Réductions chimique et électrochimique de 5H-benzopyranno[4,3-d]pyrimidines

The chemical reduction of 5H-[1]benzopyranno[4,3-d]pyrimidines with lithium aluminum hydrrde leads to 3,4-dihydro derivations. The electro-chemical reducation in acidic medium shows two monoelectronic cathodic waves. In netural or basic medium, substituted compounds in 2 position show a single bielectronic wave while two bielectronic waves are observed for unsubstituted compounds In all cases, preparative electrolysis lead to a hydrodimer in the 4,4′-positiom.     

Ultrafast nonadiabatic dynamics of [Fe(II)(bpy)(3)](2+) in solution

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.     

The renaissance of H/D exchange

The increasing demand for stable isotopically labeled compounds has led to an increased interest in H/D-exchange reactions at carbon centers. Today deuterium-labeled compounds are used as internal standards in mass spectrometry or to help elucidate mechanistic theories. Access to these deuterated compounds takes place significantly more efficiently and more cost effectively by exchange of hydrogen by deuterium in the target molecule than by classical synthesis. This Review will concentrate on the preparative application of the H/D-exchange reaction in the preparation of deuterium-labeled compounds. Advances over the last ten years are brought together and critically evaluated.     

Fast and efficient in silico 3D screening: toward maximum computational efficiency of pharmacophore-based and shape-based approaches

In continuation of our recent studies on the quality of conformational models generated with CATALYST and OMEGA we present a large-scale survey focusing on the impact of conformational model quality and several screening parameters on pharmacophore-based and shape-based virtual high throughput screening (vHTS). Therefore, we collected known active compounds of CDK2, p38 MAPK, PPAR-gamma, and factor Xa and built a set of druglike decoys using ilib:diverse. Subsequently, we generated 3D structures using CORINA and also calculated conformational models for all compounds using CAESAR, CATALYST FAST, and OMEGA. A widespread set of 103 structure-based pharmacophore models was developed with LigandScout for virtual screening with CATALYST. The performance of both database search modes (FAST and BEST flexible database search) as well as the fit value calculation procedures (FAST and BEST fit) available in CATALYST were analyzed in terms of their ability to discriminate between active and inactive compounds and in terms of efficiency. Moreover, these results are put in direct comparison to the performance of the shape-based virtual screening platform ROCS. Our results prove that high enrichment rates are not necessarily in conflict with efficient vHTS settings: In most of the experiments, we obtained the highest yield of actives in the hit list when parameter sets for the fastest search algorithm were used.     

AFM imaging of immobilized fibronectin: does the surface conjugation scheme affect the protein orientation/conformation?

Covalent grafting of biomolecules could potentially improve the biocompatibility of materials. However, these molecules have to be grafted in an active conformation to play their biological roles. The present work aims at verifying if the surface conjugation scheme of fibronectin (FN) affects the protein orientation/conformation and activity. FN was grafted onto plasma-treated fused silica using two different crosslinkers, glutaric anhydride (GA) or sulfosuccinimidyl 4-(p-maleimidophenyl)butyrate (SMPB). Fused silica was chosen as a model surface material because it presents a roughness well below the dimensions of FN, therefore allowing AFM analyses with appropriate depth resolution. Cell adhesion assays were performed to evaluate the bioactivity of grafted FN. Cell adhesion was found to be higher on GA-FN than on SMPB-FN. Since FN-radiolabeling assays allowed us to rule out a surface concentration effect (approximately 80 ng/cm2 of FN on both crosslinkers), it was hypothesized that FN adopted a more active conformation when grafted via GA. In this context, the FN conformation on both crosslinkers was investigated through AFM and contact angle analyses. Before FN grafting, GA- and SMPB-modified surfaces had a similar water contact angle, topography, and roughness. However, water contact angles of GA-FN and SMPB-FN surfaces clearly show differences in surface hydrophilicity, therefore indicating a dependence of protein organization toward the conjugation strategy. Furthermore, AFM results demonstrated that surface topography and roughness of both FN-conjugated surfaces were significantly different. Distribution analysis of FN height and diameter confirmed this observation as the protein dimensions were significantly larger on GA than SMPB. This study confirmed that the covalent immobilization scheme of biomolecules influences their conformation and, hence, their activity. Consequently, selecting the appropriate conjugation strategy is of paramount importance in retaining molecule bioactivity.     

Novel aflatoxin derivatives and protein conjugates

Aflatoxins, a group of structurally related mycotoxins, are well known for their toxic and carcinogenic effects in humans and animals. Aflatoxin derivatives and protein conjugates are needed for diverse analytical applications. This work describes a reliable and fast synthesis of novel aflatoxin derivatives, purification by preparative HPLC and characterisation by ESI-MS and one- and two-dimensional NMR. Novel aflatoxin bovine serum albumin conjugates were prepared and characterised by UV absorption and MALDI-MS. These aflatoxin protein conjugates are potentially interesting as immunogens for the generation of aflatoxin selective antibodies with novel specificities.     

Heavy Grignard reagents: challenges and possibilities of aryl alkaline earth metal compounds

Compounds of the type aryl--M--X, with M=Ca, Sr, Ba and X as any kind of ligand (such as halide, phosphanide, amide, aryl), are presented. The low reactivity of the heavy alkaline earth metals calcium, strontium, and barium enforces an activation prior to use for the direct synthesis. The insertion of these metals into C--I bonds of aryl iodides (direct synthesis) yields aryl metal iodides and has to be performed at low temperatures and in THF. Aryl alkaline-earth-metal compounds show some characteristics: 1) the ease of ether cleavage enforces low reaction temperatures, 2) for Sr and Ba the Schlenk equilibrium is shifted towards homoleptic MI2 and MPh2, 3) high solubility of diaryl alkaline-earth-metal derivatives in THF even at low temperatures initiated quantum chemical investigations on the aggregation behavior, and 4) a strong low field shift of the 13C resonances of the ipso carbon atoms in NMR spectra was observed. First results from quantum chemical calculations on diaryl dicalcium(I) suggest a long Ca--Ca bond with a considerable Ca--Ca bond dissociation energy. Initial results on a selection of applications such as metallation, metathesis, and addition reactions of aryl calcium compounds are presented as well.