Catalytic synthesis of enantiopure mixed diacylglycerols – synthesis of a major M. tuberculosis phospholipid and platelet activating factor

Concise Synthesis of Glycerophospholipids

Glycerophospholipids are major components of cellular membranes and provide important signaling molecules. Besides shaping membrane properties, some bind to specific receptors to activate biological pathways. Untangling the roles of individual glycerophospholipids requires clearly defined molecular species, a challenge that can be best addressed through chemical synthesis. However, glycerophospholipid syntheses are often lengthy due to the contrasting polarities found within these lipids. We now report a general strategy to quickly access glycerophospholipids via opening of a phosphate triester epoxide with carboxylic acids catalyzed by Jacobsen's Co(salen) complex. We show that this method can be applied to a variety of commercially available fatty acids, photoswitchable fatty acids, and other carboxylic acids to provide the corresponding glycerophosphate derivatives.

The impact of lysyl-phosphatidylglycerol on the interaction of daptomycin with model membranes

Daptomycin is an important clinical antibiotic for which resistance is rising. Daptomycin resistant strains of S. aureus often have increased 1,2-diacyl-sn-glycero-3-[phospho-1-(3-lysyl(1-glycerol))] (lysyl-PG) and mutations to the proteins directly involved in the synthesis and translocation of lysyl-PG are implicated in resistance mechanisms. To study the interaction of daptomycin with lysyl-DMPG-containing model membranes a new stereospecific and regioselective synthesis of lysyl-DMPG was developed. Studies on model membranes containing lysyl-DMPG demonstrate that: (1) daptomycin is not significantly repelled by the cationic charge of lysyl-DMPG; (2) daptomycin binds less avidly to lysyl-DMPG compared to DMPG; (3) the presence of lysyl-DMPG does not impact the membrane bound backbone conformation of daptomycin in a significant way; (4) lysyl-DMPG increases oligomer formation; (5) lysyl-DMPG does not impact model membrane fluidity at lysyl-PG : PG ratios that are relevant to daptomycin resistance. The results of these studies suggest that increased lysyl-PG content does not confer resistance to daptomycin by altering membrane fluidity or reducing membrane affinity but may confer resistance by altering the structure of daptomycin oligomers.

Synthesis of phosphatidic acids via cobalt(salen) catalyzed epoxide ring-opening with dibenzyl phosphate

With a CoIII(salen)OTs catalyst, dibenzyl phosphate ring-opens a variety of terminal epoxides with excellent regio-selectively and yields up to 85%. The reaction is used in a highly efficient synthesis of enantiopure mixed-diacyl phosphatidic acids, including a photoswitchable phosphatidic acid mimic.

A cobalt-catalyzed, phosphate-induced, ring-opening reaction is used in an efficient synthesis of enantiopure mixed-diacyl phosphatidic acids, including a photoswitchable phosphatidic acid mimic.

From the banal to the bizarre: unravelling immune recognition and response to microbial lipids

Microbes produce a rich array of lipidic species that through their location in the cell wall and ability to mingle with host lipids represent a privileged class of immune-active molecules. Lipid-sensing immunity recognizes microbial lipids from pathogens and commensals causing immune responses. Yet microbial lipids are often heterogeneous, in limited supply and in some cases their structures are incompletely defined. Total synthesis can assist in structural determination, overcome supply issues, and provide access to high-purity, homogeneous samples and analogues. This account highlights synthetic approaches to lipidic species from pathogenic and commensal bacteria and fungi that have supported immunological studies involving lipid sensing through the pattern recognition receptor Mincle and cell-mediated immunity through the CD1-T cell axis.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

The late stage of COPI vesicle fission requires shorter forms of phosphatidic acid and diacylglycerol

Studies on vesicle formation by the Coat Protein I (COPI) complex have contributed to a basic understanding of how vesicular transport is initiated. Phosphatidic acid (PA) and diacylglycerol (DAG) have been found previously to be required for the fission stage of COPI vesicle formation. Here, we find that PA with varying lipid geometry can all promote early fission, but only PA with shortened acyl chains promotes late fission. Moreover, diacylglycerol (DAG) acts after PA in late fission, with this role of DAG also requiring shorter acyl chains. Further highlighting the importance of the short-chain lipid geometry for late fission, we find that shorter forms of PA and DAG promote the vesiculation ability of COPI fission factors. These findings advance a general understanding of how lipid geometry contributes to membrane deformation for vesicle fission, and also how proteins and lipids coordinate their actions in driving this process.

Total Synthesis of an Immunogenic Trehalose Phospholipid from Salmonella Typhi and Elucidation of Its sn-Regiochemistry by Mass Spectrometry

Diphosphatidyltrehalose (diPT) is an immunogenic glycolipid, recently isolated from Salmonella Typhi. Despite rigorous structure elucidation, the sn-position of the acyl chains on the glycerol backbone had not been unequivocally established. A stereoselective synthesis of diPT and its regioisomer is reported herein. Using a hybrid MS³ approach combining collisional dissociation and ultraviolet photodissociation mass spectrometry for analysis of the regioisomers and natural diPT, the regiochemistry of the acyl chains of this abundant immunostimulatory glycolipid was established.

Synthesis of acylglycerol derivatives by mechanochemistry

In recent times, many biologically relevant building blocks such as amino acids, peptides, saccharides, nucleotides and nucleosides, etc. have been prepared by mechanochemical synthesis. However, mechanosynthesis of lipids by ball milling techniques has remained essentially unexplored. In this work, a multistep synthetic route to access mono- and diacylglycerol derivatives by mechanochemistry has been realized, including the synthesis of diacylglycerol-coumarin conjugates.

Discovery of Salmonella trehalose phospholipids reveals functional convergence with mycobacteria

Salmonella species are among the world's most prevalent pathogens. Because the cell wall interfaces with the host, we designed a lipidomics approach to reveal pathogen-specific cell wall compounds. Among the molecules differentially expressed between Salmonella Paratyphi and S. Typhi, we focused on lipids that are enriched in S. Typhi, because it causes typhoid fever. We discovered a previously unknown family of trehalose phospholipids, 6,6'-diphosphatidyltrehalose (diPT) and 6-phosphatidyltrehalose (PT). Cardiolipin synthase B (ClsB) is essential for PT and diPT but not for cardiolipin biosynthesis. Chemotyping outperformed clsB homology analysis in evaluating synthesis of diPT. DiPT is restricted to a subset of Gram-negative bacteria: large amounts are produced by S. Typhi, lower amounts by other pathogens, and variable amounts by Escherichia coli strains. DiPT activates Mincle, a macrophage activating receptor that also recognizes mycobacterial cord factor (6,6'-trehalose dimycolate). Thus, Gram-negative bacteria show convergent function with mycobacteria. Overall, we discovered a previously unknown immunostimulant that is selectively expressed among medically important bacterial species.

Total synthesis and mass spectrometric analysis of a Mycobacterium tuberculosis phosphatidylglycerol featuring a two-step synthesis of (R)-tuberculostearic acid

We report the total synthesis of (R)-tuberculostearic acid-containing Mycobacterium tuberculosis phosphatidylglycerol (PG). The approach features a two-step synthesis of (R)-tuberculostearic acid, involving an (S)-citronellyl bromide linchpin, and the phosphoramidite-assisted assembly of the full PG structure. Collision-induced dissociation mass spectrometry of two chemically-synthesized PG acyl regioisomers revealed diagnostic product ions formed by preferential loss of carboxylate at the secondary (sn-2) position.