Heterocyclic bis-cations as starting hits for design of inhibitors of the bifunctional enzyme histidine-containing protein kinase/phosphatase from Bacillus subtilis.

The main mechanism of carbon catabolite repression/activation in low-guanine and low-cytosine Gram-positive bacteria seems to involve phosphorylation of HPr (histidine-containing protein) at Ser-46 by the ATP-dependent HPr kinase, which in Bacillus subtilis, Lactobacillus casei, and Staphylococcus xylosus also exhibits phosphatase activity and is thus a bifunctional enzyme (HPrK/P). Since deficiency of HPrK/P in S. xylosus, L. casei, and B. subtilis mutants leads to severe growth defects, inhibitors of the enzyme could form a new family of antibiotic drugs. The aim of the study was to screen an in-house chemical library for identification of hits as inhibitors of HPrK/P in B. subtilis and to further extract additional information of structural features from hit optimization using a radioactive in vitro assay. A symmetrical bis-cationic compound LPS 02-10-L-D09 (2a) with a 12-carbon alkyl linker bridging the two 2-aminobenzimidazole moieties was identified as a non-ATP mimetic compound exhibiting an EC(50) value of 10 microM in a kinase assay with HPr as substrate. The substance also inhibited the phosphatase activity of HPrK/P triggered by the addition of inorganic phosphate. Similar results were obtained with 2a and catabolite repression HPr, which, like HPr, can be phosphorylated at Ser-46 by HPrK/P and is involved in catabolite repression. Structure-activity relationship analysis indicated the importance in its structure of a substituted 2-aminobenzimidazole. This typical heterocycle is linked through a C12 alkyl chain to a second scaffold that can bear a cationic or a noncationic moiety but in all cases should present an aromatic ring in its vicinity.

Fluorescent derivatives of the GFP chromophore give a new insight into the GFP fluorescence process.

The photophysical properties of synthetic compounds derived from the imidazolidinone chromophore of the green fluorescent protein were determined. Various electron-withdrawing or electron-donating substituents were introduced to mimic the effect of the chromophore surroundings in the protein. The absorption and emission spectra as well as the fluorescence quantum yields in dioxane and glycerol were shown to be highly dependent on the electronic properties of the substituents. We propose a kinetic scheme that takes into account the temperature-dependent twisting of the excited molecule. If the activation energy is low, the molecule most often undergoes an excited-state intramolecular twisting that leads it to the ground state through an avoided crossing between the S(1) and S(0) energy surfaces. For a high activation energy, the torsional motion within the compounds is limited and the ground-state recovery will occur preferentially by fluorescence emission. The excellent correlation between the fluorescence quantum yields and the calculated activation energies to torsion points to the above-mentioned avoided crossing as the main nonradiative deactivation channel in these compounds. Finally, our results are discussed with regard to the chromophore in green fluorescent protein and some of its mutants.