Marker atom

Fig. 6.16 the use of a marker atom on the alpha-carbon in an arginine residue may lead to a significant electrostatic interaction being neglected because the distance between the marker atoms exceeds the cutoff. 

Na and Nb are the numbers of atoms in the two groups A and B and S is the switching function. With the group-based switching function, it is necessary to define the distance between the two groups (i.e. the two points Ta and Tb). There is no definitive way to do this. As with cutoffs, a special marker atom can be nominated within each residue, or the centre of mass, centre of geometry or centre of charge may be used. 

Similar in principle to this is the use of two different X-ray wavelengths on the same crystal, one near to and the other far from the absorption edge of a marker atom, to give different diffracting powers this is the "ideal isomorphism method advocated by Lipson and Cochran (195 ) and Pepinsky (1956). A comparison of the vector maps for the two wavelengths, or a vector map based on the differences of structure amplitudes for corresponding reflections for the two wavelengths, indicates which peaks are due to the marker atoms. 

During self-diffusion in a pure material, whether a gas, liquid, or solid, the components diffuse in a chemically homogeneous medium. The diffusion can be measured using radioactive tracer isotopes or marker atoms that have chemistry identical to that of their stable isotope. The tracer concentration is measured and the tracer diffusivity (self-diffusivity) is inferred from the evolution of the concentration profile. 

These questions can be answered by using diffusion markers. Atoms of one element (or both) can be marked by a radioactive or stable isotope. Radioactive Si (half... 

For most proteins, the phase information is obtained from isomorphous heavy atom derivatives. A heavy atom (e.g., Hg, U or Pt), introduced into the protein, scatters more than the light atoms (C,N,0) of the protein and is used as a marker atom. 

Equation (13.5) and the marker data presented in Fig. 13.5 can be used to estimate the average atomic displacement distance of a marker atom in a collision cascade formed in a matrix of amorphous Si. For example, from the temperature-independent data in Fig. 13.5a, a typical value of DiJlcj), for both Sn and Sb markers, is 4(10 cm" ), or 0.4 nm". From Fig. 13.5b, the corresponding damage energy is 1,500 eV nm Using the atomic density of crystalline Si, 50 atoms nm for the amorphous Si value of N, the ratio of Fj lN will be 30 eV nm This indicates that should be approximately 1.6 nm for a Si displacement energy of Ad = 13 eV. 

By irradiating dust in a neutron flux [109, 112, 113], dust tagged with, for example, %a, and other isotopes has been obtained, For modeling radioactive dust formed by the processing of radioactive ore >in isotope laboratories, nuclear reactors, etc., Semashko and Plievskii [114] used phosphoric acid (H3PO4) with a marker atom of P, The acid particles had the following fractional composition less than 60 m, 40% 60-80 M, 45% 80-100 M, 15%. 

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