Immunoelectron microscopic studies on the localization of peptidoglycan peptide subunit pentapeptides in bacterial cell walls

The peptide subunit pentapeptide H-L-Ala-D-Glu(L-Lys-D-Ala-D-Ala-OH)-NH₂ of peptidoglycan was localized in the cell walls of several Gram-positive bacteria employing the indirect immunoferritin technique. Specific antibodies to the D-alanyl-D-alanine moiety of non-crosslinked peptide subunit pentapeptide were raised in rabbits by immunization with synthetic immunogen albumin-(CH₂CO-Gly-L-Ala-L-Ala-D-Ala-D-Ala-OH)₃₉. Specificity of these antibodies for the peptide subunit pentapeptide and not for the peptide subunit tetrapeptide was corroborated in a Farr-type radio-active hapten binding assay. Specificity of labelling with ferritin was established by immunoelectron microscopic controls, and by the excellent correlation between specific labelling of cells with ferritin and the particular peptidoglycan primary structure of bacterial strains investigated. Cells of Lactobacillus gasseri, Streptococcus pyogenes and Staphylococcus aureus revealing non-crosslinked peptide subunit pentapeptides in their peptidoglycans could specifically be labelled. Lactobacillus acidophilus and Bacillus subtilis, on the contrary, missing such pentapeptides, failed in labelling.The implication of this method to possibly localize the points of attack of penicillin or cycloserine is discussed.Abbreviations used meso-A₂pm meso-diaminopimelic acid - DSM Deutsche Sammlung für Mikroorganismen, Göttingen, FRG This paper is dedicated to Professor Gerhart Drews on the occasion of his 60th birthday     

Scrutiny of chain-length and N-terminal effects in α-helix folding: a molecular dynamics study on polyalanine peptides

Protein folding remains an unsolved problem as main-chain, side-chain, and solvent interactions remain entangled and have been hard to resolve. Polyalanines are promising models to analyze protein folding initiation and propagation structurally as well as energetically. In the present work, the effect of chain-length and N-terminal residue stereochemistry in polyalanine peptides are investigated for their role in the nucleation of α-helical conformation. The end-protected polyalanine peptides, tetra-alanine, Ac-^LAla₄-NHMe (Ia) and Ac-^DAla-^LAla₃-NHMe (Ib), hexa-alanine, Ac-^LAla₆-NHMe (IIa) and Ac-^DAla-^LAla₅-NHMe (IIb), and octa-alanine, Ac-^LAla₈-NHMe (IIIa) and Ac-^DAla-^LAla₇-NHMe (IIIb), are assessed as chain-length and stereochemical-structure perturbed models. The appreciable variations in the sampling of α-helical conformation, including a sampling of α-helix folds, due to the cooperative effect of chain-length and N-terminal residue stereochemistry have been noted. The electrostatics of α-helical conformation rather than the conformational entropy of the main-chain appear to be decisive in the initiation of α-helix folding. The results of the present work will enhance our understanding on the nucleation of α-helical conformation in short peptides and aid in the design of novel peptides with α-helical structure that can modulate disease-related protein–protein interactions.     

Dynamic behavior and selective adsorption of a methanol/water mixture inside a cyclic peptide nanotube

Present molecular dynamics simulations indicate that the methanol component in a methanol/water mixture is more likely to be trapped in a cyclic peptide nanotube (CPNT), while water molecules tend to be present at the channel mouths as transient guests. Channel water resides mainly between methanol and the CPNT wall, resulting in a distinct decrease in the H-bond number per channel methanol. Six designed CPNTs with different channel diameters and outer surface characteristics all possess distinct selectivity to methanol over water. Of these, the amphipathic 8?×?(AQ)₄-CPNT exhibits the best performance. Results in this study provide basic information for the application of a CPNT to enrich methanol from a methanol/water mixture.

