NaBH4 reduction of alkenones to the corresponding alkenols: a useful tool for their characterisation in natural samples

During a search for phytoplanktonic lipid photoproducts in sediment extract s that had been reduced with NaBH4, we observed that the alkenones present had been reduced quantitatively to the corresponding unsaturated alcohols ( alkenols). The silylated alkenols display better chromatographic characteri stics than the corresponding alkenones and very useful El mass spectra, whi ch show strong fragment ions at m/z 117 or m/z 131 due to cleavage alpha to the functional group, allowing methyl and ethyl alkenols (and hence the pa rent alkenones) to be readily differentiated. Calculations of values using the alkenols formed by reduction agreed within experimental error (0.01 uni ts) with values calculated from the original alkenones, suggesting that thi s might be a useful method for samples containing low contents of alkenones or where co-elution is a problem. Although small amounts of alkenols have been found in some sediments their presence does not appear to affect U-37( K) calculations. We have found large amounts of alkenols (up to 20% of the amount of the corresponding alkenone) in extracts of some sediments from Ca margue (France), which we suggest are probably formed by a non-selective ba cterial reduction of alkenones under anoxic conditions. The possibility of a natural contribution of alkenols from haptophytes was tested and we were able for the first time to detect traces (less than 1% of the amount of the corresponding alkenone) of alkenols in non-reduced extracts of Gephyrocaps a oceanica and Isochrysis galbana. We presume that their biosynthesis is cl osely related to that of the alkenones. Studies of other haptophytes (parti cularly those from benthic or coastal environments) are needed to ascertain whether other species might contain higher contents of alkenols, The reduc tion-silylation technique has also proven to be very useful for the charact erisation of previously unreported alkenones in microalgae, sediments and s eawater. A re-examination of extracts of several haptophytes after this tre atment allowed us to detect small amounts of a 35:1 methyl alkenone and 2 i somers of a 35:2 alkenone in G. oceanica, and monounsaturated C-35-C-38 alk enones in Emiliania huxleyi. (C) 2001 Published by Elsevier Science Ltd.