2)S)(2)B(12)H(8) react with RMgX (R = Me, Ph, Bn; X = Cl, Br) in THF to yield the corresponding B-alkyl- and B-aryl-substituted products in good yields without using CuI as a cocatalyst. The bromo derivative 9-Br-1,7-(Me(2)S)(2)B(12)H(9) did not react under similar conditions. No interference from the nearby Me(2)S substituent was observed in palladium-catalyzed substitution of iodide in 2-I-1,12-(Me(2)S)(2)B(12)H(9). Presumably due to the intramolecular activation of an aryl C-H bond of the benzyl substituent in the intermediate palladium complex, the yield of 9,10-Bn(2)-1,7-(Me(2)S)(2)B(12)H(8) was significantly lower than those of the dimethyl and diphenyl derivatives. The molecular structures of 9-R-1,7-(Me(2)S)(2)B(12)H(9) (R = Ph, Bn) and 2-Bn-1,12-(Me(2)S)(2)B(12)H(9) were obtained by single-crystal X-ray analysis.

Non-peptidic small-molecule antagonists of the human platelet thrombin receptor PAR-1.

The thrombin receptor on human platelets is the first member identified of a new family of G-protein coupled receptors referred to as protease activated receptors (PARs). These receptors are activated by a unique mechanism involving proteolytic cleavage of a portion of the extracellular domain to generate a new N-terminus which then acts as a tethered or intramolecular ligand (agonist) for the receptor. The hexapeptide SFLLRN-NH2 comprising the new N-terminus is referred to as the Thrombin Receptor Activating Peptide, or "TRAP" Thrombin is the most potent agonist for platelet aggregation and selective blockade of the intramolecular activation step without effecting the proteolytic activity of thrombin should result in moderation of platelet activation and aggregation without interfering with the other coagulation cascade effects of thrombin. Screening of combinatorial libraries identified a novel, non-peptide PAR-1 thrombin receptor antagonist. Examination of structure-activity relationships revealed that portions of the molecule could be replaced resulting in simpler molecules of lower molecular weight that were at the same time more potent. Molecules in this series were effective antagonists of TRAP-stimulated platelet activation, but had limited activity when thrombin was the agonist. Additional directed screening and subsequent lead refinement resulted in a second series of isoxazole based compounds. Some of the resultant molecules were potent PAR-1 antagonists that were effective against both TRAP- and thrombin-stimulated receptor activation. These compounds do not inhibit the proteolytic effects of thrombin but rather interfere with the intramolecular binding of the tethered ligand (SFLLRN) to the transmembrane portion of the thrombin receptor. They represent promising leads for future explorations of antithrombotic activity of thrombin receptor antagonists.

A theoretical study on the complete catalytic cycle of the hetero-Pauson-Khand-type [2+2+1] cycloaddition reaction of ketimines, carbon monoxide and ethylene catalyzed by iron carbonyl complexes.

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.

Radiationless Deactivation Process of 1-Dimethylamino-9-fluorenone Induced by Conformational Relaxation in the Excited State: A New Model Molecule for the TICT Process

The deactivation process of excited 1-(dimethylamino)-9-fluorenone (1-DMAF) was investigated by means of steady-state and time-resolved fluorescence spectroscopy. Fluorescence decay profiles for 1-DMAF, which has a relatively short lifetime (several tens of picoseconds in low viscosity solvents at ambient temperature) are much affected by the fluidity of the surrounding solvent. The lifetimes increase in glassy solvents at 77 K as well as with increasing viscosity. These results indicate that conformational relaxation by the dimethylamino group plays an important role in the deactivation process of 1-DMAF. Detailed analysis of the temperature dependence of the fluorescence lifetime revealed that the conformational relaxation process has a very small intramolecular activation barrier (5.4 kJ/mol). Results of MO calculations suggest that the dimethylamino moiety of 1-DMAF is pretwisted in the ground state and that the most stable structure in the excited state is a twisted intramolecular charge transfer (TICT) state. 1-DMAF could be a new model molecule that exhibits the TICT phenomena.

