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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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142 Trewhella and Krueger<br />

Table 1<br />

Coherent Neutron Scattering, bcoh, and X-Ray Scattering, fX-ray, Amplitudes<br />

for Biologically Relevant Nuclei<br />

fX-ray for θ = 0 in electrons<br />

Atom Nucleus bcoh (10 –12 cm) (and in units of 10 –12 cm)<br />

Hydrogen 1 H – 0.3742 1.000 (0.28)<br />

Deuterium 2 H 0.6671 1.000 (0.28)<br />

Carbon 12 C 0.6651 6.000 (1.69)<br />

Nitrogen 14 N 0.940 7.000 (1.97)<br />

Oxygen 16 O 0.5804 8.000 (2.25)<br />

Phosphorous 31 P 0.517 15.000 (4.23)<br />

Sulfur Mostly 32 S 0.2847 16.000 (4.5)<br />

ent neutron-scattering properties. For neutrons, one of the largest differences is<br />

between the isotopes of hydrogen ( 1 H = hydrogen, and 2 H = deuterium). Table 1<br />

gives the coherent, elastic X-ray, and neutron-scattering amplitudes for the<br />

atoms commonly found in biological systems. Note that the neutron scattering<br />

amplitudes for most nuclei, including 2 H, are positive and approximately equal.<br />

The exception is the negative scattering amplitude for 1 H that results from a<br />

180° phase shift between neutrons scattered by 1 H compared to the other nuclei.<br />

As a consequence, the neutron scattering density of a molecule depends<br />

strongly on its mean hydrogen content, and deuterium substitution can be used<br />

to manipulate neutron scattering densities.<br />

3.1.2. Neutron Scattering and Contrast Variation<br />

Equation 1 shows that the intensity of the scattering from a protein in solution<br />

depends upon the difference in scattering density between the particle and the<br />

solvent, i.e., its “contrast.” If a complex is made using one deuterated and one<br />

nondeuterated protein, the two proteins will have very different neutron scattering<br />

densities. Further, by changing the deuterium level in the solvent, the neutron<br />

scattering contrast of each protein can be systematically varied. By adjusting the<br />

deuteration level in the solvent so that the mean solvent density matches that of<br />

either the deuterated or the nondeuterated protein, it is possible to “solvent match”<br />

that protein such that it has zero contrast and becomes “invisible” in the neutron<br />

experiment. Solvent matching thus provides a means for extracting structural<br />

information on the individual components within a complex.<br />

A more robust approach to utilizing contrast variation methods with neutron<br />

scattering for extracting structural information on the components within the<br />

macromolecular complexes is to measure a “contrast series” in which the solvent<br />

deuteration level is systematically varied over he widest range possible.

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