Showing papers on "Steric effects published in 1986"

Probing steric and hydrophobic effects on enzyme-substrate interactions by protein engineering

Abstract: Steric and hydrophobic effects on substrate specificity were probed by protein engineering of subtilisin. Subtilisin has broad peptidase specificity and contains a large hydrophobic substrate binding cleft. A conserved glycine (Gly(166)), located at the bottom of the substrate binding left, was replaced by 12 nonionic amino acids by the cassette mutagenesis method. Mutant enzymes showed large changes in specificity toward substrates of increasing size and hydrophobicity. In general, the catalytic efficiency (k(cat)/K(m)) toward small hydrophobic substrates was increased (up to 16 times) by hydrophobic substitutions at position 166 in the binding cleft. Exceeding the optimal binding volume of the cleft ( approximately 160 A(3)), by enlarging either the substrate side chain or the side chain at position 166, evoked precipitous drops in catalytic efficiency (k(cat)/K(m)) (up to 5000 times) as a result of steric hindrance.

Steric and electronic substituent effects on the carbon-carbon bond

Abstract: Correlations of various indices of the stability and reactivity of carbon-centered radicals with ESR hyperfine splitting constants have been examined. For a large number of monoand disubstituted radicals there is a moderately good linear correlation of R-proton hyperfine splitting constants (a(HR)) with radical stabilization enthalpies (RSE) and with BDE(C-H), the C-H bond dissociation energies for the corresponding parent compounds determined from thermodynamic and kinetic studies of C-C homolysis reactions. There is a similarly satisfactory linear correlation of a(HR) with BDE(C-H) determined by Bordwell’s electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, â-proton hyperfine splitting constants (a(HâMe)) for radicals with a freely rotating methyl substituent are less sensitive to deviations from planarity and give better linear correlations with RSE and BDE(C-H). The correlations cover a range of more than 20 kcal/mol and are reliable predictors of RSE and BDE(C-H) for a variety of radicals including captodative species. However, the correlations fail for significantly nonplanar radicals and for radicals with cyclic delocalized systems, e.g., cyclopentadienyl. The ratio a(HâMe)/ a(HR) for suitably substituted radicals provides an index of pyramidalization and allows one to decide for which compounds values of RSE and BDE(C-H) can be confidently estimated.

Steric effects on the solution chemistry of nickel(II) complexes with N-monomethylated 14-membered tetraaza macrocycles. The blue-to-yellow conversion and the oxidation and reduction behavior

Abstract: Preparation de 3 macrocycles azotes dans lesquels un groupe CH 3 a ete introduit sur un atome N du cyclane ou de l'isocyclane. Effet de ce groupe sur le comportement en solution des complexes de Ni

Ligand Effects on Metallocene Catalyzed Ziegler-Natta Polymerizations

Abstract: The effects of the chiralities, steric requirements and basicities of ligands attached to soluble Ti and Zr metallocene catalysts on propylene and ethylene homo- and copolymerizations have been reviewed. Isotactic polypropylene configurational structures are controlled by chiral cyclopentadienyl (Cp) ligands and chiral chain-ends; possibly both individually and simultaneously. Ethylene-propylene copolymerization reactivity ratios are predominantly influenced by ligand steric effects. The polymerization rates and polymer molecular weights obtained in Zr catalyzed ethylene polymerizations vary according to both Cp basicities and steric requirements. Novel polypropylene microstructures, narrow polydispersities (Mw/Mn), bimodal polyethylene molecular weight distributions and tailored copolymer composition distributions have been obtained.

Intramolecular van der Waals interactions and 13C chemical shifts: Substituent effects in some cyclic and bicyclic systems

Abstract: In recent work, a reasonably good linear correlation was found to exist between the local van der Waals steric energy as calculated by the MM2 force field and the chemical shift of the resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. This study extends this work to some additional cyclic systems: 91 trans- and 31 cis-decalin carbons, 57 bicyclo[2.2.1]heptane and 27 bicyclo[2.2.2]octane nuclei have been investigated and yield root-mean-square errors of 3.3–3.7 ppm and correlation parameters in good agreement with those found previously for other carbon species. In the more strained bicyclo systems it is found that additional class parameters must be defined. The theory is applied to a variety of α,β,γ,… ubstituents effects in cyclohexane and decalin systems, and correlated these resonant shifts under one simple, physical model.

Molecular mechanics and molecular shape. Part 4. Size, shape, and steric parameters

Abstract: To deal with steric effects of substituents on reaction rates, two types of descriptor are proposed. Shape is characterised by the ratio (G) of the substituent's surface area to its volume. Bulk is characterised by the volume (Va) of the portion of the substituent that is within 0.3 nm of the reaction centre. For alkyl substituents, rate constants for several series of reactions (including the classical series, acid hydrolysis of the esters RCO2Et) correlate fairly well with Va and G. Taft's steric parameters (Es) correlate less well. Values of Es for hetero-substituents are found to be contaminated by non-steric factors. It is possible to construe whether the contaminant acts to hinder or to enhance reaction.

Barriers to rotation adjacent to double bonds. 2. n-Propyl versus isopropyl groups

Abstract: The barriers to rotation about the C-C bonds adjacent to the carbonyl groups of isobutyraldehyde, methyl isopropyl ketone, and isobutyric acid were calculated. The 3-21G basis set was used for the geometry optimizations, and the 6-31G* basis set was used to obtain the energies. The differences in energy between R = n-propyl and isopropyl also were calculated and reproduced the observed energy differences. Whereas the more branched isomer had a significantly lower energy for the aldehydes and acids, the difference in energy was very small with the ketones. The components of the barrier are discussed. The traditional decomposition into 1-, 2-, and 3-fold terms does not provide a useful representation of the interactions which are involved. Besides the 3-fold barrier observed with compounds having R = CH3, the major contributions to the barrier arise from the stabilizing interaction between an alkyl group and the carbonyl (- 1 kcal/mol) and from the repulsive interaction between one of the methyls of the isopropyl group and the other substituent at the carbonyl. A hydroxy group (Le., in a carboxylic acid) leads to a significantly smaller steric interaction than found with a methyl group (i.e,, in a methyl alkyl ketone).

