Showing papers on "Side chain published in 1986"

Dimerization energetics of benzene and aromatic amino acid side chains

Abstract: Interactions between aromatic side chains in protein crystal structures have been studied by geometric analysis and nonbonded interaction energy calculations, based on ab initio quantum mechanical calculations of dibenzene. Aromatic side chains in proteins pair with preferred centroid separations of between 3.4 and 6.5 A. Eighty-four percent of these aromatic pairs make enthalpically favorable edge-to-face interactions, which bring a delta(+) hydrogen atom of one aromatic ring near to the delta(-) ..pi..-electron cloud of the other aromatic ring. The distribution of observed interaction geometries differs substantially from random and depends critically on the spatial arrangement of the two aromatic rings. These data demonstrate that the edge-to-face interaction of two aromatic side chains makes an enthalpic contribution of between -1 and -2 kcal/mol to the energy stabilization of a protein and does not arise solely as a function of packing constraints.

An efficient, enantioselective synthesis of the taxol side chain

Abstract: Synthese de la chaine du taxol a partir du phenylacetylene avec comme etapes-cle une epoxydation asymetrique et la scission regioselective de l'epoxyde

Stereochemically constrained peptides. Theoretical and experimental studies on the conformations of peptides containing 1-aminocyclohexanecarboxylic acid

Abstract: Conformational energy calculations on the model system N-acetyl- 1 -aminocyclohexanecarboxylic acid N'methylamide (Ac-Acc6-NHMe), using an average geometry derived from 13 crystallographic observations, establish that the Acc6 residue is constrained to adopt conformations in the helical regions of In contrast, the a,a-dialkylated residue with linear hydrocarbon side chains, a,a-di-n-propylglycine favors fully extended backbone structures (6 1= $ = 180'). The crystal structures of two model peptides, Boc-(Acc6),-OMe (type 111 @-turn at -Acc6(1)-Acc6(2)-) and Boc-Pro-Acc6-Ala-OMe (type I1 P-turn at -Pro-Acc6-), establish that Acc6 residues can occupy either position of type 111 P-turns and the i + 2 position of type I1 @-turns. The stereochemical rigidity of these peptides is demonstrated in solution by NMR studies, which establish the presence of one intramolecular hydrogen bond in each peptide in CDCI, and (CDJ2S0. Nuclear Overhauser effects permit characterization of the @-turn conformations in solution and establish their similarity to the solid-state structures. The implications for the use of Acc6 residues in conformational design are considered.

Studies of chain conformational kinetics in poly(di‐n‐alkylsilanes) by spectroscopic methods 2. Conformation and packing of poly(di‐n‐hexylsilane)

Abstract: Unoriented and highly oriented films of poly(di‐n‐hexylsilane) have been studied by Raman scattering, wide angle x‐ray diffraction and optical absorption measurements. From a comparison of group theoretical predictions with those bands observed in the Raman experiments and from the x‐ray layer line spacing from an oriented sample, a planar zig–zag conformation for the silicon backbone was deduced. X‐ray reflections were suitably indexed by a monoclinic unit cell containing two molecules. Polarized Raman studies on uniaxially oriented samples also revealed that the hexyl side chains are not orthogonal to the silicon backbone but may be slightly tilted in order to minimize intramolecular steric interactions.

Use of binding energy in catalysis analyzed by mutagenesis of the tyrosyl-tRNA synthetase

Abstract: The utilization of enzyme-substrate binding energy in catalysis has been investigated by experiments on mutant tyrosyl-tRNA synthetases that have been generated by site-directed mutagenesis. The mutants are poorer enzymes because they lack side chains that form hydrogen bonds with ATP and tyrosine during stages of the reaction. The hydrogen bonds are not directly involved in the chemical processes but are at some distance from the seat of reaction. The free energy profiles for the formation of enzyme-bound tyrosyl adenylate and the equilibria between the substrates and products were determined from a combination of pre-steady-state kinetics and equilibrium binding methods. By comparison of the profile of each mutant with wild-type enzyme, a picture is built up of how the course of reaction is affected by the influence of each side chain on the energies of the complexes of the enzyme with substrates, transition states, and intermediates (tyrosyl adenylate). As the activation reaction proceeds, the apparent binding energies of certain side chains with the tyrosine and nucleotide moieties increase, being weakest in the enzyme-substrate complex, stronger in the transition state, and strongest in the enzyme-intermediate complex. Most marked is the interaction of Cys-35 with the 3'-hydroxyl of the ribose. Removal of the side chain of Cys-35 leads to no change in the dissociation constant of ATP but causes a 10-fold lowering of the catalytic rate constant. It contributes no net apparent binding energy in the E X Tyr X ATP complex and stabilizes the transition state by 1.2 kcal/mol and the E X Tyr-AMP complex by 1.6 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Optical biaxiality of nematic LC-side chain polymers with laterally attached mesogenic groups

