Investigation of Conformational Changes in Spin-Labeled Myosin: Implications for the Molecular Mechanism of Muscle Contraction

Spin labels, stable compounds that contain an unpaired electron, have become important tools in the investigation of biological systems. These probes, which are analogous to optical probes or reporter groups, are usually attached covalently to a molecule of biological interest and can be used for the study of individual protein molecules or to obtain information on the structure of more complex systems, e.g., membranes (Hamilton and McConnell, 1968; McConnell and McFarland, 1970; McConnell, 1971). When these stable radicals are placed in a steady magnetic field (H0), they absorb the energy of an oscillating electromagnetic field the same way as chromophores absorb light. On varying the strength of H0 one obtains the so-called electron spin resonance (ESR) spectrum which is usually displayed as the derivative of the absorption.