The Escherichia coli inducible lysine decarboxylase, LdcI/ CadA, together with the inner-membrane lysine-cadaverine antiporter, CadB, provide cells with protection against mild acidic conditions (pHB5). To gain a better understanding of the molecular processes underlying the acid stress response, the X-ray crystal structure of LdcI was determined. The structure revealed that the protein is an oligomer of five dimers that associate to form a decamer. Surprisingly, LdcI was found to co-crystallize with the stringent response effector molecule ppGpp, also known as the alarmone, with 10 ppGpp molecules in the decamer. ppGpp is known to mediate the stringent response, which occurs in response to nutrient deprivation. The alarmone strongly inhibited LdcI enzymatic activity. This inhibition is important for modulating the consumption of lysine in cells during acid stress under nutrient limiting conditions. Hence, our data provide direct evidence for a link between the bacterial acid stress and stringent responses.

The monomeric zinc compounds with threecoordinated zinc (dipp)NacNacZnCl ( 1), (dipp)NacNac-ZnMe ( 2), (dipp)NacNacZntBu (3), and (dipp)NacNac-ZnPh (4) were prepared under mild conditions in strong coordinating donor solvents. Compounds 2-4 have been structurally characterized. The β-diketoiminate ligand (2- ((2,6-diisopropylphenyl)amino)-4- ((2,6-diisopropylphenyl)imino)pent-2-ene) ((dipp)NacNac) acts as a chelating ligand through its two N atoms, forming a sixmembered ring with the zinc atom.

The reaction of the b-diketoiminate lithium complex (dipp)NacNacLi´OEt 2 ((dipp)NacNac = 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-pent-2-enyl) with iPrMgCl and MgI 2 yield the corresponding (dipp)NacNacMgiPr´OEt 2 (1) and (dipp)NacNacMgI´OEt 2 (2). The reaction of 2 with NaBH 4 in diethylether gives (dipp)NacNacMg(l-H) 3 BH´OEt 2 (3). The core element of compounds 1±3 is a six-membered ring formed by N(1)±C(1)±C(2)±C(3)±N(2) and magnesium. The structures of 1 and 2 show the b-diketoiminate backbone in a boat-conformation with the tetrahedrally coordinated metal center at the prow and the opposing carbon atom at the stern. The magnesium atom in 3 is octahedrally coordinated and out of the b-diketoiminate plane. Synthese und Struktur von b-Diketiminato Komplexen des MagnesiumsInhaltsu È bersicht. Die Reaktion des b-Diketiminato-LithiumKomplexes (dipp)NacNacLi´OEt 2 ((dipp)NacNac = 2-((2,6-Diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-pent-2-enyl) mit iPrMgCl bzw. mit MgI 2 fu È hrt zur Bildung des entsprechenden (dipp)NacNacMgiPr´OEt 2 (1) und (dipp)NacNacMgI´OEt 2 (2). Durch Umsetzung von 2 mit NaBH 4 in Diethylether wird das (dipp)NacNacMg-(l-H) 3 BH´OEt 2 (3) erhalten. Das zentrale Strukturelement der Verbindungen 1±3 ist ein sechsgliedriger Ring bestehend aus N(1)±C(1)±C(2)±C(3)±N(2) und Magnesium. Die Strukturen von 1 und 2 stellen die b-Diketiminatoverbindungen in einer Bootkonformation dar, mit dem tetraedrisch koordinierten Metallatom auf der einen und dem gegenu È berliegenden Kohlenstoffatom auf der anderen Seite. Das Magnesiumatom in 3 ist oktaedrisch koordiniert und liegt auûerhalb der Ebene, die durch den b-DiketiminatoLiganden gebildet wird.

Keywords: Acetylides / Copper / N ligands / ZincThe reaction of the lithium β-diketoiminate complex (dipp)NacNacLi·OEt 2 {(dipp)NacNac = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-enyl} with ZnCl 2 yielded the corresponding (dipp)NacNacZnCl (1) and (dipp)NacNacZn(µ-Cl) 2 Li(OEt 2 ) 2 (1a). Treatment of 1 with LiCϵCPh and NaBH 4 in diethyl ether gave (dipp)NacNacZnCϵCPh (2) and (dipp)NacNacZn(µ-H) 2 BH 2 (3). The core element of compounds 1a−3 is a six-membered ZnN 2 C 3 ring. The structure of 1a shows the β-diketoiminate backbone in a boat conformation with the tetrahedrally coordinated metal centre in 1a at the prow and the opposing carbon atom at the stern. The zinc atoms in 2 and 3 show a planar

The crystal structure of H-(L-Tyr-D-Tyr)(4)-L-Lys-OH has been determined to 1.3 A resolution. The D,L-alternating peptide crystallizes in the tetragonal system, space group P4(3)2(1)2, with unit-cell parameters a = b = 27.99 (3), c = 78.93 (8) A. The crystals contain two molecules in the asymmetric unit that form a double-stranded right-handed antiparallel beta-helix. The structure has been solved by SIRAS using a crystal soaked in an iodide-containing solution for 1 min. The programs SHELXD and SHELXE were used to determine the iodide substructure and also the experimental electron-density map. Using the coordinates of known D,L-peptides deposited in the PDB, several attempts were made to solve the structure by molecular-replacement techniques. Although the backbone of the MR model selected shows great similarity and was used to trace the actual peptide structure in the map, it was not possible to obtain the correct solution before the experimental phases became available. The correct fragment orientations are easily determined, but the same does not apply to the translation search. Nevertheless, insights into fragment search and expansion were gained from the tests described in this paper. The correlation coefficient calculated with the resolution shell of data around 2.4 A, a distance corresponding to most 1-3 interatomic vectors, is a particularly good discriminator of correct orientations in the rotation search of small fragments.