The Rigid Layer

In our studies, several approaches were employed to probe the nature 

Our hypothesis of multiple-growth centers is supported by the results of van Tubergen and Setlow, who showed that radioactive DAP of parental cell walls is broken down into at least 300 units which are randomly distributed over the progeny cell population. Recently, Lin et 

A corollary question that arises is what effect does interference of peptidoglycan synthesis have on the outer membrane In DAP auxotrophs of E. coli, we observed an extensive overproduction of surface material under conditions of limited supply of DAP. This effect appears to be analogous to that observed with a DAP-decarboxylase mutant which excretes large amounts of predominantly LPS under conditions of lysine limitation. The excretion of surface material was also observed when protein synthesis was blocked, and when it was not restricted to mutant strains only, but occurred in all of the E. coli strains tested/ In addition to LPS, which accounted for 50% of the excreted material, the cells also released phospholipids (35%) and protein (15%). It is interesting that these authors also found some shedding of the LPS-lipid-protein complex into the medium under normal conditions of growth. These observations may be related to the occasional bleb formation and detachment we had seen in earlier work on the structure of unfixed cell walls of E. coli.  

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It is useful to calibrate the EQCM, i.e., to determine Cf, e.g., by electrodeposition and electrodissolution of silver. The rigidity layer behavior can be tested by depositing films of different thicknesses. Usually relatively thin films (10 nm-some hundreds nm) show rigid layer behavior. The deviation from linearity regarding the Am vs. Q function is related to the appearance of the viscoelastic effect. By the help of impedance measurements... 

The Raman spectrum of NbSej single crystals exhibit three first order lines at 29.6, 230.9 and 238.3 cm. The low frequency line at 29.6 cm is due to the rigid layer vibration mode (Ej ) which accounts for the weak interlayer bonding. The high-frequency lines at 230.9 and 238.3 cm are due to the A,g and Ejg modes, respectively. The high frequency Raman modes in the NbScj nanostructures were found to be identical to those of the bulk crystal. Bulk NbSc2 shows a photoluminescence band of very weak intensity band at around 825 nm possibly due to trapped states. The band is shifted to 820 nm in the nanostructures. 

It is useful to calibrate the EXJCM, i.e., to determine Cf, e.g., by electrodeposition and electrodissolution of silver. The rigidity layer behavior can be tested by depositing films of different thicknesses. Usually relatively thin films (10 nm - some hundreds nm) show rigid layer behavior. The deviation from the linearity regarding the Am vs. Q function is related to the appearance of the viscoelastic effect. By the help of impedance measurements the viscoelastic characteristics of the surface film can also be tested [4, 5, 6, 7, 10]. In the absence of any deposition the change of the density and viscosity in the double layer or in the diffusion layer may cause 0.1-10 Hz frequency change. It may interfere with the effect caused by the deposition of monolayers or submonolayers. In some cases other effects, e.g., stress, porosity, pressure, and temperature, should also be considered. 

Aspects of loading with nonrigid films have been considered by several authors [18-22]. The primary case of interest in the electrochemical context is loading with a rigid layer (the elechode), a viscoelastic film (commonly, though not necessarily, a polymer), and then a Newtonian fluid, schematically illustrated in Fig. 3. (The rigid layer component may also include material entrapped within sm-face features [23].) The characteristic mechanical impedances of the two nonrigid... 

Fig. 3 Schematic diagram of the propagation of an acoustic shear wave launched by a TSM resonator loaded with a viscoelastic overlayer and exposed to a fluid. Note the progressive zero, significant, and dramatic attenuations of the wave on moving from the rigid layer (electrode plus surface feature-entrapped material) to the viscoelastic solid to the fluid. The acoustic decay lengths in these three regions are, respectively, infinity, [2C/ 1 — C7C ] / /(ft)ypf), and t A[G"//(o in the latter two instances, typical values are 2 and 0.2 pm.

Two separate sets of simulations are performed one where the nanotube is flexible and one where the nanotube is kept rigid. The atoms in the flexible nanotube are partitioned in much the same way as the atoms in the substrate (39). In both cases, indentation (extraction) is accomplished by moving all rigid atoms at a constant velocity toward (away from) the monolayer surface. The load on the probe tip is taken to be the total force on the rigid-layer atoms of the nanotube. Moving the rigid-layer atoms of the nanotube parallel to the monolayer surface at a constant velocity simulates friction. Unless otherwise indicated, all indentation and sliding velocities are 100 m/s.. (See Ref. (39) for a discussion of how these speeds compare to those used in AFM experiments.)... 

The rigid layer of the wall is found in all procaryotic cells (with the exception, however, of the halobacteria), where it functions as an insoluble, supporting structure allowing the bacteria to live under hypotonic environmental conditions. This layer is visualized as a network of glycan strands interlinked by means of peptide chains. It is thus a peptidoglycan polymer. (The terms mucopeptide. gjycopeptide and murein used by some authors are synonymous with peptidoglycan.)... 

Recalling that crystal-medium interfacial layer is composed of the rigid layer 1 - 2 atomic diameters wide and the diffuse layer extends up to some 10 cm deep into the solution, we may view the diffusion-controlled dissolution process as one in which the reactant species and reaction products have to traverse the wide diffuse layer in order to enable the surface reactions to proceed. In this case, the rate of dissolution, expressed as the amount, m, of material removed per unit time, t, is described by Pick s first law, namely ... 

Fig. 5. Diagram showing the various cleavage planes of the bacterial envelope. OM Outer membrane, cleaving in plane I. IM Inner membrane, cleaving in plane II. The filled circles in OM represent proteins of the rigid layer. The open circles in IM represent particles typical of plane II.

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