Use of defined-function mutants to access receptor–receptor interactions

This article describes a novel method to access functional interactions of two defective mutant receptors. As a model, luteinizing hormone receptor, a G-protein-coupled receptor, was used by coexpressing two different mutants, one defective in hormone binding and the other defective in signal generation. When these two mutants were coexpressed in a cell, the cell responded to the hormone and induced the hormone action, indicating the interaction of the two receptors and rescue of the activity. The luteinizing hormone receptor consists of a 350-amino-acid extracellular N-terminal domain (exodomain), followed by seven transmembrane domains and connecting loops (endodomain). Hormone binds to the exodomain, whereas hormone signals are generated in the endodomain. Here, we show that binding of hormone to one receptor can activate adenylyl cyclase through its transmembrane bundle, intramolecular activation ( cis-activation), as well as intermolecular activation ( trans-activation) through the transmembrane bundle of an adjacent receptor, without forming a stable receptor dimer. Our observations provide new insights into the mechanism of receptor activation mechanisms, and have implications for the treatment of inherited disorders of glycoprotein hormone receptors.

Cis- and trans-activation of hormone receptors: the LH receptor.

G protein-coupled receptors (GPCRs) accommodate a wide spectrum of activators from ions to glycoprotein hormones. The mechanism of activation for this large and clinically important family of receptors is poorly understood. Although initially thought to function as monomers, there is a growing body of evidence that GPCR dimers form, and in some cases that these dimers are essential for signal transduction. Here we describe a novel mechanism of intermolecular GPCR activation, which we refer to as trans-activation, in the LH receptor, a GPCR that does not form stable dimers. The LH receptor consists of a 350-amino acid amino-terminal domain, which is responsible for high-affinity binding to human CG, followed by seven-transmembrane domains and connecting loops. This seven-transmembrane domain bundle transmits the signal from the extracellular amino terminus to intracellular G proteins and adenylyl cyclase. Here, we show that binding of hormone to one receptor can activate adenylyl cyclase through its transmembrane bundle, intramolecular activation (cis-activation), as well as trans-activation through the transmembrane bundle of an adjacent receptor, without forming a stable receptor dimer. Coexpression of a mutant receptor defective in hormone binding and another mutant defective in signal generation rescues hormone-activated cAMP production. Our observations provide new insights into the mechanism of receptor activation mechanisms and have implications for the treatment of inherited disorders of glycoprotein hormone receptors.

Unconventional Activation Mechanisms of MMP-26, a Human Matrix Metalloproteinase with a Unique PHCG XXD Cysteine-switch Motif

ProMMP-26 has the unique Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp cysteine-switch motif that discriminates this protease from all other matrix metalloproteinases (MMPs) known so far. The conserved, free cysteine residue of the conventional PRCXXPD sequence interacts with the zinc ion of the catalytic domain and provides the fourth coordination site for the catalytic zinc, thereby preventing latent proMMPs from becoming active. MMPs become functionally active when proteolytic cleavage releases the prodomain and the PRCXXPD sequence and exposes the zinc atom. Here, we report that the Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp motif is not functional in proMMP-26 and consequently is not involved in the activation mechanisms. Organomercurial treatment failed to activate proMMP-26. The autolytic Lys-Lys-Gln(59) downward arrow Gln(60)-Phe-His cleavage upstream of the Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp motif induced the proteolytic activity of recombinant proMMP-26 whereas any further cleavage inactivated the enzyme. The His(81) --> Arg(81) mutation restored the conventional cysteine-switch sequence in the prodomain but failed to induce the cysteine-switch activation mechanism. These data and computer modeling studies allowed us to hypothesize that the presence of His(81) significantly modified the fold of proMMP-26, abolished the functionality of the cysteine-switch motif, and stimulated an alternative intramolecular activation pathway of the proenzyme.

