Intramolecular motions and lateral diffusion of lipid through the membrane, as well
as the effect of proteins and other factors on these parameters may be observed.
Quantification of effects often involves calculation of theorder parameterZ. Spin-
labelled lipids are either concentrated into one region of the bilayer or randomly
incorporated into model membranes. The diffusion of spin labels allows them to come
into contact with each other, which causes line-broadening in the spectrum. Labelling
of phospholipids with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) is used for
measurement of the flip rate of phospholipids between inner and outer surfaces as
well as lateral diffusion.Free radicals
Molecules in theirtriplet states(Fig. 12.8) have unpaired electrons and thus are amen-
able to EPR spectroscopy. Such molecules possess the property of phosphorescence(a)(b)(c)BEBBDE+1/ 2–1/ 2(d)BFig. 13.6Instead of the absorption signal (a), EPR spectrometry records its first derivative (b). (c) The energy
of the two spin states of a free electron is shown as a function of the external magnetic field B.
Resonance happens when the energy of the applied microwave radiation is the same as the energy difference
DE. (d) Hyperfine splitting due to coupling of an unpaired electron with a nuclear spin of 1/2. For the hydrogen
atom, the distance between the two signals is 50.7 mT.534 Spectroscopic techniques: II Structure and interactions