The core particle

Another method to synthesize hollow nanocapsules involves the use of nanoparticle templates as the core, growing a shell around them, then subsequently removing the core by dissolution [30-32]. Although this approach is reminiscent of the sacrificial core method, the nanoparticles are first trapped and aligned in membrane pores by vacuum filtration rather than coated while in aqueous solution. The nanoparticles are employed as templates for polymer nucleation and growth Polymerization of a conducting polymer around the nanoparticles results in polymer-coated particles and, following dissolution of the core particles, hollow polymer nanocapsules are obtained. 

Using long-chain alkylsulfobetaines as the stabilizer, a number of highly water soluble nanometal colloids have been isolated in excellent yields (see Figure 8). The core particle size can be tailored between 1 and 10 nm. TEM examinations have shown that the resulting materials are generally mono-disperse. Further, a combination of spectroscopic methods confirmed the zerovalent nature of the metal cores [200]. 

Best-fit parameters for the earlier FPA and TPD data(59,62) are in reasonable accord, but differ significantly from those for the most recent FPA experiments.(235) For the TPD data,(62) a must be at least 2.5 times smaller than that of the free DNA, provided y is not smaller than for free DNA in solution/63 Assuming only that one or both ends of the mobile region are clamped to the core particle, Genest et a/.<5<>> concluded that a is reduced by... 

Several steps were needed to determine the structure of the core particle to higher resolution (Fig. Id). The X-ray phases of the low-resolution models were insufficient to extend the structure to higher resolution, since the resolution of the early models of the NCP was severely limited by disorder in the crystals. The disorder was presumed to derive from both the random sequences of the DNA and from heterogeneity of the histone proteins caused by variability in post-translational modification of the native proteins. One strategy for developing an atomic position model of the NCP was to develop a high-resolution structure of the histone core. This structure could then be used with molecular replacement techniques to determine the histone core within the NCP and subsequently identify the DNA in difference Fourier electron density maps. 

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. 

One consequence of the molecular asymmetry is that the core particle presents two distinct faces, arbitrarily labeled ventral and dorsal in our images. These two faces have subtle yet distinct dilferences in the electrostatic surface potentials they present. 

This problem first emerged from the necessity to reconcile topological and structural data of nucleosomes and chromatin. As soon as a minichromosome could be reconstituted from pure DNA and histones, the total reduction of the DNA linking number (Lk) was found to be equal to the number of nucleosomes, which was also true for the native Hl-bearing SV40 minichromosome [12]. On the other hand, the first low-resolution crystal of the core particle showed that DNA was wrapped with 1 3/4 turns of a left-handed superhelix. Assuming linker DNAs... 

Fig. 1. The core particle, the DNA superhelix and H2B and H3 N-terminal tails, (a) Space-filling representation of the 2.8 A crystal structure of the 146 bp human a-satellite nucleosome core particle [22]. The dyad is in the plane of the paper and the superhelix axis slightly off that plane. Positive and negative numbers mark the superhelix locations (SHL) in the upper and lower gyres, respectively, and the dotted curve follows the path of the double helix axis, (b) Ribbon representation of the DNA superhelix slit along a line parallel to its axis, opened out and laid flat on the paper surface. SHL are also indicated, together with H2B and H3 tails passage points between the gyres. (From Fig. 5 in Ref [29].)...

The accepted involvement of CpG methylation in transcription regulation has guided various lines of research into the direct effects methylation may have on chromatin structure. As early as 1982, Felsenfeld and co-workers looked for effects of DNA methylation on the affinity of DNA for histone octamers they reported a slight increase in this parameter [163]. Nightingale and Wolffe [164] using mononucleosome reconstitution on a specific sequence, the 5S rDNA from X. borealis, failed to see such an effect. More recently, Travers (personal communication) has seen clear-cut differences in the affinity of the histone octamer for a number of methylated versus unmethylated sequences. The fine structure of the core particle was, however, unaffected by the methylation status of the DNA [165]. 

The geometrical structure of the chain in Fig. 2 is determined entirely by 6, (j), and B. The model only describes the linker geometry and does not account for excluded volume effects and other forms of nucleosome-nucleosome interaction it assumes that the core particles are point-like and that they are located at the joints of the linkers, which are straight rods. 

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