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<strong>Calcium</strong>-43 NMR 217<br />

18<br />

<strong>Calcium</strong>-43 NMR of <strong>Calcium</strong>-<strong>Binding</strong> <strong>Protein</strong>s<br />

Torbjörn Drakenberg<br />

1. Introduction<br />

The Ca 2+ -ion, with its closed-shell electronic structure, has very few spectroscopic<br />

properties that are useful in studies of calcium-binding proteins and<br />

their interactions with calcium ions. Therefore, it is even more important to<br />

exploit the few possibilities for spectroscopy that are available. The most abundant<br />

calcium isotope, 42 Ca, has a spin quantum number I = 0 and is therefore<br />

not observable by NMR. However, 43 Ca has I = 7/2 and is in principle NMR<br />

active. The natural abundance of this istope is only 0.15% and 43 Ca NMR can<br />

therefore most likely never be used to study biological systems without isotope<br />

enrichment. Nowadays, 43 Ca enriched to 50% is readily available, though at a<br />

rather high cost. It seems that most research groups working with calciumbinding<br />

proteins consider the 43 Ca isotope as an exotic one and only a limited<br />

number of groups have been involved in 43 Ca NMR studies of biological systems<br />

(1–6). Only the Swedish group in Lund and the Japanese group in Sendai<br />

have been using 43 Ca NMR more consistently.<br />

Below I will first present, very briefly, what is special about the application<br />

of 43 Ca NMR as compared to typical spin = 1/2 nuclei like 1 H, 13 C, and 15 N and<br />

than discuss some typical studies.<br />

2. Experimental and Theoretical Considerations<br />

Because 43 Ca is a quadrupolar nucleus with a quite high spin quantum number<br />

(I = 7/2) it has properties that are in some respects different from the dipolar<br />

nuclei that are mostly considered in studies of macromolecules. From an<br />

experimental point of view it is essential to recognize that sensitivity is the<br />

most important fact to consider because the natural width of the 43 Ca NMR<br />

resonance from a 43 Ca 2+ ion bound to a macromolecule is hundreds, if not thou-<br />

From: Methods in Molecular Biology, vol. 173:<br />

<strong>Calcium</strong>-<strong>Binding</strong> <strong>Protein</strong> <strong>Protocols</strong>, Vol. 2: Methods and Techniques<br />

Edited by: H. J. Vogel © Humana Press Inc., Totowa, NJ<br />

217

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