Disordering substrates

Cluster 2 appears to be unique among Fe-S-containing proteins whose structures have so far been determined, emd it has been termed the hybrid cluster (6) because of its diverse chemical nature. Figure 14 is a schematic drawing of the cluster as interpreted from the final electron density synthesis and the structure refinement. The cluster contains both oxygen and sulfur bridges, and X represents a site whose precise nature has not been determined, but which may contain a partially occupied and/or disordered substrate molecule (see Section III,B,3,b). The environments of the four iron atoms can be described as follows. 

Figure 25 Loop ordering caused by triclosan binding to ecFabi. (a) Structure of the ecFabi monomer in compiex with NAD" (siate). Active site residues are shown in gray. The ends of the disordered substrate-binding ioop are shown in red.

At this step, then, the differences between the effects of surface chemistry and secondary molecular motions as dictated by morphological order can be observed on thrombogenesis. In the case of the sterically ordered substrate, in conjunction with the subsurface cationic array, the sorbed pro-tein(s) can assume a paracrystalline state, and subsequently be subjected to further pertubations. However, in the case of the disordered substrate, the sorbed proteins can not assume a paracrystalline state and, therefore, will not, per se, be subjected to any further conformational changes. 

Fig. 8.6. Profiles of the lowest (thick line) and of one of the highly excited modes of biaxial (P i,n) and director (P 2,n) fluctuations in the LC heterophase system in contact with (a) ordering and (b) disordering substrates. Dfished lines correspond to the mean-field scalar order parameter. In all cases T —5- Tni-...

Fig. 8.4. Portredt of profiles of the scalar order parameter for various values of the reduced temperature 0 = T — T )/ Tni — T ) in the vicinity of the isotropic nematic transition for (a) ordering and (b) disordering substrates. The substrate induced order is fixed to 1.1 Sc and 0, respectively.

Finally, one can treat the fractal lattice as a simple model for a disordered substrate on which the polymer is adsorbed, and use the exact results found for polymers on fractals to develop some understanding about real experimental systems. But for this, this article... 

Figure 1. The relation between the two distance metrics, the Euclidean distance r and the topological distance , exemplified using two sites (marked in white) on a disordered substrate (a percolation cluster, shown in grey).

Disorder Substrate Enzyme source Activity (% of normal)... 

Filler et al (1969) observed changes in the electroreflectance of Ge-films with increasing disorder in these films. They found that when the film is fully disordered (substrate temperature during evaporation below 200 C) all structure disappears except for the peak corresponding to the transitions from the top of the valence bands (both the highest band and the split-off band) into the conduction band. The peaks move towards smaller energies as disorder increases. This result was interpreted as evidence that the split-off valence band exists in a-Ge (cf Section 4.3), however, some objections have been raised against this interpretation (Fischer (1971)). 

As for the LB structure, no birefringence is measurable therefore a planar structure is discarded. As the trilayer becomes unstable, this suggests that a homeotropic configuration is not plausible. It is therefore tempting to refer to the Landau-de Gennes analysis and assume that the structure of the LB is close to isotropic, with a low value of the order parameter, as expected close to a disordering substrate. This is quite probable but does not help to write equations explicitly. 

Diffraction is not limited to periodic structures [1]. Non-periodic imperfections such as defects or vibrations, as well as sample-size or domain effects, are inevitable in practice but do not cause much difSculty or can be taken into account when studying the ordered part of a structure. Some other forms of disorder can also be handled quite well in their own right, such as lattice-gas disorder in which a given site in the unit cell is randomly occupied with less than 100% probability. At surfaces, lattice-gas disorder is very connnon when atoms or molecules are adsorbed on a substrate. The local adsorption structure in the given site can be studied in detail. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Cohen S R, Naaman R and Sagiv J 1986 Thermally induced disorder in organized organic monolayers on solid substrates J. Phys. Chem. 90 3054-6... 

Using this strategy, constmction of multilayer films of - O. fim thickness by self-assembly of methyl 23-ttichlorosilyltticosanoate (MTST) on siUcon substrates has been demonstrated (Fig. 9) (165). The linear relationship between the film thickness and the layer number showed a slope of 3.5 nm /layer. Filipsometry data, absorbance intensities, and dichroic ratios for the multilayers all suggest that the samples were composed of distinct monolayers. However, ir data indicated that there maybe more tilting or disordering of the alkyl chains in the seven-layer sample than for the monolayer samples. 

Another class of therapeutic agents is used for the treatment of certain genetic diseases or other enzymatic disorders caused by the dysfunction or absence of one particular enzyme. This often leads to an unwanted accumulation or imbalance of metaboUtes in the organism. Eor example, some anticonvulsive agents are inhibitors for y-aminobutyric acid aminotransferase [9037-67-6]. An imbalance of two neurotransmitters, glutamate and y-aminobutyric acid, is responsible for the symptoms. Inhibition of the enzyme leads to an increase of its substrate y-aminobutyric acid, decreasing the imbalance and subsequently relieving the symptoms of the disease. 

In Fig. 1 is shown a HRTEM image of part of the end of a PCNT. The initial material consisted of carbon nanotubes upon which bi-conical spindle-like secondary growth had deposited[21], apparently by inhomogeneous deposition of aromatic carbonaceous, presumably disordered, layers on the primary substrate nanotube. Prior to further heat treatment, the second-... 

The order-disorder transformation is not unique to two-layer fluids, which is readily concluded from the second maximum of n in the vicinity of s 3.55 where the fluid consists of three strata. However, it turns out that only the innermost, middle stratum undergoes the same kind of structural reorganization just explained for the two-layer fluid the two contact strata (i.e., the strata closest to the substrate) do not participate in the transformation. The intensity of the second maximum in n is therefore reduced by roughly 2/3 compared with the first one, as one would expect. 

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