Nitroxide fatty acid spin-label

Monolayer Studies of Pure Nitroxide Fatty Acid Spin-Label Probes... 

To determine the shape of the hydrophobic barrier of bilayer membranes, fatty acids and PC molecules spin labeled with nitroxides at various positions along the lipid chains were diffused into vesicles and their solvent-sensitive isotropic coupling constants were measured [54]. Results are plotted in Figure 5 in terms of distance of the probe from the bilayer center. Also shown is the profile of the dielectric constant along the membrane normal evaluated from the fluorescence lifetime distribution of fluorescence probes in PC liposomes [55]. These data correlate well with results from neutron diffraction studies that map the positional distribution of water and lipid moieties along the bilayer normal [56]. 

The localization of a nitroxide in a membrane [30] depends upon the relative polarity of the nitroxide moiety and other polar groups that may be present in the molecules. If the nitroxide group is the only polar group, then the molecule will orient itself in the membrane so that the nitroxide is located in the aqueous phase. The cholestane spin label is one such example. If groups more polar than the nitroxide are present, then the nitroxide will be found within the bilayer. For example, doxyl-labeled fatty acids are oriented such that the carboxyl group is in the aqueous layer and the nitroxide is down within the membrane. 

For fatty acids bound to serum albumin, the fractions of singly and doubly hydrogen-bonded spin labels vary between yiw = 0.34 and 0.46, and w = 0.05 and 0.12, at the C7 and C16 positions, respectively. On the other hand, higher values of /iw = 0.48 and w=0.15 are found for spin labels bound covalently at the surface of serum albumin, a site with maximum exposure to water. Note that maximum two waters can be hydrogen bonded to a single nitroxide. 

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