Open image in new window

Graphical Abstract Typical overview of water and methanol molecular distribution in cyclic peptide nanotubes

     

On the specificity of the uridine diphospho-N-acetylmuramyl-alanyl-D-glutamic acid: Diamino acid ligase of Bifidobacterium globosum

The peptidoglycan of Bifidobacterium globosum contains ornithine and lysine alternately in the same position of the peptide subunit. The uridine diphospho-N-acetylmuramyl-alanyl-D-glutamic acid: diamino acid ligase of this organism was purified 700-fold. Since the activities for the incorporation of ornithine and lysine into uridine diphospho-N-acetylmuramyl-tripeptide did not separate during purification and since the incorporation of ornithine is competitively inhibited by lysine and vice versa, both ornithine and lysine are assumed to be incorporated by one single enzyme. Studies on the specificity of the ligase toward analogs of ornithine have shown that the enzyme requires a diamino, monocarboxylic acid with 4–6 carbon atoms. Methylation of the -amino group or hydroxylation of the -carbon atom of lysine decreases the competitive properties of the analog, whereas the substitution of the -methylen group by sulfur (S-2-aminoethyl cysteine) results in a highly competitive compound.Abbreviations BSA bovine serum albumine - MurNAc N-acetyl-muramyl - DA diamino acid - Ala-DGlu--L-DA-DAla-D-Ala pentapeptide - Ala-DGlu--LDA tripeptide - Ala-DGlu dipeptide - DSM Deutsche Sammlung von Mikroorganismen - CEM clostridial enrichment medium     

Chemical composition of Eubacterium nodatum cell wall peptidoglycan

The structure of Eubacterium nodatum cell wall peptidoglycan was investigated. The peptide subunit of E. nodatum peptidoglycan has the following structure: L-Ala-D-Glu (Gly)-L-Orn-D-Ala. The carboxyl group of alanine occupying position 4 is attached to the -amino group of ornithine of an other subunit by the cross-linking bridge L-Ala-L-Ala-L-Orn. All glycine molecules are connected with the -carboxyl group of glutamic acid with the ratio being 0.5–1. The hydrolysis of E. nodatum peptidoglycan by the S. albus G enzyme proceeds primarily due to the activity of alanyl-alanine endopeptidase, ornithyl-ornithine endopeptidase, ornithyl-alanine endopeptidase, N-acetyl-muramyl-alanine amidase, N-acetylmuramidase and N-acetylglucosaminidase.     

Die Aminosäuresequenz des Mureins von Staphylococcus epidermidis (Winslow and Winslow) Evans,Stamm 66

Zusammenfassung Das Murein eines aus Milch isolierten Stammes von Staphylococcus epidermidis weist folgende Molverhältnisse auf (auf- bzw. abgerundete Zahlen): Mur:GlcNH₂:Ala:Glu:Lys:Gly=1:1:3:1:1:4. Das Verhältnis D-Ala:L-Ala ist 1:2,03. Die Glutaminsäure liegt in der D-Konfiguration und als Amid vor.Durch die Isolierung und Identifizierung der Peptide des Partialhydrolysats des Mureins konnte die Aminosäuresequenz erschlossen werden. Die Sequenz des an die Muraminsäure gebundenen Tetrapeptides (L-Ala-D-GluNH₂-L-Lys-D-Ala) stimmt mit dem der meisten anderen Bakterien überein. Die Quervernetzung wird durch das Peptid (Gly)_4–5-L-Ala hergestellt, das mit dem N-terminalen Glycin an die Carboxylgruppe des D-Alanins und mit dem C-terminalen L-Alanin an die -Aminogruppe des Lysins zweier benachbarter Tetrapeptide gebunden ist. Die Dinitrophenylierung des Mureins ergab, daß 2% des Lysins (-Aminogruppe), 3% des gesamten Alanins und 7% des gesamten Glycins N-terminal vorliegen. Demnach ist die Quervernetzung nur zu rund 60% realisiert. Neben unvernetzten mehr oder weinger vollständigen Interpeptidbrücken kommen auch unvollständige Peptide vor, bei denen nur L-Alanin an die -Aminogruppe des Lysins gebunden ist. In mindestens 2% der Fälle fehlt die Interpeptidkette völlig.