ChemInform Abstract: Ligand Oxidation as a Method for Intramolecular Activation of Metal Complexes

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Mechanism-based Suppression of Dideoxynucleotide Resistance by K65R Human Immunodeficiency Virus Reverse Transcriptase Using an α-Boranophosphate Nucleoside Analogue

The amino acid change K65R in human immunodeficiency virus type 1-reverse transcriptase (RT) confers viral resistance to various 2',3'-dideoxynucleoside drugs in vivo. Using pre-steady state kinetic methods, we found that K65R-reverse transcriptase is 3.2-14-fold resistant to 2',3'-dideoxynucleotides in vitro relative to wild-type reverse transcriptase, in agreement with resistance levels observed in vivo. A decreased catalytic rate constant k(pol) mostly accounts for the lower incorporation efficiency observed for 2',3'-dideoxynucleotides. Examination of the crystal structure of the RT.DNA.dNTP complex suggested that both the charge at position 65 and the 3'-OH of the incoming nucleotide act in synergy during the creation of the phosphodiester bond, resulting in a more pronounced decreased catalytic rate constant for 2',3'-dideoxynucleotides than for dNTPs. This type of intramolecular activation of the leaving phosphate by the 3'-OH group appears to be conserved in several nucleotide phosphotransferases. These data were used to design dideoxynucleotide analogues targeting K65R RT specifically. alpha-Boranophosphate ddATP was found to be a 2-fold better substrate than dATP and inhibited DNA synthesis by K65R RT 153-fold better than ddATP. This complete suppression of drug resistance at the nucleotide level could serve for other reverse transcriptases for which drug resistance is achieved at the catalytic step.

Ligand oxidation as a method for intramolecular activation of metal complexes

The results of investigations of organometallic and coordination compounds whose ligands contain redox fragments based on 2,6-di-tert-butylphenol and linked to the metal by various coordinating groups are summarized. The redox properties of σ-aryl, π- and σ-allyl complexes, β-diketonates, hydrazides, hydroxamates, glyoximates, and metalloporphyrins and metallophthalocyanines are considered. The possibility of changing the reactivity of organometallic and coordination compounds by changing the magnetic state due to oxidation of the redox-active phenolic group in the ligand is demonstrated. It is proposed to use ligand oxidation as a method for intramolecular activation of metal complexes.

New metallocenes of early transition metals with functional groups in organic ligands. Red-ox transformations

Abstract The reduction behavior of the novel zirconocene dichlorides, (h5-C5Me5) (h5-C 5Me4CH2CH2ER 2) ZrCl2 (ER2 = NMe2, PMe2, PPh2) , (h5-C5Me4CH2 CH2PMe2) 2ZrCl 2, and (h5-C5Me4CH2 CH2PPh2) (h5-C 5Me4CH2CH2PMe 2) ZrCl2, together with the new types of intramolecular activation of inert C-H and C-heteroatom bonds are presented and discussed. A remarkably thermally stable zirconocene alkylhydryde type framework complex (h5-C5Me5) [h5 ,s-C5Me4CH2CH 2N (Me) CH2-]ZrH, zirconocene arylhydryde complex (h5-C5Me5) [h5 ,s-C5Me4CH2CH 2P (Ph) -o-C6H4-]ZrH, tetramethylfulvene hydride-type complexes (h5-C5Me5) [h6 :h1- (CH2C5Me3 ) CH2CH2PR2]ZrH (R = Me, Ph) and (h5-C5Me4CH2 CH2PPh2) [h6:h 1- (CH2C5Me3) CH 2CH2PMe2]ZrH are reported. The first unusually thermally stable zirconocene (h5,h1-C5Me4 CH2CH2PMe2) 2 Zr with the Zr (II) center stabilized by intramolecular coordination of two Me2P-groups is described.

