Ferrier Carbocyclization-Mediated Synthesis of Enantiopure Azido Inositol Analogues.

Azide-modified inositol (InoAz) analogues are valuable as inhibitors and have shown promise as metabolic chemical reporters (MCRs) for labeling inositol-containing glycoconjugates in eukaryotic cells and potentially in mycobacteria, but the synthesis of enantiomerically pure InoAz analogues via traditional approaches is challenging. As a complementary route, here we investigated the application of the Ferrier carbocyclization reaction to the synthesis of enantiopure InoAz analogues starting from readily available azido glucosides. Using this approach combined with a para-methoxybenzyl protecting group strategy, 3-azido-3-deoxy- and 4-azido-4-deoxy-d-myo-inositol were efficiently synthesized. 5-Azido-5-deoxy-d-myo-inositol was inaccessible due to an unusual ß-elimination reaction, wherein the azide anion acted as the leaving group. The reported strategy is expected to facilitate continued development of synthetic InoAz analogues as inhibitors or MCRs of inositol-containing glycoconjugates in eukaryotic and mycobacterial systems.

Deoxyfluoro-d-trehalose (FDTre) analogues as potential PET probes for imaging mycobacterial infection.

Mycobacterium tuberculosis, the etiological agent of human tuberculosis, requires the non-mammalian disaccharide trehalose for growth and virulence. Recently, detectable trehalose analogues have gained attention as probes for studying trehalose metabolism and as potential diagnostic imaging agents for mycobacterial infections. Of particular interest are deoxy-[(18)F]fluoro-d-trehalose ((18)F-FDTre) analogues, which have been suggested as possible positron emission tomography (PET) probes for in vivo imaging of M. tuberculosis infection. Here, we report progress toward this objective, including the synthesis and conformational analysis of four non-radioactive deoxy-[(19)F]fluoro-d-trehalose ((19)F-FDTre) analogues, as well as evaluation of their uptake by M. smegmatis. The rapid synthesis and purification of several (19)F-FDTre analogues was accomplished in high yield using a one-step chemoenzymatic method. Conformational analysis of the (19)F-FDTre analogues using NMR and molecular modeling methods showed that fluorine substitution had a negligible effect on the conformation of the native disaccharide, suggesting that fluorinated analogues may be successfully recognized and processed by trehalose metabolic machinery in mycobacteria. To test this hypothesis and to evaluate a possible route for delivery of FDTre probes specifically to mycobacteria, we showed that (19)F-FDTre analogues are actively imported into M. smegmatis via the trehalose-specific transporter SugABC-LpqY. Finally, to demonstrate the applicability of these results to the efficient preparation and use of short-lived (18)F-FDTre PET radiotracers, we carried out (19)F-FDTre synthesis, purification, and administration to M. smegmatis in 1 hour.

Bioorthogonal Chemical Reporters for Selective In†Situ Probing of Mycomembrane Components in Mycobacteria.

The global pathogen Mycobacterium tuberculosis and other species in the suborder Corynebacterineae possess a distinctive outer membrane called the mycomembrane (MM). The MM is composed of mycolic acids, which are either covalently linked to an underlying arabinogalactan layer or incorporated into trehalose glycolipids that associate with the MM non-covalently. These structures are generated through a process called mycolylation, which is central to mycobacterial physiology and pathogenesis and is an important target for tuberculosis drug development. Current approaches to investigating mycolylation rely on arduous analytical methods that occur outside the context of a whole cell. Herein, we describe mycobacteria-specific chemical reporters that can selectively probe either covalent arabinogalactan mycolates or non-covalent trehalose mycolates in live mycobacteria. These probes, in conjunction with bioorthogonal chemistry, enable selective in situ detection of the major MM components.

Peptidoglycan precursor synthesis along the sidewall of pole-growing mycobacteria.

Rod-shaped mycobacteria expand from their poles, yet d-amino acid probes label cell wall peptidoglycan in this genus at both the poles and sidewall. We sought to clarify the metabolic fates of these probes. Monopeptide incorporation was decreased by antibiotics that block peptidoglycan synthesis or l,d-transpeptidation and in an l,d-transpeptidase mutant. Dipeptides complemented defects in d-alanine synthesis or ligation and were present in lipid-linked peptidoglycan precursors. Characterizing probe uptake pathways allowed us to localize peptidoglycan metabolism with precision: monopeptide-marked l,d-transpeptidase remodeling and dipeptide-marked synthesis were coincident with mycomembrane metabolism at the poles, septum and sidewall. Fluorescent pencillin-marked d,d-transpeptidation around the cell perimeter further suggested that the mycobacterial sidewall is a site of cell wall assembly. While polar peptidoglycan synthesis was associated with cell elongation, sidewall synthesis responded to cell wall damage. Peptidoglycan editing along the sidewall may support cell wall robustness in pole-growing mycobacteria.

A trifunctional cyclooctyne for modifying azide-labeled biomolecules with photocrosslinking and affinity tags.

A bicyclo[6.1.0]nonyne (BCN)-based cyclooctyne reagent bearing a photocrosslinking diazirine (DAz) group and a biotin affinity handle, BCN-DAz-Biotin, is reported. BCN-DAz-Biotin is capable of simultaneously delivering photocrosslinking and affinity tags to azide-labeled biomolecules, enabling photoactivated capture and enrichment/detection of interacting species in native contexts.