---

The amino acid sequence of the murein of Staphylococcus epidermidis (winslow and winslow) evans, strain 66

Summary A strain of Staphylococcus epidermidis was isolated from raw milk. Its murein contained muramic acid, glucosamine, alanine, D-glutamic acid, L-lysine and glucine at a molar ratio of about 1:1:3:1:1:4. The ratio D-Ala: L-Ala is 1:2.03. D-glutamic acid is present as an amide.By partial acid hydrolysis of the cell wall and subsequent isolation and identification of the peptides the amino acid sequence of the murein was elucidated. The tetrapeptide, bound to muramic acid is identical with that of most bacteria: L-Ala-D-GluNH₂-L-Lys-D-Ala. The crosslinking of the murein is performed by the peptide (Gly)_4–5-L-Ala. L-Ala is attached to the -aminogroup of lysine, while the N-terminal glycine is bound to the C-terminal D-alanine of an adjacent tetrapeptide. About 2% of lysine, 3% of alanine and 7% of glycine of the murein are dinitrophenylizable, indicating that about 2% of the tetrapeptides are not substituted by an interpeptide chain, and that 40% of the interpeptide chains are more or less incomplete (10% consist of L-alanine only) and are not bound to a C-terminal D-alanine.

     

Synthesis and biological activity of aryl S-β-glycosides of 1-thio-N-acetylmuramyl-L-alanyl-D-isoglutamine

Phenyl, p-tolyl, and p-tert-butylphenyl β-1-thio-N-acetylglucosaminides were synthesized by the treatment of thiophenols with peracetate of α-D-glucosaminyl chloride in the presence of triethylamine or under the conditions of phase-transfer catalysis with quaternary ammonium salts. The compounds synthesized were used for obtaining of glycosides of 4,6-O-isopropylidene-N-acetylmuramic acid, which were coupled with L-Ala-D-Glu(NH₂)-OBzl and then deprotected to obtain the target aryl β-thioglycosides of N-acetylmuramyl-L-analyl-D-isoglutamine (MDP). The aryl β-thioglycosides of MDP were found to stimulate an antibacterial resistance toward Staphylococcus aureus in mice. The reliable induction of the spontaneous activity of natural killers in the population of blood mononuclear cells was observed only for phenyl β-thio-MDP at a dose of 200 μg/ml. Original Russian Text ? A.E. Zemlyakov, V.N. Tsikalova, L.R. Azizova, V.Ya. Chirva, E.L. Mulik, M.V. Shkalev, O.V. Kalyuzhin, M.V. Kiselevsky, 2008, published in Bioorganicheskaya Khimiya, 2008, Vol. 34, No. 2, pp. 245–251.     

Structure of a self-assembled single nanotube of cyclo[(-d-Ala-l-Ala)4-]

Diameter and wall thickness of self-assembled peptide nanotube of cyclo[(-d-Ala-l-Ala)₄-] were characterised by molecular simulation. In order to verify the existence of peptide nanotube of cyclo[(-d-Ala-l-Ala)₄-], cyclo[(-d-Ala-l-Ala)₄-] was firstly synthesised through Fmoc solid-phase synthesis method and then self-assembled in trifluoroacetic acid. Based on the results of experiment, the single nanotube structure was further characterised by molecular dynamics (MD) employing the COMPASS force field. The results indicate that cyclo[(-d-Ala-l-Ala)₄-] is self-assembled into nanotube bundles of about 0.5 μm in diameter and 10 μm in length; the inner and outer diameter of the single nanotube is 8.5 and 15.9 Å, respectively, and the nanotube wall thickness is 3.3 Å.     

Water-soluble titanocene complexes with sulfur-containing aminoacids: synthesis, spectroscopic, electrochemical and Ti(IV)–transferrin interaction studies

Three new water soluble titanocene–aminoacid complexes have been synthesized via the reaction of Cp₂TiCl₂ and two equivalents of aminoacid (L) in methanol, affording [Cp₂TiL₂]Cl₂, L=L-cysteine (2), D-penicillamine (3) and L-methionine (4). These complexes have been characterized by ¹H, IR and UV-Vis spectroscopies, elemental analysis and cyclic voltammetry. Kinetic studies of ligand hydrolysis have been monitored at low pH using UV-Vis and ¹H NMR spectroscopies to assess their stability in aqueous solution. At low pH, aminoacid ligands are lost one order of magnitude faster than cyclopentadienyl. However, at physiological pH, in Tris buffer solution, the complexes decompose rapidly to form an insoluble titanium compound. The affinity of these complexes to apo-transferrin was also investigated to elucidate how the ancillary aminoacid ligands affect the titanium intake by apo-transferrin.Electronic Supplementary Material Supplementary material is available for this article at     

