Dynamics and Spin Labels

4 Dynamics and Spin Labels. Nitroxide spin labels have been used for some time to study dynamics and the local environment of the probe. However, this area has received a very significant new impetus with the introduction of site- 

The technique was beautifully illustrated in a recent combined 250 GHz and 9 GHz study on membrane vesicles composed of pure lipid (DPPC) and of DPPC with cholesterol in a 1 1 molar ratio using the end-chain labelled lipid 16-PC. Using sophisticated computer modelling it was shown that the 250 GHz spectra were only sensitive to rapid internal dynamics of the end chain whereas the 9 GHz spectra were also sensitive to the overall motion of the lipid and the primary effect of the cholesterol was to restrict the range of motion whilst simultaneously allowing faster motion of the end-chain dynamics. 

A detailed simulation study of spin-labelled lipids in membranes at 94 GHz also concluded that cholesterol causes in-plane ordering of the lipid chains, restricting the range of axial rotation, but that the rotation frequency could still be relatively rapid compared with phospholipids membranes not containing cholesterol. Dynamics were also explored using the spin-labelled pentamer TTC TT and a new cytosine spin label, both in aqueous solution at 220 GHz.  

A variable-temperature study again demonstrated that high-field EPR is capable of discriminating between local motion and overall motion of spin-labelled macromolecules. 

Another interesting and potentially important study showed that by addition of the lanthanide YbCh it is possible to align phospholipid bilayers magnetically, either perpendicular or parallel to the field, using a static magnetic field of 3.4 T. This was clearly shown at 95 GHz with the nitroxide spin probe 3P-doxyl-5a-cholestane, where excellent sensitivity and resolution were demonstrated.  

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