Abstract: Liquid crystalline polymers with mesogenic groups laterally attached to the polymer backbone were investigated by polarizing microscopy with respect to their optical properties. The nematic polymers were macroscopically oriented in a magnetic or electric field. Polarization microscopic investigations with convergent polarized light show optical biaxial behavior of the nematic phase.

Metal complexes with macrocyclic ligands. Part XXV. One‐step synthesis of mono‐N‐substituted azamacrocycles with a carboxylic group in the side‐chain and their complexes with Cu2+ and Ni2+

Abstract: Mono-N-substituted azamacrocycles 2–7, containing a carboxyalkyl or carboxyaryl side-chain, are obtained by reacting a five-fold excess of the macrocycle with 1 equiv. of a suitable halogenocarboxylic acid in alkaline aqueous EtOH. For halogenocarboxylic acids, which easily lactonize under alkaline conditions, a variant with the corresponding ester or nitrile as alkylating agent is also described. The salient point of this synthesis lies in the use of an excess of the macrocycle over the alkylating agent, thus reducing the amount of polyalkylation to a minimum, and in the easy separation of the excess of unreacted educt from the aminocarboxylic acid. These new ligands form Ni2+ and Cu2+ complexes, the spectral properties of which have been studied. In the case of the Cu2+ complexes with ligand 2, 3, and 6, a pH-dependent color change is observed. This is explained with an equilibrium between a species, in which the carboxylate group is bound to the metal, and one, in which it is protonated and non-coordinated. In the case of the Ni2+ complexes with the same ligands, only the species with a coordinated carboxylate was observed. In the Cu2+ and Ni2+ complexes with ligands 4 and 5, however, the carboxylate group does not coordinate at all, because of the length or the special structure of the chain.

Poly(vinyl ether)s and poly(propenyl ether)s containing mesogenic groups: a new class of side-chain liquid-crystalline polymers

Abstract: Preparation des polymeres cristaux liquides thermotropes contenant le mesogene methoxy-4 hydroxy-4' biphenyle dans la chaine laterale et differents espaceurs entre les mesogenes et le squelette et caracterisation par DSC et microscopie optique

Effects of substituent-induced strain on the electronic structure of polydiacetylenes

Abstract: The valence effective Hamiltonian technique (VEH), and modified neglect of differential overlap (MNDO) calculations are used to study the influence of strain induced by side chains on the geometry of polydiacetylene backbones and the resulting polymer band structure, band gap, and ionization potential. Simulations of strain effects on the polymer backbone yield variations in optical properties which are similar to those observed experimentally during thermochromic phase changes in urethane‐substituted polydiacetylenes. Our results suggest that these changes in optical properties are related to strain at points of substituent attachment and not to fundamental changes in the backbone geometry such as an acetylenic‐to‐butatrienic transformation.

Liquid Crystalline Side Chain Polymers Derived From Poly-acrylate, Poly-methyacrylate and Poly-α-chloroacrylate

Abstract: Interest in liquid crystalline side-chain polymers has grown tremendously in recent years because of their theoretical aspects and their potential applications. Over the past few years, a considerable research effort has been made in electro-optic display using nematic or smectic. A mesogenic side-chain polymers. On the other hand, non linear optical effects have been obtained with a nematic side-chain copolymer doped with a molecule exhibiting an extremely large molecular hyperpolarizability β and poled with a D.C. electricfield.