Synthesis and Protective Activity of β-Glycosides of <Emphasis Type="Italic">N</Emphasis>-Acetylmuramyl-<Emphasis Type="Italic">L</Emphasis>-Alanyl-<Emphasis Type="Italic">D</Emphasis>-Isoglutamine with Alkylalicyclic and Arylaliphatic Aglycons

The following glycosides of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) were synthesized: β-4-tert-butylcyclohexyl MDP, β-2-(adamant-1-yl)ethyl MDP, β-2,2-diphenylethyl MDP, and β-2-(p-biphenyl) ethyl MDP. The starting peracetylated β-N-acetylglucosaminides were prepared by the oxazoline method. They were converted into 4,6-O-isopropylidene-N-acetyl-D-muramic acids, which were coupled with L-Ala-D-Glu(NH₂)OBn. The target glycopeptides were obtained after their deprotection. The stimulation of the anti-infection resistance of mice against Staphylococcus aureus by the MDP glycosides was studied.     

Preparative Separation of Enantiomers Based on Functional Nucleic Acids Modified Gold Nanoparticles

The preparative‐scale separation of chiral compounds is vitally important for the pharmaceutical industry and related fields. Herein we report a simple approach for rapid preparative separation of enantiomers using functional nucleic acids modified gold nanoparticles (AuNPs). The separation of _DL‐tryptophan (_DL‐Trp) is demonstrated as an example to show the feasibility of the approach. AuNPs modified with enantioselective aptamers were added into a racemic mixture of _DL‐Trp. The aptamer‐specific enantiomer (_L‐Trp) binds to the AuNPs surface through aptamer‐_L‐Trp interaction. The separation of _DL‐Trp is then simply accomplished by centrifugation: the precipitate containing _L‐Trp bounded AuNPs is separated from the solution, while the _D‐Trp remains in the supernatant. The precipitate is then redispersed in water. The aptamer is denatured under 95 °C and a second centrifugation is then performed, resulting in the separation of AuNPs and _L‐Trp. The supernatant is finally collected to obtain pure _L‐Trp in water. The results show that the racemic mixture of _DL‐Trp is completely separated into _D‐Trp and _L‐Trp, respectively, after 5 rounds of repeated addition of fresh aptamer‐modified AuNPs to the _DL‐Trp mixture solution. Additionally, the aptamer‐modified AuNPs can be repeatedly used for at least eight times without significant loss of its binding ability because the aptamer can be easily denatured and renatured in relatively mild conditions. The proposed approach could be scaled up and extended to the separation of other enantiomers by the adoption of other enantioselective aptamers. Chirality 25:751–756, 2013. © 2013 Wiley Periodicals, Inc.     

New derivatives of eremomycin containing 15N or F atoms for NMR study

New semisynthetic derivatives of eremomycin containing ¹⁵N or F atoms were obtained for studying the antibiotic-target interaction in intact cells of Gram-positive bacteria by REDOR NMR method. Interaction of the terminal carboxyl group of amino acid 7 (AA7) of eremomycin with amines in the presence of PyBOP and TBTU reagents resulted in the corresponding [¹⁵N]-amide, p-fluorobenzylamide, p-fluorophenylpiperazide, and 6-N-(p-fluorobenzyl)aminohexylamide. A selective method of [¹⁵N]-amidation of carboxyl group of amino acid 3 (AA3) of carboxyeremomycin was developed, and the amide of eremomycin containing [¹⁵N] in AA3 amide group near the antibiotic binding pocket was obtained. Carboxyeremomycin bisamides substituted at AA3 and AA7 and containing two atoms of [¹⁵N] or F were obtained from carboxyeremomycin and [¹⁵N]NH₄Cl or the corresponding p-fluorobenzylamine hydrochloride in the presence of PyBOP at pH ～8. The Edman degradation of eremomycin p-fluorobenzylamide gave de-(D-MeLeu)-eremomycin p-fluorobenzylamide, a hexapeptide derivative incapable of the antibiotic binding with-D-Ala-D-Ala fragment of growing cell wall peptidoglycan. Among the compounds studied, carboxyeremomycin bis-p-fluorobenzylamide showed the best activity against both the glycopeptides-sensitive and glycopeptides-resistant strains of staphylococci and enterococci.     

