Being a major cause of eutrophication and subsequent loss of water quality, the turnover of phosphorus (P) in lake sediments is in need of deeper understanding. A major part of the flux of P to eutrophic lake sediments is organically bound or of biogenic origin. This P is incorporated in a poorly described mixture of autochthonous and allochthonous sediment and forms the primary storage of P available for recycling to the water column, thus regulating lake trophic status. To identify and quantify biogenic sediment P and assess its lability, we analyzed sediment cores from Lake Erken, Sweden, using traditional P fractionation, and in parallel, NaOH extracts were analyzed using 31P NMR. The surface sediments contain orthophosphates (ortho-P) and pyrophosphates (pyro-P), as well as phosphate mono- and diesters. The first group of compounds to disappear with increased sediment depth is pyrophosphate, followed by a steady decline of the different ester compounds. Estimated half-life times of these compound groups are about 10 yr for pyrophosphate and 2 decades for mono- and diesters. Probably, these compounds will be mineralized to ortho-P and is thus potentially available for recycling to the water column, supporting further growth of phytoplankton. In conclusion, 31P NMR is a useful tool to asses the bioavailability of certain P compound groups, and the combination with traditional fractionation techniques makes quantification possible.
The antibacterial activities of 31 different b-, mixed a/B-, and y-peptides, as well as of B-peptides derived from B2-3-aza- and B3-2-methylidene-amino acids were assayed against six pathogens (Enterococcus faecails, STaphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa), and the results were compared with literature data. The interaction of these peptides with mammalian cells, as modeled by measuring the hemolysis of human erythrocytes, was also investigated. In addition to those peptides designed to fold into amphiphilic helical conformations with positive charges on one face of the helix, one new peptide with hemolytic activity was detected within the sample set. Moreover, it was demontrated that neither cationic peptides used for membrane translocation (B3-oligoarginines), nor mixeda/B- or y-peptides with somatostatin-mimicking activities display unwanted hemolytic activity.
Thiol-functionalized cobalt porphyrins were used as a model system for investigating catalytic activity in homogeneous and heterogeneous oxidation catalysis. Self-assemble monolayers of thiol-functionalized cobalt porphyrins were prepared on a gold surface and served as heterogenous catalysts. These immoblilized molecules prevented the strong inactivation observed for their homogeneous congener. As a result, the turnover number per molecule in heterogeneous catalysis was at least 100 times higher than that of the corresponding homogeneous catalyst. It is atypical for a heterogenized catalyst to outperform its homogeneous congener. The properties of the molecular layers were characterized on the molecular level by means of X-ray photoelectorn spectroscopy (XPS) and scanning tunneling microscopy (STM). The results demonstrate that the performance of these biomimetic catalysts can be dramatically improved if the catalyst arangement can be controlled on the molecular level.
Peptidomimetics incorporating two amino acids 1 and 2 with a stiff stilbene chromophore have been screened by a computational study and compared to a previously investigated analog 3 with stilbene chromophore. The effect of E-Z isomerization of the chromophores on the conformational properties of the petidomimetics was assessed via the frequency of hydrogen bonding between the two peptide strands attached to either side of the chromophore. Substantial differences between the three amino acids were thus indicated, in line with the anticipated effect of chromophore rigidity variation.
Bisporphyrin molecular tweezers with an enediyne (1) or a stiff stilbene (2) photoswitchable spacer are proposed as systems for modulation of bitopic binding to diamine guests via E/Z photoisomerization. The photoisomerization has been monitored by UV-Vis and 1H NMR spectroscopy and occurs without side reactions such as Bergman cyclization. Possible applications are rationalized in terms of competitive binding involving monoamine/diamine mixtures, and are supported by conformational analysis of the envisioned host-guest complexes. Binding dynamics for conformationally flexible guests show significantly different performance of aliphatic 1,w-diamine guests with varying N-N distance.
The solution-phase synthesis of the simplest cyclic B-tetrapeptide, cyclo(B-Ala)4 (4), as well as the solidphase syntheses through side chain anchoring and on-resin cyclization of the cyclic B3-tetrapeptide cyclo-(B3hPhe-B3hLeu-B3hLys-B3hGln-) (14) and the first cyclic B3-pentapeptide cyclo- (B3hVal-B3hPhe-B3Leu- B3hLys-B3hLys-) (19) are reported. Extensive computational as well as spectroscopic studies, including X-ray and NMR spectroscopy, were undertaken to determine the preferred conformations of these unnatural oligomers in solution and in the solid state. cyclo(B-Ala)4 (4) with no chiral side chains is shown to exist as a mixture of rapidly interchanging conformers in solution, whereas inclusion of chiral side chains in the cyclo-B3-tetrapetpride causes stabilizaton of one dominating conformer. The cyclic B3-pentapeptide on the other hand shows larger conformational freedom. The X-ray structure of achiral cyclo(B-Ala)4 (4) displays a Ci-symmetrical 16-membered ring with adjacent C=O and N-H atoms pointing pair wise up and down with respect to the ring plane. CD spectorscopic examinations of all cyclic B-peptides were undertaken and revealed results valuable as starting point for further structural investigations of these entities.
Stereocontrolledp alladium(I1)-catalyzed1 ,4-chloro-and 1,4-acetoxylactonizations o f conjugated cyclic dienes have been developed to give stereodefined fused lactones. The stereochemistry of the 1,4- acetoxylactonization was controlled by the ligand on the metal catalyst, and in this way either a cisor truns-acetoxylactonization was obtained. This dual stereoselectivity is explained by a stereocontrolled acetate attack (trans or cis, respectively) on the allyl group in the catalytic (π-allyl)- palladium intermediate. To further strengthen the mechanism the intermediate (π-ally1)palladium complex was isolated and fully characterized. A stereospecific synthesis of cis- and truns-2-[6- (benzyloxy)-2,4-heptadien-l-yllacetic a cid (cis- and truns-9) followed by stereoselective Pd(I1)- catalyzed chloro- and acetoxylactonization in acetonelacetic acid resulted in highly functionalized fused lactones with control of the relative stereochemistry at four different carbons.
New thiazole-based chiral N,P-ligands that are open-chain analogues of known cyclic thiazole ligands have been synthesized and evaluated in the iridium-catalyzed asymmetric hydrogenation of trisubstituted olefins. Chirality was introduced into the ligands through a highly diastereoselective alkylation using Oppolzer's camphorsultam as chiral auxiliary. In general, the new catalysts are as reactive and selective as their cyclic counterparts for the asymmetric hydrogenation of various trisubstituted olefins.
The crystal structure of 2-[(N,N-dimethylamino)methyl]benzenetellurenyl chloride (2), a compound previously formulated as bis [[2-(N,N-dimethylamino)methyl]phenyl] ditelluride bis hydrochloride (1a), was determinded. In the molecule 2, tellurium is bonded to the carbon of the phenyl group [2.120(3)Å], the nitrogen o fthe ortho dimethylamino substituent [2.362(3)Å], and the chlorine atom [2.536[1]Å]. There also is an intermolecular interaction of the tellurium atom with the phenyl ring of a neighbouring molecule [3.655(1)Å], resulting in the formation of zigzag chains along the b axis. The noncentorsymmetric space group of the crystal can be explained by the chiral surrounding of tellurium.