The Molecular Architecture

The molecular architecture defines the geometry of the chain. In general, the polymer chain is described by a coil that is capable of adopting various forms in accordance with the ambient conditions, solvents, and temperature. Under conditions where there is no net interaction with the surroundings (so-called theta state, or temperature), a model of a randomly condensed coil 

The branched chain contains side subchains, mainly obtained by the chain transfer mechanism. Most branches are short but yet are able to decrease the crystallinity in the polymer, due to steric hindrance. Existence of longer branches affects the flow properties by decreasing viscosity. This 

The highly ordered planar stmctures, as obtained by stereospecific polymerization, were mentioned in Section 2.3. In contrast to the branched chains, we deal here with linear chains where side substitute groups appear regularly on the same side of the plane, or alternatively. These structures are named isotactic or syndiotactic, respectively, and the degree of order is termed tacticity. In both cases, good conditions for crystallization exist, even when the polymer itself is known as difficult to crystallize (as in the case of polystyrene or polymethylmethacrylate). Any orderless structure is named atactic. 

A rotational angle between 0 and the 180 prevails in most cases, thus affecting the practical dimensions of the chains, namely, the end-to-end distance. The most remote distance appears in the trans configuration, and shortest distance is in the cis configuration. The stiffness of the chain and the ambient conditions (temperature) affect the configuration of the chain and, as a result, its properties. 

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Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

A multitude of different variants of this model has been investigated using Monte Carlo simulations (see, for example [M])- The studies aim at correlating the phase behaviour with the molecular architecture and revealing the local structure of the aggregates. This type of model has also proven useful for studying rather complex structures (e.g., vesicles or pores in bilayers). 

These chain models are well suited to investigate the dependence of tire phase behaviour on the molecular architecture and to explore the local properties (e.g., enriclnnent of amphiphiles at interfaces, molecular confonnations at interfaces). In order to investigate the effect of fluctuations on large length scales or the shapes of vesicles, more coarse-grained descriptions have to be explored. 

R. D. Preston, The Molecular Architecture of Plant Cell flY/A, John Wiley Sons, Inc., New York, 1952. 

The Molecular Architecture of Photo.synthedc Reaction Centers... 

Eukaryotic Reaction Centers The Molecular Architecture of PSII... 

FIGURE 22.19 The molecular architecture of PSII. The core of the PSII complex consists of the two polypeptides (D1 and D2) that bind P680, pheophytin (Pheo), and the quinones, Qb- Additional components of this complex include cytochrome -6559,... 

FIGURE 22.20 The molecular architecture of PSI. PsaA and PsaB constitute the reaction center dimer, an integral membrane complex P700 is located at the lumenal side of this dimer. PsaC, which bears Fe-S centers and Fb, and PsaD, the interaction site for ferre-doxin, are on the stromal side of the thylakoid membrane. PsaF, which provides the plasto-cyaiiin interaction site, is on the lumenal side. (Adapted from Golbeck, J. H., 1992. Amiual Review of Plant Physiology and. Plant Molecular Biology 43 293-324.)... 

The properties of a molecule are primarily deter- and by the molecular architecture. By archi-mined by the bond types which hold it together tecture we mean the structure of the molecule—... 

From the data presented here, the orbitals involved in bonding correlate with the molecular architecture. The relationships are summarized in Table 16-IV. 

Consider the fluorides of the second-row elements. There is a continuous change in ionic character of the bonds fluorine forms with the elements F, O, N, C, B, Be, and Li. The ionic character increases as the difference in ionization energies increases (see Table 16-11). This ionic character results in an electric dipole in each bond. The molecular dipole will be determined by the sum of the dipoles of all of the bonds, taking into account the geometry of the molecule. Since the properties of the molecule are strongly influenced by the molecular dipole, we shall investigate how it is determined by the molecular architecture and the ionic character of the individual bonds. For this study we shall begin at the left side of the periodic table. 

Inhibitors must possess chemical and physical properties that will ensure absorption by root tips or penetration by foliar surfaces, and translocation to the active site. Once there they must assume the precise spatial configuration required to complement the molecular architecture of the active center if they are to block the key reaction. A comprehension of comparative biochemistry and information on how plants differ in the architecture of the reactive sites should assist in developing truly selective herbicides. 

This is a very new field, but given the current interest in both dendrimer chemistry and supramolecular chemistry, it is one that is likely to receive attention in the future. The prospect of preparing well-defined molecular assemblies, rather than ill-defined clusters, is attractive, and is expected to give access to new properties that can be controlled by the molecular architecture of the assembly. New and useful materials seem likely to emerge as this chemistry grows and is exploited in the years to come. 

Changes in cell-wall protein composition may regulate the molecular architecture of protein networks in a manner that allows new developmental outcomes for both fungal cell adhesion and root colonization. Further investigation of the structure and regulation of SRAP wall proteins will provide a more complete picture of their role in developing ectomycorrhizal tissues. Incompatibility between ectomycorrhizal hyphae and the host roots detected during the initial con-... 

Table II also shows the calculated Mcs of the PHEMA segments. The Mc of the PHEMA segments and the Mn of PIB determine the molecular architecture of PHEMA-l -PIBs. The architecture of PHEMA-1-PIB networks can be controlled by the concentration and the Mn of the MA-PIB-MA employed in the synthesis which in turn controls the Mc of the PHEMA segments.

How does the molecular architecture of the bisphenol molecule affect the physical properties of the final polycarbonate polymer ... 

Lysozyme generates a carbocation within the molecular architecture of the bacterial cell wall. 

In their study of branched PSA, Maniar et al. (1990) found that the molecular architecture of branched polymers affects the release kinetics in a variety of ways. They found that the branched polymers degraded faster than linear PSA of comparable molecular weight (Maniar et al., 1990). They also noted that drug (morphine) release profiles were more characteristic of bulk erosion than surface erosion An initial lag time during which very little drug was released was associated with the time required for water to swell the polymer. This was followed by a period of relatively fast release, which tapered off as the device disintegrated. The polymer matrix lost its mechanical integrity before the release experiment was complete (Maniar et al., 1990). Despite the increase... 

HUANG W, JIA, J EDWARDS, P., DEHESH, K., SCHNEIDER, G., LINDQVIST, Y., Crystal structure of P-ketoacyl-acyl carrier protein synthase II from E. coli reveals the molecular architecture of condensing enzymes, EMBO J., 1998,17, 1183-1191. 

Barnard, E. A. (2001) The molecular architecture of GABAa receptors, in Pharmacology of GABA and Glycine Neurotransmission, vol. 150 (Mohler, H ed.), Springer-Verlag, Berlin, pp. 79-100. 

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