Influence of the route of administration on the pharmacokinetics of pirprofen enantiomers in the rat

The pharmacokinetics of the enantiomers of the non-steroidal anti-inflammatory drug pirprofen were studied in male Sprague-Dawley rats after oral and intravenous (iv) doses of the racemate. No significant differences were detected between the enantiomers after oral or iv dosing in t_½, Vd, or ∑Xu. However, the R:S area under the plasma concentration (AUC) ratio after oral doses (0.92 ± 0.13) was slightly but significantly lower than after matching iv doses (1.05 ± 0.036). The absolute bioavailability of the active S-enantiomer (78.5%) after oral doses was higher than the inactive R-enantiomer (69.3%). The plasma protein binding of both enantiomers was saturable over a fivefold range of plasma concentrations. At higher plasma concentrations, the S-enantiomer was less bound than the R-enantiomer. In an in vitro experiment using everted rat jejunum, no chiral inversion was discernible. The dependency of the AUC ratio of the enantiomers on the route of administration may be due to stereoselective first-pass metabolism. © 1993 Wiley-Liss, Inc.     

Coordinative versatility of 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline (L) to different metal ions: syntheses, crystal structures and properties of [Cu(I)L]2 and [ML] (M=Cu(II), Ni(II), Zn(II) and Co(II))

The complexes of Cu(I), Cu(II), Ni(II), Zn(II) and Co(II) with a new polypyridyl ligand, 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline (L), have been synthesized and characterized. The crystal structures of these complexes have been elucidated by X-ray diffraction analyses and three types of coordination modes for L were found to exist in them. In the dinuclear complex [Cu(I)L(CH₃CN)]₂·(ClO₄)₂ (1), L acts as a tridentate ligand with two Cu(I) centers bridged by two L ligands to form a box-like dimeric structure, in which each Cu(I) ion is penta-coordinated with three nitrogen atoms and a methoxyl oxygen atom of two L ligands, and an acetonitrile. In [Cu(II)L(NO₃)₂]·CH₃CN 2, the Cu(II) center is coordinated to the two nitrogen atoms of the two pyridine rings of L which acts as a bidentate ligand. The structures of [Ni(II)L(NO₃)(H₂O)₂]·2CH₃CN·NO₃ (3), [Zn(II)L(NO₃)₂ (H₂O)]·2CH₃CN (4) and [Co(II)LCl₂(H₂O)] (5) are similar to each other in which L acts as a tridentate ligand by using its half side, and the metal centers are coordinated to a methoxyl oxygen atom and two bipyridine nitrogen atoms of L in the same side. The formation of infinite quasi-one-dimensional chains (1, 4 and 5) or a quasi-two-dimensional sheet (2) assisted by the intra- or intermolecular face-to-face aryl stacking interactions and hydrogen bonds may have stabilized the crystals of these complexes. Luminescence studies showed that 1 exhibits broad, structureless emissions at 420 nm in the solid state and at 450 nm in frozen alcohol frozen glasses at 77 K. Cyclic voltammetric studies of 1 show the presence of an irreversible metal-centered reduction wave at approximately −0.973 V versus Fc^+/0 and a quasi-reversible ligand-centered reduction couple at approximately −1.996 V versus Fc^+/0. The solution behaviors of these complexes have been further studied by UV-Vis and ESR techniques.     

