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of their oxidised states. The appearance and disappearance of their EPR signals are

used to monitor the activity of these proteins in the multi-enzyme systems of intact

mitochondria and chloroplasts, as well as in isolated enzymes. In many metallo-

proteins, the ligands coordinating the metal ion are the amino acid residues of the

protein. Coordination chemistry requires a specific stereochemical structure of the

ligands, and EPR studies show that the geometry is frequently distorted in proteins

when compared to model systems. Such distortions may be related to biological

function.

Spin labels

Spin labels are stable and non-reactive unpaired electrons used as reporter groups or

probes for EPR. The procedure of spin labelling is the attachment of these probes to

biological molecules that lack unpaired electrons. The label can be attached to either a

substrate or a ligand. Often, a spin label contains the nitric oxide moiety. These labels

enable the study of events that occur with a frequency of 10^7 to 10^11 s^1. If the motion

is restricted in some directions, only anisotropic motion (movement in one particular

direction) may be studied, for example in membrane-rigid spin labels in bilayers. Here,

the label is attached so that the NO group lies parallel to the long axis of the lipid.

Klystron

oscillator

Crystal

detector

Computer

Sweep supply

Magnet

pole

Sample

Magnet

pole

Reflected

microwave

radiation

Incident

microwave

radiation

Fig. 13.5Diagram of an EPR spectrometer.

533 13.4 Electron paramagnetic resonance