Molecular junctions

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Bazzoni G, Dejana E (2004) Endothelial cell-to-cell junctions molecular organization and role in vascular homeostasis. Physiol Rev 84 869-901... 

Troisi, A. and Ramer, M.A. (2006) Molecular signatures in the transport properties of molecular wire junchons what makes a junction molecular . Small, 2,172-181. 

Raman spectra were taken of duplex oligomers with the sequences, 15 -d(TN(CG)8AN)J2 in aqueous 6M NaCl solutions. These oligomers showed the presence of two B/Z junctions. Molecular modeling was used to show that the oligomer conformation went abruptly from the B to the Z form and from the Z back to the B form inside die (CG)g tract. 

Volume 49 Gap Junctions Molecular Basis of Cell Communication in... 

Fig. 7. Transport pathways of a polymer in the liver 1 — diffusion through theintercellular junctions (molecular-size limited process) 2 — transcellular route of polymer transport into the bile including pinocytosis into hqtatocyte and exocytosis of the vesicles or residual bodies at the lateral side of the cell 3 — pinocytosis into the Kupffer cells occurs regularly, from either the central or the interstitial compartment. H.C. — hepatocyte E.C. — endothelial cell of capillary wall K.C. — Kupffer cell I.S. — interstitial space B.C. — bile canaliculi P — pinocytic vesicle L — lysosomes primary) S.L. — secondary lysosome R.B. — residual body N — nudeus...

The probability matrix plays an important role in many processes in chemical physics. For chemical reactions, the probability of reaction is often limited by tunnelling tlnough a barrier, or by the fonnation of metastable states (resonances) in an intennediate well. Equivalently, the conductivity of a molecular wire is related to the probability of transmission of conduction electrons tlttough the junction region between the wire and the electrodes to which the wire is attached. 

An expression for the current across a molecular junction is developed by analogy with the description of unimolecular solution phase electron transfer. The conduction is written 1201... 

Using both condensation-cured and addition-cured model systems, it has been shown that the modulus depends on the molecular weight of the polymer and that the modulus at mpture increases with increased junction functionahty (259). However, if a bimodal distribution of chain lengths is employed, an anomalously high modulus at high extensions is observed. Finite extensibihty of the short chains has been proposed as the origin of this upturn in the stress—strain curve. 

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

Structures considered are quinolizinium (187), and 1,5- (188), 1,6- (189), 1,7- (190), 1,8- (191), and 2,7- (192) naphthyridines. In the naphthyridines the 10 7r-electrons are delocalized in five bonding molecular orbitals, which are distorted by the annular nitrogens in such a way that positions ortho and para to those nitrogens are less likely to be electrophilically haloge-nated than meta carbons. Compounds with a nitrogen at the ring junction carry a positive charge and will be naturally resistant to electrophilic attack. 

Cholinesterases (ChEs), polymorphic carboxyles-terases of broad substrate specificity, terminate neurotransmission at cholinergic synapses and neuromuscular junctions (NMJs). Being sensitive to inhibition by organophosphate (OP) poisons, ChEs belong to the serine hydrolases (B type). ChEs share 65% amino acid sequence homology and have similar molecular forms and active centre structures [1]. Substrate and inhibitor specificities classify ChEs into two subtypes ... 

Mc averaged molecular mass of strands between crosslinks, f functionality of junction (f = 3),... 

Crosslinked polymers were denser than the thermoplastic. The densities of the polymers increased proportional to the number of crosslinks ( Mc ) as shown by the two straight lines in Fig. 5.1. The volume occupied by the polymer was reduced by 0.008 nm3 for each junction introduced in the network. The change of volume was deduced from the slope in Fig. 5.1. Likewise, small voids close to the ends of the molecular chains may well be responsible for the lower density of the Phenoxy resin (g = 1.1807 Mgm-3) as compared to polymer E. 

Left-hand side Sic < 1 kg/mol the activation volume contains several molecular junctions. Right-hand side Sic > 1 kg/mol the volume of one strand is much larger than the activation volume... 

Consistent with their chemical differences, the molecular structures of i- and K-carrageenans are not identical. A shorter pitch and an offset positioning of the two chains in the kappa helix is compatible with the lack of sulfate group on every 3,6-anhydrogalactose residue. The variations in molecular structures mirror the types of junction zones formed by these polymers and relate to the observed gelation properties. 

Burridge, K., Path, K., Kelly, T., Nuckolls, G., Turner, C. (1988). Focal adhesions Transmembrane junctions between the extracellular matrix and the cytoskeleton. Ann. Rev. Cell Biol. 4,487-525. Cleveland, D.W. Sullivan, K.F. (1985). Molecular biology and genetics of tubulin. Ann. Rev. Biochem. 54,331-365. 

---