Distinctive and Synergistic Signaling of Human Adenosine A2a and Dopamine D2L Receptors in CHO Cells

Adenosine A_2a receptor (A_2aR) colocalizes with dopamine D₂ receptor (D₂R) in the basal ganglia and modulates D₂R-mediated dopaminergic activities. A_2aR and D₂R couple to stimulatory and inhibitory G proteins, respectively. Their opposing roles in regulating neuronal activities, such as locomotion and alcohol consumption, are mediated by their opposite actions on adenylate cyclase, which often serves as “co-incidence detector” of various activators. On the other hand, the neural actions of A_2aR and D₂R are also, at least partially, independent of each other, as indicated by studies using D₂R and A_2aR knock-out mice. Here we co-expressed human A_2aR and human D_2LR in CHO cells and examined their signaling characteristics. Human A_2aR desensitized rapidly upon agonist stimulation. A_2aR activity (80%) was diminished after 2 hr of pretreatment with its agonist CGS21680. In contrast, human D_2LR activity was sustained even after 2 hr and 18 hr pretreatment with its agonist quinpirole. Long-term (18 hr) stimulation of human D_2LR also increased basal cAMP levels in CHO cells, whereas long-term (18 hr) activation of human A_2aR did not affect basal cAMP levels. Furthermore, long-term (18 hr) activation of D_2LR dramatically sensitized A_2aR-induced stimulation of adenylate cyclase in a pertussis toxin-sensitive way. Forskolin-induced cAMP accumulation was significantly increased after short-term (2 hr) human D_2LR stimulation and further elevated after long-term (18 hr) D_2LR activation. However, neither short-term (2 hr) nor long-term (18 hr) stimulation of A_2aR affected the inhibitory effects of D_2LR on adenylate cyclase. Co-stimulation of A_2aR and D_2LR could not induce desensitization or sensitization of D_2LR either. In summary, signaling t hrough A_2aR and D_2LR is distinctive and synergistic, supporting their unique and yet integrative roles in regulating neuronal functions when both receptors are present.     

千烟洲针叶林的比叶面积及叶面积指数

根据实测数据计算了湿地松(Pinus elliotii)、马尾松(P. massoniana) 和杉木(Cunninghamia lanceolata)不同年龄、不同类型叶片的生物量和比叶面积,并结合样地调查数据和相对生长方程计算了中国科学院千烟洲试验站20年生湿地松林、马尾松林、杉木林和针叶混交林的叶面积指数。根据拟合结果,选择如下方程计算3个树种的叶生物量:湿地松W=12.074 1D^{2.151 5}、马尾松W=6.972 7D^{2.197 3}和杉木W=5.261 9D^{2.302 7}。湿地松林的叶生物量(0.822 kg·m^-2)最大,其次为针叶混交林(0.679 kg·m^-2),马尾松林和杉木林相差不大(分别为林0.528和0.572 kg·m^-2)。不同树种、不同年龄、不同类型叶片的比叶面积比较发现,新叶的比叶面积大于老叶,三针一束叶的比叶面积略大于两针一束叶,马尾松的平均半比表面积(8.62 m²·kg^-1)大于湿地松(6.04 m²·kg^-1)和杉木(7.91 m²·kg^-1)。胸径与单木叶片半表面积之间的经验方程为:湿地松LA=0.073D^{2.151 5}、马尾松LA=0.060D^{2.197 3}和杉木LA=0.042D^{2.302 7}。据此计算湿地松林的叶面积指数为5.03,马尾松林和杉木林为4.31,针叶混交林为4.77,该结果比利用CI-110植被冠层数字图像仪测得的结果偏大。     

Kinetics of Thermostable Alanine Racemase of Bacillus stearothermophilus

Unlabeled D- and L-alanine were racemized in deuterium oxide with an alanine racemase of Bacillus stearothermophilus at saturated concentration of substrate, and various p²H and temperature. Samples of the solution were taken at intervals, and all alanine isomers in the samples were transformed into a mixture of diastereomeric derivatives of methyl N-(–)-camphanylalaninate. Their ratio was measured on a GC-Mass, and the relative rate was calculated at the initial stage of the reaction. There was little difference in the decrease rate of the optical rotation between the enantiomers. Internal proton-transfer to the antipode was almost zero for either substrate. The α-hydrogen was abstracted 1.2–2.3 times faster from D-alanine than from L-alanine. D-Alanine gave an almost even mixture of deuterium labeled D- and L-alanine, while L-alanine gave a mixture of labeled D- and L-alanine at a ratio of 3:1. These results suggest the racemase builds two different bases in the active site. The base for D-alanine may be closer to the enzyme surface, and that for L-alanine inside.     