Intramolecular Activation of a Ca2+-Dependent Protein Kinase Is Disrupted by Insertions in the Tether That Connects the Calmodulin-like Domain to the Kinase

Ca(2+)-dependent protein kinases (CDPK) have a calmodulin-like domain (CaM-LD) tethered to the C-terminal end of the kinase. Activation is proposed to involve intramolecular binding of the CaM-LD to a junction sequence that connects the CaM-LD to the kinase domain. Consistent with this model, a truncated CDPK (DeltaNC) in which the CaM-LD has been deleted can be activated in a bimolecular interaction with an isolated CaM-LD or calmodulin, similar to the activation of a calmodulin-dependent protein kinase (CaMK) by calmodulin. Here we provide genetic evidence that this bimolecular activation requires a nine-residue binding segment from F436 to I444 (numbers correspond to CPK-1 accession number L14771). Two mutations at either end of this core segment (F436/A and VI444/AA) severely disrupted bimolecular activation, whereas flanking mutations had only minor effects. Intramolecular activation of a full-length kinase was also disrupted by a VI444/AA mutation, but surprisingly not by a F436/A mutation (at the N-terminal end of the binding site). Interestingly, intramolecular but not bimolecular activation was disrupted by insertion mutations placed immediately downstream of I444. To show that mutant enzymes were not misfolded, latent kinase activity was stimulated through binding of an antijunction antibody. Results here support a model of intramolecular activation in which the tether (A445 to G455) that connects the CaM-LD to the kinase provides an important structural constraint and is not just a simple flexible connection.

Intramolecular activation of a Ca(2+)-dependent protein kinase is disrupted by insertions in the tether that connects the calmodulin-like domain to the kinase.

Ca(2+)-dependent protein kinases (CDPK) have a calmodulin-like domain (CaM-LD) tethered to the C-terminal end of the kinase. Activation is proposed to involve intramolecular binding of the CaM-LD to a junction sequence that connects the CaM-LD to the kinase domain. Consistent with this model, a truncated CDPK (DeltaNC) in which the CaM-LD has been deleted can be activated in a bimolecular interaction with an isolated CaM-LD or calmodulin, similar to the activation of a calmodulin-dependent protein kinase (CaMK) by calmodulin. Here we provide genetic evidence that this bimolecular activation requires a nine-residue binding segment from F436 to I444 (numbers correspond to CPK-1 accession number L14771). Two mutations at either end of this core segment (F436/A and VI444/AA) severely disrupted bimolecular activation, whereas flanking mutations had only minor effects. Intramolecular activation of a full-length kinase was also disrupted by a VI444/AA mutation, but surprisingly not by a F436/A mutation (at the N-terminal end of the binding site). Interestingly, intramolecular but not bimolecular activation was disrupted by insertion mutations placed immediately downstream of I444. To show that mutant enzymes were not misfolded, latent kinase activity was stimulated through binding of an antijunction antibody. Results here support a model of intramolecular activation in which the tether (A445 to G455) that connects the CaM-LD to the kinase provides an important structural constraint and is not just a simple flexible connection.

Insertion of O2 into a Pd(i)–Pd(i) dimer and subsequent C–O bond formation by activation of a C–H bond

The reaction between O2 and the dimer [Pd2(μ-Br)2(PBut3)2 ] leads to the formation of the novel dimeric compound [Pd(μ-OCH2{CMe2}PBu t2)Br]2 by coordination of O2 to the original palladium dimer and further formation of two C–O bonds by intramolecular activation of C–H bonds.

Stimulation of lysosomal sphingomyelin degradation by sphingolipid activator proteins

Lysosomal breakdown of glycosphingolipids with short hydrophilic carbohydrate headgroups is achieved by the simultaneous action of specific hydrolases and sphingolipid activator proteins (SAPs). Activator proteins are considered to facilitate the enzyme/substrate interaction between water-soluble enzymes and membrane-bound substrates. Sphingomyelin, containing the small hydrophilic phosphorylcholine moiety, is hydrolysed by acid sphingomyelinase (acid SMase). Recent experimental data on the in vivo and in vitro role of activator proteins in sphingomyelin breakdown by acid SMase are reviewed. These data combined with the results using homogenous protein preparations as well as a liposomal assay system mimicking the physiological conditions suggest that lysosomal sphingomyelin degradation is not critically dependent on any of the known activator proteins. Moreover, evidence is provided that the assumed intramolecular activator domain of acid SMase and especially the presence of negatively charged lipids in the lysosomes are sufficient for sphingomyelin turnover.

