Conductivity kinks

Chemical bonds are defined by their frontier orbitals. That is, by the highest molecular orbital that is occupied by electrons (HOMO), and the lowest unoccupied molecular orbital (LUMO). These are analogous with the top of the valence band and the bottom of the conduction band in electron band theory. However, since kinks are localized and non-periodic, band theory is not appropriate for this discussion. 

Receptor sites. The intercalation and kinked sites in DNA used in this study are listed in Table V. Three theoretically determined intercalation sites (I, II and III) permit the study to be conducted with the DNA unwound by 7°-12°, lU-l8° and 25°-32° with parallel base pairs separated by 6.76 A and with alternating (a) sugar puckers (67,68) Attention will be confined to site I because it was found to be the most favorable in the present studies. Several kink (K) sites have been identified (66). The constraint that proper hybridization exists about N2(g) and CIO of BPDEs stimulated an investigation in kinked DNA. In an idealized structure the pyrene moiety is approximately parallel to one of the base pairs as shown... 

We note that at the consolute point the conductivity is still metallic, the appearance of an activation energy e2 occurring for somewhat lower concentrations. The reason for this, in our view, is as follows. The consolute point should occur approximately at the same concentration as the kink in the free-energy curve of Fig. 4.2, namely that at which the concentration n of carriers is of order given by n1/3aH 0.2. Above the consolute point there is no sudden disappearance of the electron gas as the concentration decreases its entropy stabilizes it, so metallic behaviour extends to lower concentrations, until Anderson localization sets in. Conduction, then, is due to excited electrons at the mobility edge, as discussed above. 

A number of different X-ray structures of bacterial potassium channels reveal the detailed atomic picture of the pore-forming part, helices S5 and S6 [9]. KcsA, which is crystallized in the closed conformation, has an overall structure similar to an inverted teepee [9a], Four identical subunits surround the ion-conducting pathway (Figure 8.2). Each subunit contains two full transmembrane helices, S5 and S6, as well as the P loop. The S6 helices line the central cavity, whereas the S5 helices are involved in interactions with the lipid environment. In the closed channel conformation the transmembrane helices meet at the cytosolic side to block the ion conduction path. In the open conformation of the channel, the S6 helix kinks at a conserved glycine residue to open the ion conduction path, as shown in the structure of the bacterial channel MthK [10], The ion conduction path is formed by the selectivity filter and the large water-filled central cavity. 

Polythiazyl. This polymer, (SN), known since 1910, can now be obtained in a pure state. It is golden bronze in color and displays metallic type electrical conductance more remarkable still is the fact that at 0.26 K it becomes a superconductor. In the crystal the kinked, nearly planar, chains (12-IX) lie parallel and conductance takes place along the chains, in which n electrons are extensively delocalized according to molecular quantum mechanical calculations. A partially brominated substance, (SNBr04) , is an even better conductor. 

The results in Table VII depict how the band structure of carbyne [-CaC] (corresponding to Y - in Table VII) is affected by periodic inclusion of selected atoms or groups which, by introducing a kink into the otherwise rectilinear chain of cart e, would provide greater conformational versatility and therefore possibly improve processability. The chains were considered in their trans planar zig-zag conformation. It is seen that any of the modifica-tions to carb e indicated in Table VII produces an increased E value and a decreased BW value. That electrical conductivity would be adversely affected is reasonable given the disruption of the conjugated system caused 1 such modifications. As yet, no clear relationship is apparent between the specific molecular nature of Y and the effect of its inclusion on E and BW the relatively minor effect for the case Y - CH2 is notewSrthy and under continued study. 

Fig. 4.10. Magnetoresistance of 0 -(ET)2KHg(SCN)4 for B applied perpendicular to the conducting plane. The large arrows indicate the kink field. The small arrows mark the SdH oscillation peaks. Prom [289]...

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