Synthesis, structures and properties of macrocyclic nickel(II) supramolecules with imidazole pendants

Two new nickel(II) complexes with the composition [Ni(L+H)(CH₃CN)₂](ClO₄)₃ (1) and [Ni(L)(tp)]·6H₂O (2), (L = 3,10-bis{3-(1-imidazolyl)propyl}-1,3,5,8,10,12-hexaazacyclotetradecane, tp = terephthalate) have been synthesized and structurally characterized by a combination of analytical, spectroscopic and X-ray diffraction methods. The structure of 1 consists of monomeric cations of the formula [Ni(L+H)(CH₃CN)₂]³⁺ and perchlorate ions. The nickel(II) ion is six-coordinate with bonds to the four nitrogen atoms of the macrocycle and two nitrogen atoms of the axial acetonitrile ligands. One of the protonated imidazole pendants of the macrocycle is hydrogen bonded to the imidazole group of the neighboring nickel(II) macrocycle, forming an undulated 1D supramolecule. Then, the two 1D supramolecular chains are further interconnected by C-H···π interactions between the methyl group of the acetonitrile ligand and one of the imidazole groups to form a 2D double stranded supramolecular polymer. In the structure of 2, the 1D coordination polymer is formed with nickel(II) macrocycles and bridging terephthalate ions, where each 1D chain is interconnected with π-π interactions of pendant imidazole moieties of the macrocycles, resulting in the formation of a 2D supramolecule.     

A proposal for a convenient notation for P-chiral nucleotide analogues. Part 3. Compounds with one nucleoside residue and nonnucleosidic derivatives

Recently, we have proposed a new D_P/L_P stereochemical notation for P-chiral dinucleoside monophosphate analogues that permits simple correlation between spatial arrangement of the substituents and the configuration at the phosphorus center. As an extension of this work, we present here applications of the D_P/L_P notation to derivatives containing only one nucleoside unit (e.g., alkyl nucleoside phosphodiesters, nucleoside phosphomonoesters, cyclic phosphate derivatives, nucleoside di-, and triphosphates) and to nonnucleosidic phosphorus compounds.     

Influence of trifluoroethanol on membrane interfacial anchoring interactions of transmembrane alpha-helical peptides

Interfacial anchoring interactions between aromatic amino acid residues and the lipid-water interface are believed to be important determinants for membrane protein structure and function. Thus, it is possible that molecules that partition into the lipid-water interface can influence membrane protein activity simply by interfering with these anchoring interactions. Here we tested this hypothesis by investigating the effects of 2,2,2-trifluoroethanol (TFE) on the interaction of a Trp-flanked synthetic transmembrane peptide (acetyl-GW₂(LA)₈LW₂A-NH₂) with model membranes of dimyristoylphosphatidylcholine. Two striking observations were made. First, using ²H nuclear magnetic resonance on acyl chain deuterated lipids, we found that addition of 4 or 8 vol % of TFE completely abolishes the ability of the peptide to order and stretch the lipid acyl chains in these relatively thin bilayers. Second, we observed that addition of 8 vol % TFE reduces the tilt angle of the peptide from 5.3° to 2.5°, as measured by ²H NMR on Ala-d₄ labeled peptides. The “straightening” of the peptide was accompanied by an increased exposure of Trp to the aqueous phase, as shown by Trp-fluorescence quenching experiments using acrylamide. The observation of a reduced tilt angle was surprising because we also found that TFE partioning results in a significant thinning of the membrane, which would increase the extent of hydrophobic mismatch. In contrast to the Trp-flanked peptide, no effect of TFE was observed on the interaction of a Lys-flanked analog (acetyl-GK₂(LA)₈LK₂A-NH₂) with the lipid bilayer. These results emphasize the importance of interfacial anchoring interactions for membrane organization and provide new insights into how molecules such as TFE that can act as anesthetics may affect the behavior of membrane proteins that are enriched in aromatic amino acids at the lipid-water interface.