Thioglycosides Act as Metabolic Inhibitors of Bacterial Glycan Biosynthesis

"Illuminated Glycoporphyrins": A photodynamic approach for Candida albicans inactivation

The continuous increase in the incidence of invasive mycoses, particularly those caused by Candida albicans, is a relevant health issue worldwide due to the lack of effective antifungals and the constant emergence of resistant strains. One of the most promising therapies to treat infections caused by resistant microorganisms is photodynamic inactivation (PDI). The development of novel photosensitizers (PSs) with suitable properties is a key factor to consider when optimizing this therapy. In this work, we designed, synthesized, and characterized four glycoporphyrins functionalized with S-galactose (acetylated and deacetylated) and varying the number of tertiary amino groups as precursors of cationic centers, which can be activated by protonation at physiological pH. The amino and glycosyl groups were introduced to enhance interaction with the microbial cell wall, increase hydrophilicity, and evaluate their combined effect on PS efficiency in photoinactivation. All derivatives presented the characteristic absorption and emission properties of the porphyrin macrocycle. Moreover, the glycoporphyrins were capable of generating singlet oxygen and superoxide anion radical. The photophysical and photodynamic properties were not affected by the different substitution patterns on the porphyrin core. PDI treatments of C. albicans cultures, treated with 5 μM of the PS and irradiated for 30 min, produced cellular inactivation of ∼3.5 log for glycoporphyrins with cationic centers. Furthermore, PDI of C. albicans mediated by glycoporphyrins was potentiated by the addition of KI. Under these conditions, a significant enhancement in cellular death was observed, achieving complete eradication of the treated cell suspensions. Moreover, glycoporphyrins containing pH-activable groups, combined with KI, showed outstanding efficacy against C. albicans pseudohyphae. These in vitro findings underscore the significant impact of substitution patterns on antimicrobial action. To our knowledge, this study marks the first application of glycosylated porphyrin derivatives containing pH-activatable cationic groups in the photoinactivation of C. albicans, paving the way for the development of novel derivatives with potential applications as effective antifungal PSs.

Synthesis of C-glycoside analogues of isopropyl β-D-1-thiogalactopyranoside (IPTG) and 1-β-D-galactopyranosyl-2-methylpropane. Conformational analysis and evaluation as inhibitors of the lac repressor in E. coli and as galactosidase inhibitors

Isopropyl 1-thio-β-D-galactopyranoside (IPTG, 1) is used widely as an inducer of protein expression in E. coli and 1-β-D-galactopyranosyl-2-methylpropane (2), a C-glycoside analogue of 1, has also been identified as an inducer. Here, synthesis and study of mimetics of 1 and 2, 1-β-D-galactopyranosyl-2-methylpropan-1-ols and two cyclic acetals derivatives, that constrain the presentation of the iPr group in various geometries is described. Conformational analysis of C-glycosides in protic solvent is performed using (i) Desmond metadynamics simulations (OPLS4) and (ii) use of 3JHH values obtained by 1H-NMR spectroscopy. 1-β-D-Galactopyranosyl-2-methylpropane (2) is an effective protein expression inducer when compared to the new mimetics, which were less effective or did not induce expression. 1-β-D-Galactopyranosyl-2-methylpropane (2) led to significantly reduced proteolysis during protein expression, compared to IPTG suggesting that recombinant protein purification will be easier to achieve with 2, yielding proteins with higher quality and activity. IPTG reduced bacterial growth to a greater degree than 2 compared to the control. IPTG's isopropyl group was observed by molecular dynamics (MD) simulations to be flexible in the binding pocket, deviating from its crystal structure binding mode, without impacting other interactions. The MD simulations predicted that 1-β-D-galactopyranosyl-2-methylpropane (2) was more likely than IPTG to bind the repressor with a conformation favoured in protic solvent, while maintaining interactions observed for IPTG. MD simulations predicted that isobutanol derivatives may disrupt interactions associated with IPTG's binding mode. The compounds were also evaluated as inhibitors of galactosidases, with 2 being the more potent inhibitor of the E. coli β-galactosidase. The constrained cyclic acetals showed similar inhibition constants to IPTG indicating E. coli β-galactosidase can recognize galactopyranoses with varying presentation of the iPr group.

Chemical biology tools to probe bacterial glycans

Bacterial cells are covered by a complex carbohydrate coat of armor that allows bacteria to thrive in a range of environments. As a testament to the importance of bacterial glycans, effective and heavily utilized antibiotics including penicillin and vancomycin target and disrupt the bacterial glycocalyx. Despite their importance, the study of bacterial glycans lags far behind their eukaryotic counterparts. Bacterial cells use a large palette of monosaccharides to craft glycans, leading to molecules that are significantly more complex than eukaryotic glycans and that are refractory to study. Fortunately, chemical tools designed to probe bacterial glycans have yielded insights into these molecules, their structures, their biosynthesis, and their functions.