Intramolecular Hydrogen Bonds of the CO…HO Type as Studied by 17O-NMR

Abstract: The 17O-NMR spectra of 1,4-naphthoquinone and 5-hydroxy-1,4-naphthoquinone (juglone) have been recorded in CDCl3 solution at 40°. In juglone the 17O resonance of the carbonyl peri to the OH group was displaced by 70 ppm to low frequency relative to the resonance in the para-position. It is shown that this chemical shift arises mainly from intramolecular H-bonding, the substituent and steric effects being one order of magnitude smaller. Large carbonyl 17O chemical shifts between −34 and −100 ppm were also observed in a series of aromatic aldehydes and ketones where intramolecular H-bonds of the CO…HO type are formed. The H-bond-induced carbonyl 17O chemical shifts were linearly correlated with both the 17O and 1H chemical shifts of the OH groups. They represent a most sensitive measure of the strength of intramolecular H-bonds. The 17O resonances of the OH groups were directed to high frequency on H-bonding. Analysis of the 17O chemical shifts in 2,2′-dihydroxy-benzophenone showed clearly that the two OH groups build H-bonds simultaneously to the single carbonyl group. The 17O linewidths decreased strongly on H-bonding; the linewidth of the H-bonded carbonyl O-atom in juglone, for example, was reduced by 25% with respect to that of the free carbonyl O-atom. The carbonyl O-atom quadrupole coupling constants in juglone, evaluated from the combined use of 13C and 17O relaxation times, were 9.5 and 11.0 MHz, respectively. No correlation was observed between the H-bond-induced 17O chemical shifts and the variations in 17O quadrupole coupling constants.

Radical cation chain oxygenation of dienes by triplet oxygen

Abstract: Alkylated 1,3-dienes are shown to oxygenate to endoperoxides by a cation radical chain mechanism. Steric slowing of Diels-Alder dimerization appears to be necessary; 1,3-cyclohexadiene gave only dimer under conditions where its 1-isopropyl-4-methyl derivative gave a 96% yield of ascaridole. The cation radical chain conditions tolerate more steric crowding than does the singlet oxygen reaction, as 4,4'-bihomoadamantenyl gave an 84% yield of its endoperoxide while singlet oxygen gave a mixture of several products. Oxygenation products were not observed from biadamantylideneethane which is sterically prohibited from assuming an s-cis conformation.

Cyan dye-forming couplers and photographic materials containing same

Abstract: Novel phenolic cyan dye-forming couplers contain a p-cyanophenylureido group in the 2-position and in the 5-position an acylamino group containing bulky alicyclic or heterocyclic substituents sufficient to provide steric interaction within the coupler molecule and the dye molecule derived therefrom. The couplers are useful in photographic emulsions.

Modelling the interaction of small organic molecules with biomacromolecules. I. Interaction of substituted pyridines with anti-3-azopyridine antibody.

Abstract: An approach is presented for modelling the biological activity of organic molecules. This approach requires a consideration of the influence of all factors (topological, steric, hydrophobic, electronic) which determine the bioactivity. In this work, the interaction between substituted pyridines and antibodies generated by anti-3-azapyridine is studied. The stereoelectronic interactions are responsible for the reaction. Meta-positions to nitrogen are found to be the most probable positions for attack. The most likely reaction products are pi-complexes with charges transfer from the biomolecule to the pyridine derivatives followed by the formation of covalent-type bonds.

Stereoelectronic effects in the hydrolysis of bicyclic and acyclic phosphates and phosphorothionates

Abstract: Mise en evidence d'une augmentation de la vitesse pour les phosphate et thiophosphate bicycliques trioxa-2,6,7 phospha-1 bicyclo [2.2.2] octanes oxo-1 (ou thioxo-1) methyl-4 substitues compares a leurs analogues acycliques, les phosphate et thiophosphate de triethyle

Steric and electronic effects on atom transfer reactions of Re(CO)4L.cntdot. radicals with organic halides and tributylstannane

Abstract: Re(CO)/sub 4/L x (L = CO, P(OR)/sub 3/, PR/sub 3/, AsEt/sub 3/) radicals, generated by laser flash photolysis of L(CO)/sub 4/Re-Re(CO)/sub 4/L, react with halogen atom donors (CH/sub 2/Br/sub 2/, CHCl/sub 3/, CCL/sub 4/) or hydrogen atom donors (HSn(n-Bu)/sub 3/) in solution to produce cis-Re(CO)/sub 4/LX (X = H, Cl, Br). Bimolecular rate constants for halogen and hydrogen atom transfers to Re(CO)/sub 4/L x were measured by following the decay of the transient Re(CO)/sub 4/L x absorbance in the 550-nm region. The rate constants for both the halogen and hydrogen atom transfer processes are influenced by steric and electronic properties of the substituent L. Both bromine and chlorine atom transfer processes depend to a similar extent on the electronic properties of L. The bromine atom transfer reactions show a slightly greater steric dependence than the chlorine atom transfer reactions. The steric dependence of the hydrogen atom transfer reaction is similar to that of the halogen atom transfers, but the dependence on the electronic properties of L is smaller. Increasing electron density at the metal center enhances the rate of atom transfer; polar effects are more evident in the halogen atom transfer. Rate constants for both atom transfer reactions fit a more » two-parameter free energy relationship, wherein electronic and steric effects of L are represented. « less