Exploring calixarene-based clusters for efficient functional presentation of Streptococcus pneumoniae saccharides.

Calixarenes are promising scaffolds for an efficient clustered exposition of multiple saccharide antigenic units. Herein we report the synthesis and biological evaluation of a calix[6]arene functionalized with six copies of the trisaccharide repeating unit of Streptococcus pneumoniae (SP) serotype 19F. This system has demonstrated its ability to efficiently inhibit the binding between the native 19F capsular polysaccharide and anti-19F antibodies, despite a low number of exposed saccharide antigens, well mimicking the epitope presentations in the polysaccharide. The calix[6]arene mobile scaffold has been selected for functionalization with SP 19F repeating unit after a preliminary screening of four model glycocalixarenes, functionalized with N-acetyl mannosamine, and differing in the valency and/or conformational properties. This work is a step forward towards the development of new fully synthetic calixarenes comprising small carbohydrate antigens as potential carbohydrate-based vaccine scaffolds.

Molecular Architecture and Symmetry Properties of 1,3-Alternate Calix[4]arenes with Orientable Groups at the Para Position of the Phenolic Rings.

Two glycoclusters constituted by four fully acetylated ß-acetylmannosamine residues linked through trimethylenethioureido spacers to a calix[4]arene core and differing for the presence of methoxy or propoxy groups at the lower rim were synthesized. One of the two compounds is fixed in the 1,3-alternate geometry by the presence of the propoxy groups, while the other is potentially free to assume one of the different geometries allowed in calix[4]arene. Their similar NMR spectra in chloroform clearly suggest the same 1,3-alternate geometry. Both compounds were submitted to a conformational investigation with the DFT approach at the standard B3LYP/6-31G(d) level. The two glycocalixarenes showed a large conformational preference for the same geometry that put the mannosamine moiety of one substituent close to the thioureido group of the opposite substituent. This allows the formation of intramolecular hydrogen bonds and originates a series of through-space close contacts. A comparison with the NOESY maps evidence an excellent correspondence between experimental and theoretical data, thus giving an experimental validation of the highly symmetrical conformation that the two glycocalixarenes assume in apolar solvents.

A calixarene-based fluorescent ratiometric temperature probe.

We report the first macrocycle-based ratiometric molecular thermometer exploiting the conformational thermosensitivity of a calixarene functionalized with two different fluorophores. Thanks to the dependence on temperature of the efficiency of excitation energy transfer between the organic fluorophores, the thermometer works over a 60 °C-wide temperature range with a sensitivity of 4% °C-1.

Heme binding and peroxidase activity of a secreted minicatalase.

Microbial pathogens often require efficient and robust H2 O2 scavenger activities to survive in the presence of reactive oxygen species generated by inflammatory responses. In addition to catalases and peroxidases, enzymes known to scavenge H2 O2 , a novel class of secreted minicatalases is found in diderm bacteria. Here, we characterize the Helicobacter pylori (Hp) minicatalase: a monomeric hemoprotein with catalase core homology. Overexpression of Hp minicatalase rescued a catalase/peroxidase-deficient Escherichia coli phenotype under aerobic conditions and limited H2 O2 stress. The purified enzyme lacks catalase activity, but has strong (kcat > 100 s-1 ) H2 O2 -dependent peroxidase activity toward a variety of organic substrates. Our investigations into heme binding revealed that the heme cofactor is assembled in the periplasm to form the functional holoprotein. Furthermore, we observed the presence of a disulfide bond near the heme cavity of Hp minicatalase, which is conserved in secreted minicatalases and, therefore, may play a role in heme binding.