Selective deuteration for molecular insights into the digestion of medium chain triglycerides

• Two versions of deuterated tricaprin were synthesized that were selectively deuterated at the different alkyl chains.
• The pancreatic lipase hydrolysis of medium chain triglycerides was probed by using 1H and 2H NMR and mass spectrometry under biologically relevant conditions.
• The hydrolysis of the sn-2 monocaprin to glycerol and capric acid did not occur within biologically relevant timescales and fatty acid migration occurs only in limited amounts.
• Deuterium labeling, NMR and MS were able to determine reaction mechanism and identify products of similar molecular masses in the colloidal structures formed during the triglyceride digestion by pancreatic lipase.

Medium chain triglycerides (MCTs) are a unique form of dietary fat that have a wide range of health benefits. They are molecules with a glycerol backbone esterified with medium chain (6–12 carbon atoms) fatty acids on the two outer (sn-1 and sn-3) and the middle (sn-2) positions. During lipid digestion in the gastrointestinal tract, pancreatic lipase stereoselectively hydrolyses the ester bonds of these triglycerides on the sn-1 and sn-3 positions resulting in sn-2 monoglyceride and fatty acids as major products. However, the sn-2 monoglycerides are thermodynamically less stable than their sn-1/3 counterparts. Isomerization or fatty acid migration from the sn-2 monoglyceride to sn-1/3 monoglyceride may occur spontaneously and would lead to glycerol and fatty acid as final products. Here, tricaprin (C10) with selectively deuterated fatty acid chains was used for the first time to monitor chain migration and the stereoselectivity of the pancreatic lipase-catalyzed hydrolysis of ester bonds. The intermediate and final digestion products were studied using NMR and mass spectrometry under biologically relevant conditions. The hydrolysis of the sn-2 monocaprin to glycerol and capric acid did not occur within biologically relevant timescales and fatty acid migration occurs only in limited amounts as a result of the presence of undigested diglyceride species over long periods of time in the digestion medium. The slow kinetics for the exchange of the sn-2 fatty acid chain and the stereoselectivity of pancreatic lipase on MCTs is relevant for industrial processes that involve enzymatic interesterification and the production of high-value products such as specific structured triacylglycerols, confectionery fats and nutritional products.