Polymers with Terminally Cyanobiphenyl-Substituted Side Chains A Re-Entrant Polymorphism N SA NRe

Abstract: Two acrylate side chain polymers in which the mesogenic moiety is a 4′-cyano-biphenyl-4-yl group have been synthesized and characterized by d.s.c., optical microscopy and X-ray diffraction. If the mesogenic moiety is linked to the polymer backbone by a -O-CO-(CH2)-5 group, the resultant polymer is nematic. For a -O-(CH2)-6 group, the unusual NSANre sequence is observed. X-ray diffraction has shown that the SA phase has a partially bilayer structure : the molecules are arranged in an antiparallel, overlapping interdigitated structure with a layer spacing of about 1.4 times the length of the side chains. Monolayer fluctuations are also observed in this SA phase.

Some Novel Smectic* Liquid-Crystalline Side-Chain Polymers

Abstract: Three acrylate side-chain polymers in which the mesogenic moieties are based on the 4-n-alkoxyphenyl-4′-(4″-methylhexyloxy) benzoates have been characterized by differential scanning calorimetry, optical microscopy and X-ray diffraction. For shorter flexible spacers (n = 2) both smectic A and C* phases are observed thus making this polymer interesting for the fabrication of electro-optical devices based on ferroelectric properties (a smectic A phase is required for alignment purposes). For longer flexible spacers, (n = 6, 11) only the smectic A phase remains.

Synthesis and antithrombogenicity of polyetherurethaneurea containing quaternary ammonium groups in the side chains and of the polymer/heparin complex

Abstract: Novel polyetherurethaneureas containing tertiary amino groups in the side chains (PAEUU) were synthesized, quaternized with different alkyl halides (Q-PAEUU), and heparinized (H-PAEUU). The antithrombogenicity of PAEUU in vitro was improved by quaternization, and further by heparinization. The excellent antithrombogenicity of H-PAEUU was controlled by the kind of quaternizing agent through the polar effect of quaternizing agent on the water content and through the steric effect of quaternizing agent on the heparin content of H-PAEUU. The antithrombogenicity of H-PAEUU was found to be affected by the water content more strongly than by the heparin content. H-PAEUUs containing tertiary amino groups in the main chain, which were synthesized previously, showed a little better short-term antithrombogenicity than the present H-PAEUUs containing tertiary amino groups in the side chains. Since ammonium groups in the side chains of Q-PAEUU impose little steric hindrance against the heparin adsorption, the release of heparin from the side chains of H-PAEUU was slower but lasted longer than that from the main chain. Therefore, the present H-PAEUU is expected to be a long-term antithrombogenic material.

Copper(II) complexes of diastereoisomeric dipeptides in aqueous solution. Effect of side-chain groups on the thermodynamic stereoselectivity

Abstract: Preuve de la stereoselectivite thermodynamique dans la formation des complexes deprotones avec formation favorisee thermodynamique, des complexes des diastereoisomeres LL

Structure of pressinoic acid: the cyclic moiety of vasopressin.

Abstract: Arginine vasopressin consists of a 20-membered, disulfide-linked macrocyclic ring system called pressinoic acid to which is attached a COOH-terminal tripeptide. The molecular conformation of pressinoic acid has been determined from single crystal x-ray diffraction data. The 20-membered macrocyclic ring, stabilized by two intramolecular hydrogen bonds, has a type I beta-bend centered on Gln4 and Asn5 and a highly distorted type II' bend centered on Phe3 and Gln4. In vasopressin the Asn5 side chain extends away from the macrocyclic ring system and hydrogen bonds to the terminal tripeptide, but in pressinoic acid the Asn5 side chain lies over the molecule and forms a strong hydrogen bond to the nitrogen of Tyr2. The absence of pressor activity in pressinoic acid may be a result of both the loss of the COOH-terminal tripeptide and the incorrect orientation of the Asn5 side chain. Whether this class of hormones has pressor or oxytocic activity is determined by the orientation of the Tyr2 side chain, that is, whether it is extended away from or over the ring system, respectively. In pressinoic acid, the Tyr2 side chain is in the expected "pressor conformation," that is, extended away from the ring system, and is stabilized through a hydrophobic interaction with the Phe3 side chain. Thus, the conformation of the pressinoic acid molecule partly explains the activity of vasopressin-like hormones.

