Molecular constraints

Entropic trap A strategy aimed at improving the efficiency of catalytic antibodies, via the incorporation of a molecular constraint into the transition state analogue that gives the hapten a higher energy conformation than that of the reaction product. 

The molecular constraint which precludes ligand exchange unambiguously ensures that the tricyclic compounds undergo nucleophilic attack exclusively at the pseudo-equatorial carbon atom, irrespective of the nature of the carbon atom situated in the apical ligand of the TBP. The compounds 1-4 were obtained by alkylation of the chalcogen atom of the corresponding bicyclic (thio)phosphate, 6 and 7, with trialkyloxonium tetrafluoroborate at -78 °c (2). 

In the MSF theory, the function,/, in addition to simple reptation, is also related to both the elastic effects of tube diameter reduction, through the Helmholtz free energy, and to dissipative, convective molecular-constraint mechanisms. Wagner et al. arrive at two differential equations for the molecular stress function/ one for linear polymers and one for branched. Both require only two trial-and-error determined parameters. 

Given the complexity of excimer behaviour believed to be present in all the decays measured it is perhaps not surprising that there are shortcomings in the different kinetic interpretations offered by Equations 3 and 4. Indeed, particularly for BuPBD which is reluctant to form an intermolecular excimer, it is possible to envisage that there exists in Poly(VBuPBD) a distribution of preformed excimer conformations each with differing overlap, molecular constraint, chromophore separation and decay rate. What is interesting is that these features also look to be apparent in Poly (VPPO) even though PPO itself readily forms an intermolecular excimer. Even if just two excimer conformations are present in these polymers the apparent anomalies of Tables I, II and III now start to appear more rational. 

The FILTER program implements the distance constraints. The program takes a list of inter-molecular constraints which can either be residue to... 

In another example of using molecular constraints to force chair and boat transition states on a 3,3-shift, Shea determined the activation parameters for the irreversible rearrangements of the threo- and m 50-bis-2-methylenecyclopentyls to l,2-bis(l-... 

Liu W T, Katsaros K B, Businger J A. 1979. Bulk Parameterization of air-sea exchanges of heat and water vapor including the molecular constraints at the interface. J Atmos Sci, 36 1722-1735. 

In addition, the CST values were used as molecular constraints in determining the model structure that provides the best correspondence between theory and experiment using a least-squares minimization function for all three ROY polymorphs ... 

In a number of six-neoft>ered ring model systems in which the torsional angles are rigidly defined by some molecular constraint, such as the two dlastereomerlc phosphate trlesters 1 and 2... 

P. Palffy-Muhoray, M. A. Lee and R. G. Petschek, Ferroelectric nematic liquid crystals realizability and molecular constraints, Phys. Rev. Lett. 60, 2303-2306 (1988). 

To extract molecular constraints of the crystallites, a combination of DQ—SQ NMR and FSLG-HETCOR experiments was... 

This led the authors to speculate that the molecular constraints for cyclization when the alkyne is present may require too high of energy barrier for ring-closing metathesis to occur. 

Non-covalent interactions create less significant three dimensional molecular constraints. Because insulin must be free to bind its cellular receptor while hemoglobin must rearrange itself under the stress of oxygen tension to exhibit its characteristic sigmoidal affinity, non-covalent interactions were also used to immobilize these bio/molecules to the aquasomes. 

Sorrell, E.M., Wan, H., Araya, Y., Song, H., and Perez, D.R. (2009) Minimal molecular constraints for respiratory droplet transmission of an avian-human H9N2 influenza A virus. Proceedings of the National Academy of Sciences of the... 

Molecular constraints belong to the class of scleronomous (i.e., time independent) holonomic constraints of the form... 

Synthetic retinoids have effects similar to those of natural retinoids. A number of different natural or synthetic retinoids have been used in the experiments discussed. Judging from the limited number of comparative studies, no dramatic differences between different retinoids are found. In the induction of cell-mediated cytotoxicity, retinoic acid, 13-cw-retinoic acid, retinyl palmitate, and etretinate all had quite comparable stimulatory effects (Lotan and Dennert, 1979). In the DTH reaction, retinyl palmitate, motretinid (F3), and 13-c -retinoic acid had stimulatory effects, but etretinate (FI) had no effect (Athanassiades, 1981). At present, it is therefore not clear what the molecular constraints on the retinoid molecule may be for stimulatory effects on the immune system. 

Hence, the rest of the activation energy can be attributed primarily to the inter-molecular interactions. Lin [17] has demonstrated that around 18 chain strands are present in one cubed tube diameter in the melt of a linear PE (= 27 nm ). The term tube represents an approximation of the inter-molecular constraint of the chain in a melt imposed by the surromiding chains. Taking the range and shape of the inter-molecular interaction potential [15] into account, the above number of contributions to the inter-molecular interaction potential seems to be reasonable. Note, however, that these considerations represent approximations due to many simplifications involved. 

Studies on metal cluster preparation and characterization have been focused mainly on faujasites. There is a need to extend these investigations to other zeolite types, as well as new microporous and mesoporous materials, in order to prepare metal clusters in different geometric and electronic environments. Both medium-pore and mesoporous materials have a potential interest. Thus, conventional zeolites are not well suited for catalytic reactions involving fine chemicals in the liquid phase because of molecular size restriction and/or very low diffusivities. To take advantage of molecular constraints while tailoring the regio-, chemo- and stereoselectivity of metal-catalyzed reactions of... 

Bichromophores of diverse structure undergo intramolecular ortho-cycloaddition, and molecular constraints can restrict the ring opening of the initial adduct. Examples of this feature include the formation of 34 by chemo-, regio-, and stereo-selective intramolecular addition of the benzene-thymine bichromophore 35 and the conversion of the chromone 36 in benzene solution to the intramolecular adduct 37. However, carrying out the latter reaction in methanol or further irradiation of the ortho adduct gives the product 38 of a novel rearrangement. 

Orientation affeets the permeability of polyethylene by changing its degree of crystallinity, lamellar organization, and molecular constraints. Strain-induced crystallization may increase the degree of crystallinity, while noncrystalline regions become better paeked as a function of orientation [47,48], both of which reduce the free volume of the system and hence reduce its permeability. Not withstanding, the effects of orientation are complex, and contradictory results have been reported [49]. 

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