BioPHYSICAL chemistry

NMR spectrum selectively for the environment of the unpaired electron.

This technique, named ENDOR for electron nuclear double resonance,

was developed by George Feher in 1956. Whereas ENDOR is largely applied

to protons, any nuclei with a net spin can be studied using this technique.

Spin probes

In addition to studying metal- and radical-containing proteins, other bio-

logical components can be characterized by attaching spin probes; that

is, small molecules that contain an unpaired electron spin. Spin labels have

a stable free radical, which is usually a nitroxide derivative. Spin labels

can be incorporated into the fatty-acid chains of lipids in both artificial

and natural membranes where their EPR spectra give information con-

cerning the mobility of lipid chains. Alternatively, spin labels have been

designed to specifically bind to cysteine residues in proteins. By introduc-

ing reactive residues, such as cysteines, incorporated using mutagenesis,

spin-probe binding sites can be placed throughout the surface of a protein.

366 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

mS  

EPR

NMR

NMR

Flip-flop interaction

Flop-flop interactionFlop-flop interaction EPR

(^12)
m  ^12
mS  ^12
m  ^12
mS  ^12
m  ^12
mS  ^12
m  ^12
Figure 16.20Energy-level diagram for one unpaired electron and one proton. EPR transitions
correspond to vertical transitions and NMR transitions correspond to diagonal transitions.