A Mass Spectrometric Study of the Role of Water in the Paternò-Büchi (PB) Reaction between 2-Acetylpyridine and Unsaturated Fatty Acids.

Localizing the double-bond positions in unsaturated fatty acid isomers is of great importance for understanding the reaction pathways in the occurrence and development of some metabolic diseases. The Paternò-Büchi (PB) reaction using 2-acetylpyridine as the charge-tagging PB reagent in combination with tandem mass spectrometry (MS/MS) has been developed as an effective method for determining the positions of C═C bonds in unsaturated fatty acids. Recently, it was found that the presence of water in the reaction solution could greatly enhance the yield of the PB reaction. To optimize the reaction conditions, the water content of the reaction mixture was systematically varied in the reaction between 2-acetylpyridine and oleic acid using a binary acetonitrile/water solvent system. It was observed that increasing water content could increase the yield of the PB reaction. However, this effect was complicated by the Norrish type II reactions and the low water solubility of oleic acid, and the yield of PB product was found to be maximized at 30% water in a water/acetonitrile binary system. At higher water content conditions, the reaction products were dominated by the isomeric products derived from the Norrish type II-A reaction. This enhanced side reaction was tentatively attributed to the increasing production of hydroxyl radicals through the photolysis of water molecules under 254 nm UV illumination. Since there was no evidence for the direct involvement of water molecules in the photocyclization of carbonyl and olefin functionalities, the exact role of water in facilitating the PB reaction was evaluated based on the existing concepts of 'on-water' and 'in-water'. For the reaction system used in this study, our results favor the 'in-water' model, in which aggregates of oleic acid might be formed at elevated water content. Aggregates have a higher concentration of oleic acid or provide microscopic heterogeneous surfaces for promoting the PB reaction by bringing both reaction species together more efficiently. Our preliminary attempt to accelerate the PB reaction under water-free conditions using a suspension of 50 μm C-18 powders was found to be promising. Higher yield of the PB reaction product without elevating the undesirable Norrish type II reaction product was observed.