Intra-tube

Tangney et al.92 studied the friction between an inner and an outer carbon nanotube. Realistic potentials were used for the interactions within each nanotube and LJ potentials were employed to model the dispersive interactions between nanotubes. The intra-tube interaction potentials were varied and for some purposes even increased by a factor of 10 beyond realistic para-meterizations, thus artificially favoring the onset of instabilities and friction. Two geometries were studied, one in which inner and outer tubes were commensurate and one in which they were incommensurate. 

XIQT N seconds for a 100 kbp molecule, we thus have Xg 3 seconds," consistent with experiments. We thus conclude that the initial tube stretching is due to biased reptation, but that intra-tube modes also play a role on a time scale [Pg.575]

These results indicate that some fast intra-tube modes tend to disorient the molecular conformation on a very short time scale, but that only complete tube disengagement, due to the slow unbiased reptation process, can restore the true molecular randomness. 

However, three remarks concerning the effects of short wavelength modes should be made here. First, the over- and under-shoot stretching phenomena clearly point out that some intra-tube DMA modes, neglected in the BRM, are excited if the field is applied on a truly random coil DNA. This transient effect does not seem to affect the overall reptative behaviour of DNA, but may be very important in pulsed field techniques where separation is increased by coupling the external pulsed field(s) to transient effects that may affect the molecular electrophoretic mobility. 

Second, the fact that the LD orientation factor relaxes very quickly, ruling out unbiased reptation as a mechanism, indicates that the chain is not in equilibrium in its tube during continuous-field electrophoresis. The intra-tube tension, which may be due to gel inhomogeneities and tube leakages for example, is released very quickly when the field is turned off probably because this involves mainly local motion. However, the overall tube conformation cannot be renewed by these intra-tube modes. 

As we will see in the next section, when field-inversion electrophoresis is used to separate nucleic acids in certain size ranges, intra-tube relaxation effects take place. Of course, as the angle between the two fields increases from 90 to 180 , we expect that the crossed field effects discussed earlier will be modified by more and more of the intra-tube effects that sometimes dominate in field-inversion electrophoresis. It is not the goal of this article to discuss the angles between 90 to 180 . The next section presents a discussion of field-inversion electrophoresis as well as the ways to optimize the reorientation-induced separation obtained with this experimental arrangement. 

In fact, one does expect that the intra-tube molecular conformations will be influenced by the electric field, at least on short time- and length- scales. For example, electric forces may induce tube leakages. 

In continuous fields, these intra-tube effects change the effective values of the parameters Brown needs to fit experimental... 

One exception to that is apparently the steady-state orientation factor measured by LD. Our BRM predicts that this orientation factor should be slightly molecular-size dependent in the vicinity of the band-inversion phenomenon, but otherwise size-independent. Experiments showed a fairly strong size dependence up to large molecular sizes, even if these large molecules had a size-independent mobility. This is possibly due to an intra-tube effect which can induce local fluctuations in the average tube orientation but, since the tube stays oriented, retain the molecular-size independence of the mobility. 

Although intra-tube effects may be expected to play a larger role in pulsed field techniques and transient effects, the form of the model which averages over these effects offers a good framework to discuss the experimental results even in these cases. The orientation overshoot observed with linear dichroism is a transient effect that occurs at a time which is smaller than the tube-renewal time we thus conclude... 

Crossed-field electrophoresis provides an interesting alternative to intermittent-field electrophoresis. The idea is simply to change by 90 the direction of the field each time the tube is fully oriented in the field direction because we know that this orientation destroys the molecular-size dependence of the electrophoretic velocity. Although some intra-tube effects can be expected to play a role here, especially because crossed fields may induce the creation of a large number of transverse tube leakages, the BRM reproduces most experimentally observed effects up... 

The biased reptation model provides a good framework to discuss the experimental results of the various gel electrophoresis techniques used to separate nucleic acids. Although more experiments are needed to fully characterize these techniques, available results indicate that the simplified version of the model discussed in this paper is satisfactory when low-frequency pulsed fields are used, or when transient intra-tube effects are not dominant. This is the case in continuous fields, for small molecules in intermittent fields, and possibly also for crossed fields. However, intra-tube effects are observed to play a role in field-inversion electrophoresis, for long molecules in intermittent fields, and during the first stages of an experiment (where an orientation overshoot is observed). 

The large number of cases where the biased reptation model is reliable indicates that the intra-tube effects do not rule out reptation as the basic migration mechanism. Further theoretical advances will include the effects of both intra-tube molecular orientation and tube orientation on the electrophoretic properties of large nucleic acids. 

Siloxane-containing devices have also been used as contact lenses, tracheostomy vents, tracheal stents, antireflux cuffs, extracorporeal dialysis, ureteral stents, tibial cups, synovial fluids, toe joints, testes penile prosthesis, gluteal pads, hip implants, pacemakers, intra-aortic balloon pumps, heart valves, eustachian tubes, wrist joints, ear frames, finger joints, and in the construction of brain membranes. Almost all the siloxane polymers are based on various polydimethylsiloxanes. 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

A schematic representation of a TSLS complex is provided in Figure 1. On the basis of preliminary XRD and stochiometric studies, it appears that the imogolite tubes are in van der Waals contact, most likely in a log-jam-like array in the layer silicate galleries. Although the tubes stuff the galleries, two unique adsorption environments are available, namely, the intra-and inter-tube pores designated A and B in Figure 1. 

A method originally developed for terpenoid type off-flavor compounds was used [35]. By this method a JH NMR spectrum could be measured reliably from ca. 5 pg of monoterpenoid compound collected directly in the NMR solvent by preparative GC. A thick-wall (5 mm o.d., 2 mm i.d.) sample tube instead of a standard NMR tube was selected for the measurements in order to decrease the solvent volume from 700 to 100 pi. CH2C12 was found to be a suitable medium, because its JH NMR signal is locked at 5.30 ppm from tetramethylsilane (TMS). TheTi NMR spectral interpretation was mainly based on the synthetic procedure used, symmetry considerations of possible products, and characteristic intra-aromatic couplings. More details on the NMR spectroscopy of PCDTs and PCTAs are reported elsewhere [29,30,32]. 

---