Preparation of liquid crystalline polysiloxanes with terminal cyano groups in the side chains

Abstract: Preparation par polyhydrosilylation de poly(hydrogenomethylsiloxane avec une chaine laterale terminal alcenique en presence d'acide hexachloroplatinique dans le propanol-2

Production of styrene polymer

Abstract: PURPOSE: To obtain the titled polymer where the stereoregular structure of side chains have a syndiotactic configuration, by polymerizing styrene (derivative) in the presence of a catalyst component comprising a titanium compound and a specified product of contact. CONSTITUTION: An organoaluminum compound (e.g., trimethylaluminum) is contacted with a condensing agent (e.g., water) to obtain a product of contact (ii). A titanium compound (i) selected from titanium (chelate) compounds of formulas I and II [wherein R 1W3 are each H, a 1W20C alkyl, a 1W20C alkoxy, a 6W20C aryl, an alkylaryl, arylalkyl, a 1W20C acyloxy, a (substituted) cyclopentadienyl or indenyl, X 1 is a halogen, a, b and c are each 0W4, and d and e are each 0W3] is contacted with component (ii) at a molar ratio of Al of component (ii) to Ti of component (i) of 1W10 6 to obtain a catalyst component. Styrene (derivative) is polymerized at 0W90°C in the presence of this component. COPYRIGHT: (C)1987,JPO&Japio

Spectral studies on structure‐activity relationships of thromboxane synthesis inhibitors

Abstract: Thromboxane A2 synthase is a cytochrome P450-type enzyme and its interaction with imidazole or pyridinebased inhibitors could be studied by absolute and difference spectroscopy with the solubilized as well as the purified enzyme. Nitrogenous bases shift the 418-nm Soret absorption by 4–6 nm to the red and among them the best inhibitors of enzyme activity showed a stoichiometric binding to the enzyme. The structural and energetic prerequisites for such high binding affinities were primarily the liganding of the basic nitrogen to the hemin but also the attachment of a hydrophobic carboxylic side chain to the active site at an about 1 nm distance from the nitrogen. In addition, the side chain seemed to be oriented almost parallel to the plane of the heme. If this geometry was changed, a decrease in affinity was observed and if the ligand binding was sterically hindered, a spectral shift to a five-coordinated complex absorbing at 390 nm occured. This is best explained by the displacement of an endogenous oxygen ligand, presumably water, from the sixth coordination position of the heme. From these results it can be concluded that the inhibitors mimic the binding of prostaglandin H2 (PGH2) with its carboxylic group at the carboxyl side chain and the endoperoxide oxygen atom at C9 as previously reported. The methyl side chain of PGH2 does not seem to play a role in the formation of the enzyme-substrate complex.

Thermal transfer recording medium

Abstract: PURPOSE:To provide both lubricity and adhesiveness, which are generally contradictory properties, and obtain a feed-improving layer with a favorable film-forming property, by causing the feed-improving layer to comprise a graft copolymer having a polymer excellent in release properties and lubricity as a side chain and a polymer excellent in adhesiveness and film-forming properties as a backbone chain. CONSTITUTION:A feed-improving layer provided on the upper side of a base film comprises a graft copolymer having a polymer excellent in release properties and lubricity as a side chain and a polymer excellent in adhesiveness and film-forming properties as a backbone chain. The polymer excellent in release properties and lubricity is preferably a polymer having a critical surface tension of not more than 22 dyne/cm and a frictional coefficient of not more than 0.3. The polymer excellent in adhesiveness and film-forming properties is preferably a polymer having a critical surface tension of not less than 37 dyne/cm. In order that the backbone chain moiety, e.g., an acrylic polymer moiety and the side chain moiety, e.g., a silicone polymer moiety display the respective characteristics thereof, they are preferably used in a weight ratio of 2:0.5-1:3.5. The thickness of the feed-improving layer is preferably 0.1-1.5 mum. By this, release properties and lubricity are developed.

Conformation of Side Chain Nematic Polymers in Nematic Solutions: a Small-Angle X-Ray Scattering Study: SAXS

Abstract: Using a small-angle x-ray scattering technique, we have determined the sizes of side chain mesomorphic polymers diluted in a low-molecular-weight liquid crystal, respectively, parallel (R∥) and perpendicular (R⊥) to the director of the nematic medium (n0). We found a significant difference between the two values, with R∥ > R⊥. These results are a direct proof of the anisotropy of the polymer conformation in a nematic medium; they permit to check the validity of the interpretation that we have previously given for the hydrodynamical behaviour of such solutions.