Globular particles, analysis

The puriflcation of the y-secretase complex (59) has allowed the first glimpse into its structure. Electron microscopy and single particle analysis reveals that the complex has a globular structure that at low resolution (10-15 A) appears rather amorphous (66). [Another structure, elucidated in a similar manner but of poorer resolution ( 45 A), also has been reported (67).] Nevertheless, two important features can be gleaned. The first is a rather large interior cavity of 20 A diameter that is presumably where the active site resides, a characteristic reminiscent of the proteasome. The second is the presence of two small openings that may be the site of entry for water. Other structural features have been revealed by cysteine mutagenesis with... 

Core particles and nucleosomes are globular particles that have been usefully examined by several groups by neutron and X-ray scattering [382-401]. Stuhrmann plots of the core particle 200,000 45% DNA) show large positive slopes, where Rq is 3.9-4.1 nm and a is 36-51 X 10 (Table 15) [382,387,390,395]. Nucleosomes likewise exhibit a Rq oi 4.0 nm and an a of 45 X 10 [385,393]. Further analysis shows that the DNA component of core particles has an Rq of 4.9 nm, while the protein component has an Rq of 3.3 nm. This shows that the DNA has a more elongated shape in the core particle. In summary, it was shown that the DNA is external to a protein core. Other work shows that 1.8 turns of superhelical DNA are wound around the octamer of core histones. Several workers have examined individual neutron scattering curves in H20 buffers or X-ray curves in H2O buffers... 

Table 5.8 Polypeptides detected during the LC-electrospray-MS analysis of the tryptic digest from / -lactoglobulin (/ILG). Reprinted from 7. Chromatogr., A, 763, Tnrnla, V. E., Bishop, R. T., Ricker, R. D. and de Haseth, J. A., Complete structnre elncidation of a globular protein by particle beam hqnid chromatography-Fourier transform infrared spectrometry and electrospray hqnid chromatography-mass spectrometry - Seqnence and conformation of / -lactoglobulin , 91-103, Copyright (1997), with permission from Elsevier Science...

Since the PS reference sample is almost monodisperse, a cumulant analysis of that material would yield a very small Q, say Q < 0.03. That is, all the correction terms are negligible and Eqs. (17) collapse to Eqs. (12). But cumulant analysis is a useful way to handle practical samples such as pigments, inks, microemulsions, swollen micelles, globular proteins, and spherical virus particles, where there is a size distribution but one that is not very broad (say Q < 0.3). This analysis should be made for the milk data using a non-linem teast-squares fitting of Eq. (17a), neglecting /1.3 and all higher order terms. Report the F, D, and R values as well as the second cumulant /t2 aiid the polydispersity index Q. 

Fig. 4 shows the surface of a Ni36Al specimen after 96h sulfidation at 800°C in a gas mixture containing 5 vol.% H2S.The surface is covered by globular Ni3S2 particles solidified from an originally liquid layer. Both XRD and EDX analysis show that these spheres are Ni3S2. Tests at temperatures between 800 and 950°C in gas mixtures containing more than about 1 vol.% H2S produced the same type of attack. 

Thus, the analysis of the globular gel structure according to a particle packing model results in an overestimate of the particle diameter, whereas the cylindrical model results in an underestimate of the cylinder diameter. The appropriate geometry, therefore, appears to be a necked, random close-packed assemblage of particles whose relative density is 0.73. The necks are not large enough, however, to constitute smooth cylinders. 

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