I am preparing ferrous sulfate liposomes and I tested the oxidation stability by adding the liposomes into ethanol and monitoring the changes in the intensity of the absorption peak at 233nm. BUT!! there was no change at this wavelength over 12 weeks. In contrast, the intensity of the peak at 220 was increasing. Is it related to the oxidation of the lipids? or the leakage of iron? or anything else?
Thanks
About 200-210 nm there is an intense absorption peak mainly due to non-conjugated insaturations. As you say, absorption of conjungated dienes is located at 233 nm. When it appears can be displaced to a lower wavelenght because of the intensity of the 200 nm peak.
However, you have to take into account other important factors that can alter the oxidation index and that usually are not considered.
For example, the presence of salts modifies the spectrum because of the scattering they produce in ethanol, since they are not soluble. You can argue that the effect will be always the same over the time, but if iron 2+ oxidizes to iron 3+ the scattering could change. Furthermore, you have also to consider the absorption of iron 2+ and 3+.
In our case we always purify the lipids, for example, by the Bligh-Dyer method (biphasic separation)(see refs). In this way, water soluble components are segregated from lipids. Of course the process takes more time than the simple addition of one aliquot to ethanol, but the results have no artifacts.
Refs:
http://www.nrcresearchpress.com/doi/abs/10.1139/o59-099#.WuA5QzMuCM8
Liposomes, a practical approach. Oxford Univ Press, 1990. page 256. RRC New Ed.
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