Meistarafyrirlestur í efnafræði - Jóhann Daði Magnússon

Meistaranemi:  Jóhann Daði Magnússon 

Heiti verkefnis: Efnasmíðar á handhverft stöðubundnum þríglyseríðum af AAB og ABC gerð

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Deild: Raunvísindadeild 

Leiðbeinandi: Guðmundur G. Haraldsson, prófessor við Raunvísindadeild Háskóla Íslands

Einnig í meistaranefnd: Benjamín Ragnar Sveinbjörnsson, lektor við Raunvísindadeild Háskóla Íslands.

Prófdómari: Haraldur Garðarsson, gæðastjóri hjá Algalif.

Ágrip

 The aim of the research project described in this thesis was the organic synthesis of enantiomerically pure triacylglycerols (TAGs). TAGs are by far the largest class of nonpolar lipids in fats and oils of plant and animal origin. They consist of a glycerol framework to which three fatty acids are linked through carboxylic ester bonds, two to the primary alcohol end-positions and the third to the secondary alcohol mid-position. The glycerol molecule is prochiral and thus when two different fatty acids are attached to its two end-positions (sn-1 and sn-3), a chiral carbon is generated at the mid-position (sn-2), offering two enantiomers of the molecule, one of the (R)-and the other of the (S)-configuration. This is referred to by the sn-terminology (sn meaning stereospecific numbering). 

TAGs form an important part of the human and animal diet. They are not only a source of energy but also provide essential fatty acids for various biological roles including the long-chain n-3 PUFAs, alfa-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The latter two are the most prevalent of the long-chain n-3 PUFAs that are characteristic of marine fats and oils and serve as precursor to potent lipid mediators including various eicosanoids and docosanoids. 

Structured TAGs are TAGs that have selected fatty acids at predetermined positions of the glycerol framework. Herein, the ABA type TAGs (Class II TAGs) refer to symmetrically structured TAGs while the AAB and ABC type TAGs (Class I and Class III TAGs, respectively) refer to asymmetrically structured TAGs (or enantiostructured TAGs). The ABA type TAGs possess two identical saturated fatty acids at the end-positions with a different fatty acid in the mid-position. The AAB type TAGs possess two identical saturated fatty acids in one of the end-positions and the mid-position with a different fatty acid in the other end-position while the ABC type TAGs possess three different fatty acids. Herein, the synthesis of those three classes of TAGs is described with the main emphasis on the enantiostructured TAGs. Their intended uses include: (1) substrates for studies in rats; (2) enantiopure TAG standards to enable enantiospecific analysis of chiral TAG molecular species present in common fats and oils including the human breast milk fat. 

The enantiostructured TAGs intended for studies in rats possess a bioactive n-3 PUFA, including ALA, EPA and DHA, in one of the stereospecific positions of the glycerol framework to investigate their bioavailability, and subsequent distribution in the rat body, in terms of their stereospecific positioning, i.e. the mid-position compared to the end-positions and specifically one enantiomeric end-position (sn-1) compared to the other (sn-3). They belong to the Class I TAGs possessing either stearic (18:0) or palmitic acid (16:0) in one of the enantiomeric end-positions (sn-1 or sn-3) as well as in the mid-position (sn-2) with the bioactive n-3 PUFA in the other remaining enantiomeric end-position. In total of 12 enantiostructured TAGs were synthesized including both the (R)- and (S)-enantiomers. They were synthesised by a five-step chemoenzymatic approach starting from either (R)- or (S)-solketals as enantiopure chiral precursors, an immobilized Candida antarctica lipase (CAL-B) to introduce both the saturated fatty acids, and EDCI coupling agent to introduce the n-3 PUFAs in the final step. The enantiostructured TAGs were obtained optically pure (> 96% ee) in high chemical and regiopurities and high to excellent yields in all cases. 

The Class II TAGs were synthesized starting from glycerol by a two-step chemoenzymatic approach involving the highly regioselective CAL-B to introduce the saturated fatty acids exclusively to the end-positions of the glycerol framework and EDCI coupling agent to introduce the bioactive n-3 PUFAs to the mid-position. In total of 6 TAGs were synthesized in high chemical and regiopurities and high to excellent yields. 

In the case of DHA the symmetrically structured Class II TAG and both enantiomers of the enantiostructured Class I TAGs possessing stearic acid were prepared in 25 g quantities each that were submitted to rat studies performed in China and Finland. The first results from that study have been submitted for publication.

Synthesis of some of the enantiostructured TAGs intended for use as standards in the enantiospecific analysis of chiral TAGs commonly found in human breast milk fat was also addressed. They belong to the Class I and Class III categories of TAGs. Two belong to the Class I TAGs ((R)-14:0/14:0/18:1 and (S)-18:1/14:0/14:0), whereas three belong to the Class III TAGs ((R)-16:0/14:0/18:1, (S)-18:1/14:0/16:0 and (R)-18:0/16:0/18:2). They were synthesized by a six-step chemoenzymatic approach, again starting from either enantiopure (R)- or (S)-solketals involving CAL-B to regioselectively introduce one of the saturated fatty acids to one of the end-positions and EDCI coupling agent to introduce the two different fatty acids in the mid-position and the other end-position in two different steps. The enantiostructured TAGs were obtained optically pure in high chemical and regiopurities. The critical step in their synthesis involved a deprotection of a benzyl protective moiety by catalytic hydrogenolysis using a palladium catalyst and a catalytic amount of perchloric acid, conditions which induced acyl-migration. This resulted in moderate yields for the debenzylation step (40 – 60%) whereas yields in all remaining steps were high to excellent.