Asymmetric structures

From the ground to an excited electronic state the electron promotion involved is likely to be to a less strongly bonding orbital, leading to an increase in molecular size and a decrease in rotational constants. The effect on the rotational fine structure is to degrade it to low wavenumber to give a strongly asymmetrical structure, unlike the symmetrical structure typical of vibrational transitions. 

Biological membranes are asymmetric structures. There are several kinds of asymmetry to consider. Both the lipids and the proteins of membranes exhibit lateral and transverse asymmetries. Lateral asymmetry arises when lipids or proteins of particular types cluster in the plane of the membrane. 

What accounts for this stereospecificity It arises from the fact that the enzymes (and especially the active sites of enzymes) are inherently asymmetric structures. The nicotinamide coenzyme (and the substrate) fit the active site in only one way. Malate... 

Reactants with asymmetric structures (meta or ortho instead of para catenation).10,14... 

Complex and asymmetric structures, which are sometimes almost identical in gas and condensed... 

This includes negative phospholipids (PG, PS, PI), neutral phospholipids (PC, PE, sphingolipids), cholesterol, and asymmetric structure. 

Carbohydrates related to membranes can be found as lipopolysaccharides or as parts of glycoproteins. Sugars are often characteristic determinants of cell surfaces (see below). The great majority of carbohydrates are found in the outer leaflet of a membrane, resulting in an asymmetrical structure. This is especially true for many plasma membranes and the outer membrane of Gram-negative bacterial cells (see below). 

Many cells have an asymmetric structure because of the necessity for function (Drubin and Nelson, 1996). For example, (the outer surface of) the plasma membrane of epithelial cells is fenced by a tight junction so that the lipids are separated between the apical part and the basolateral part (Fig. 9) (Eaton and Simons, 1995). Therefore, some molecular mechanisms must exist to sort the plasma membrane proteins into these two parts. Some signals related to the secretory/endocytic pathways have been found important (Matter and Mellman, 1994). Their details are not described here because the area is too specific for predictive purposes. 

Despite the apparent symmetry of the histones within the core particle, deviations of the histones and DNA result in an asymmetric structure. In the histones, these deviations can occur throughout protein structure, not just in the relatively unstructured tails (Fig. 6). The most pronounced deviations appear in histone H3. 

The casting procedure consisted of drawing an aqueous solution of PVA to a thin layer, and after an evaporation period, immersing in the complexing bath. The complexing bath used in our study was basically a saturated CuSO solution, with or without a series of possible additives. After a period of equilibration of over 24 hours, the membranes were dried and subjected to dry heat treatment,in order to stabilize the asymmetric structure obtained. The preparation conditions of several membranes prepared by this method are shown in Table V. 

These findings were interpreted as indicating that a thin skinned, asymmetric structure was obtained. In order to verify... 

Staphyloferrin A (Fig. 20, 65) is a second siderophore of Staphylococcus spp. (226). D-Omithine coimects the two citric acid parts. Due to the unsymmetrical link the central C-atoms of the citric acid units are chiral, but their stereochemistry has not been determined. Another consequence of the asymmetric structure is that two mono- and one di-dehydration products are observed. Staphyloferrin A forms a 1 1 Fe -to-ligand complex, which is preferentially A-configured. For steric considerations only cis-(SR ) or cis-(RS ) arrangements can be considered. Uptake experiments with Fe showed that it is a true siderophore (193). 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

Third-order NLO behavior generally involves three photons resulting in effects similar to those obtained for second-order NLO behavior. Third-order NLO behavior does not require the presence of asymmetrical structures. 

The asymmetric structures which incorporate Al-0 edge-fused rings of different size all contain (AlO) metallocycles in which n = 2, 5 or, in an isolated instance, 8. The retention of at least one four-membered heterocycle n = 2) relates these systems to the symmetric polycycles discussed above in all but one aluminium oxide. 

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