C−H Bond Activation with Actinides: The First Example of Intramolecular Ring Bite of a Pentamethylcyclopentadienyl Methyl Group

Thermolysis of (C5Me5)2U(NAd)2, 1 (Ad = 1-adamantyl), in benzene or hexane results in the intramolecular C−H bond activation of a methyl group on a pentamethylcyclopentadienyl ligand across the two imido functional groups. The product of this reaction has been spectroscopically and structurally characterized. The activation product is a reduced U(IV) metallocene bis(amide) complex with an N-bound methylene unit derived from the methyl group attached to one amide group. The activation parameters for this process have been determined; the results are consistent with a simple unimolecular process. This is the first example of intramolecular activation of a (C5Me5) methyl C−H bond in an actinide complex.

Cyclometallation Reactions in the Fe(dprpe)2 System [dprpe = 1,2-Bis(dipropylphosphino)ethane

Thermal elimination of methane from cis-FeH(CH3)(dprpe)2 [dprpe = 1,2-bis(dipropylphosphino)ethane] or photochemical elimination of dihydrogen from FeH2(dprpe)2 leads to the formation of three cyclometallation products. The products are believed to arise via a pathway in which the coordinatively unsaturated intermediate Fe(dprpe)2 undergoes intramolecular activation of a C–H bond of a dprpe methyl group. 1H and 31P n.m.r. spectroscopy were used to characterize the products and explore the reactions leading to their formation.

Cyclometallated platinum compounds with chiral imines. Crystal structure of [PtMe(2-FC6H3CHN-(S)-CHMePh)(PPh3)

The reactions of [Pt2Me4(μ-SMe2)2] (1) with ligands ArCHN-(S)-CHMePh (Ar2-BrC6H4 (2a), 2,6-Cl2C6H3 (2b), C6F5 (2c), 2-FC6H4 (2d), 2-CF3C6H4 (2e)), give either cyclometallated compounds [PtMe2X(RCHN-(S)-CHMePh)(SMe2)] (3), by intramolecular activation of C–X (X=Br, Cl or F) bonds, or compounds [PtMe(RCHN-(S)-CHMePh)(SMe2)] (5), by orthometallation, followed by methane elimination. The reactions of these compounds with PPh3 result in the formation of [PtMe2X(RCHN-(S)-CHMePh)(PPh3)] (4), or [PtMe(RCHN-(S)-CHMePh)(PPh3)] (6), respectively. Oxidative addition of methyl iodide to compounds 6 produces the corresponding complexes 4. The NMR spectra of compounds 3 and 4 indicate that these are formed as two diastereomers. The X-ray structure of [PtMe(2-FC6H3CHN-(S)-CHMePh)(PPh3)] (6d) is reported.

Full activation of MEN2B mutant RET by an additional MEN2A mutation or by ligand GDNF stimulation

Germline mutations of RET gene, encoding a receptor tyrosine kinase, have been associated with the MEN2A and MEN2B inherited cancer syndromes. In MEN2A mutations affecting cysteine residues in the extracellular domain of the receptor cause constitutive activation of the tyrosine kinase by the formation of disulfide-bonded homodimers. In MEN2B a single mutation in the tyrosine kinase domain (Met918Thr) has been identified. This mutation does not lead to dimer formation, but has been shown (both biologically and biochemically) to cause ligand-independent activation of the Ret protein, but to a lesser extent than MEN2A mutations. Intramolecular activation by cis-autophosphorylation of RetMEN2B monomers has been proposed as a model for activation, although alternative mechanisms can be envisaged. Here we show that the activity of RetMEN2B can be increased by stable dimerization of the receptor. Dimerization was achieved experimentally by constructing a double mutant receptor with a MEN2A mutation (Cys634Arg) in addition to the MEN2B mutation, and by chronic exposure of RetMEN2B-expressing cells to the Ret ligand GDNF. In both cases full activation of RetMEN2B, measured by 'in vitro' transfection assays and biochemical parameters, was seen. These results indicate that the MEN2B phenotype could be influenced by the tissue distribution or concentration of Ret ligand(s).