β‐Peptidic Secondary Structures Fortified and Enforced by Zn2+ Complexation – On the Way to β‐Peptidic Zinc Fingers?

The correlation between β²‐, β³‐, and β^2,3‐amino acid‐residue configuration and stability of helix and hairpin‐turn secondary structures of peptides consisting of homologated proteinogenic amino acids is analyzed (Figs. 1–3). To test the power of Zn²⁺ ions in fortifying and/or enforcing secondary structures of β‐peptides, a β‐decapeptide, 1 , four β‐octapeptides, 2 – 5 , and a β‐hexadecapeptide, 10 , have been devised and synthesized. The design was such that the peptides would a) fold to a 14‐helix ( 1 and 3 ) or a hairpin turn ( 2 and 4 ), or form neither of these two secondary structures (i.e., 5 ), and b) carry the side chains of cysteine and histidine in positions, which will allow Zn²⁺ ions to use their extraordinary affinity for RS^? and the imidazole N‐atoms for stabilizing or destabilizing the intrinsic secondary structures of the peptides. The β‐hexadecapeptide 10 was designed to a) fold to a turn, to which a 14‐helical structure is attached through a β‐dipeptide spacer, and b) contain two cysteine and two histidine side chains for Zn complexation, in order to possibly mimic a Zn‐finger motif. While CD spectra (Figs. 6–8 and 17) and ESI mass spectra (Figs. 9 and 18) are compatible with the expected effects of Zn²⁺ ions in all cases, it was shown by detailed NMR analyses of three of the peptides, i.e., 2, 3, 5 , in the absence and presence of ZnCl₂, that i) β‐peptide 2 forms a hairpin turn in H₂O, even without Zn complexation to the terminal β³hHis and β³hCys side chains (Fig. 11), ii) β‐peptide 3 , which is present as a 14‐helix in MeOH, is forced to a hairpin‐turn structure by Zn complexation in H₂O (Fig. 12), and iii) β‐peptide 5 is poorly ordered in CD₃OH (Fig. 13) and in H₂O (Fig. 14), with far‐remote β³hCys and β³hHis residues, and has a distorted turn structure in the presence of Zn²⁺ ions in H₂O, with proximate terminal Cys and His side chains (Fig. 15).     

On the lattice stability of metals

The success of perturbation calculations of second order for the NFE (“Nearly Free Electron”) metals and that of the two-parameter model of Pettifor for the transition elements show that the lattice-stability of the metals has simple physical reasons. Using the results of Harrison, Heine and Weaire, Deegan, and Pettifor, a model is developed which allows to explain the stability of the three metal lattices in terms of differences in the potentials. Only those potential differences are considered which are caused by the different packing of the lattices. With the aid of the virial theorem the band structure energy is connected with the potential bandstructure energy. The sequence of stability is predicted to be body centered cubic (bcc), hexagonal close packed (hcp), face centered cubic (fcc) with increasing valence electron concentration. The ranges of stability can be expressed in simple numbers. This simple model holds in principle for NFE as well as for transition metals because it contains no assumptions restricted to only one of these metal types. Deviations of the observed lattice stability from the model can be understood from the approximations involved.     

Struktur von S-9,10-Dimethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantan und 9,10-Diethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantan

Structure of S-9,10-Dimethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantane and 9,10-Diethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantane S-9,10-Dimethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantane ( 1 ) and 9,10-diethyl-1,3,5,7-tetraarsa-2,4,6,8-tetraoxaadamantane ( 2 ) have been prepared by the reaction of propionic acid, propionic anhydride and butyric acid, butyric anhydride, respectively, with arsenic(III)-oxide. The crystals of 1 are rhombic, a = 6.902(4), b = 11.121(5), c = 13.988(8), space group P2₁2₁2₁. The crystals of 2 are monoclinic, a = 11.757(10), b = 11.255(10), c = 18.631 (18), β = 91.78(7), space group P2₁/n. The mean bond lengths and angles in 1 are AsO = 1.790 Å, AsC = 1.959 Å, OAsO = 100.60°, CAsO = 99.65°, AsOAs = 128.77°, AsCAs = 118.73°, and in 2 they are AsO = 1.780 Å, AsC = 1.978 Å, OAsO = 101.45°, CAsO = 99.55°, AsOAs = 129.64°, AsCAs = 117.72°.     

Delayed fluorescence S2 → S0 of azulene in isopentane,due to hetero-triplet—triplet annihilation of 3azulene* and 3fluoranthene*

The lowest triplet state of azulene, T₁(Az), can be populated efficiently by triplet energy transfer from the lowest triplet state of fluoranthene, T₁(F1). In isopentane at temperatures 120 K ? T ? 193 K a delayed fluorescence S₂(Az) → S₀(Az) is found, caused by hetero-triplet—triplet annihilation T₁(Az) + T₁(Fl) → S₂(Az) + S₀(F1).     

Der Stoffwechsel von Verbindungen mit Dreifachbindung. VI. Stoffwechselverhalten von Alkincarbonsäuren und Alkindicarbonsäuren

Metabolism of Acetylenic Monocarboxylic and Dicarboxylic Acids Feeding of acetylenic monoacids with chain length of 11 to 18 C-atoms to rats led to excretion of dicarboxylic acids with retained triple bonds. 10-Octadecynoic acid led to the formation of metabolites with even and odd number of C-atoms, suggesting in addition to established ω- and β-oxidation an α-oxidative pathway.     

An extended ab initio QM/MM MD approach to structure and dynamics of Zn(II) in aqueous solution

Structural and dynamical properties of Zn(II) in aqueous solution were investigated, based on an ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation at double-zeta Hartree-Fock quantum mechanical level including the first and second hydration shells into the QM region. The inclusion of the second shell in the QM region resulted in significant changes in the properties of the hydrate. The first shell coordination number was found to be 6, the second shell consists of approximately 14 water molecules. The structural properties were determined in terms of RDF, ADF, tilt and theta angle distributions, while dynamics were characterized by mean ligand residence times, ion-ligand stretching frequencies and the vibrational and librational motions of water ligands.     

Determination of aluminum in highly concentrated iron samples: Study of interferences using high-resolution continuum source atomic absorption spectrometry

High-resolution continuum source atomic absorption spectrometry (HR-CS AAS) has been used to investigate spectral and non-spectral interferences found with a conventional line source atomic absorption spectrometer in the determination of aluminum in pharmaceutical products containing elevated iron and sugar concentrations. A transversely heated graphite furnace was used as the atomizer in both spectrometers. The two most sensitive aluminum lines at 309.3 nm and 396.2 nm were investigated and it was found that an iron absorption line at 309.278 nm, in the vicinity of the aluminum line at 309.271 nm, could be responsible for some spectral interference. The simultaneous presence of iron and the organic components of the matrix were responsible for radiation scattering, causing high continuous and also structured background absorption at both wavelengths. The aluminum and iron absorption could not be separated in time, i.e., the iron interference could not be eliminated by optimizing the graphite furnace temperature program. However, an interference-free determination of aluminum was possible carrying out the measurements with HR-CS AAS at 396.152 nm after applying least squares background correction for the elimination of the structured background. Analytical working range and other figures of merit were determined and are presented for both wavelengths using peak volume registration (center pixel ± 1) and the center pixel only. Limits of detection and characteristic masses ranged from 1.1 to 2.5 pg and 13 to 43 pg, respectively. The method was used for the determination of the aluminum contamination in pharmaceutical formulations for iron deficiency treatment, which present iron concentrations from 10 to 50 g l^− 1. Spike recoveries from 89% to 105% show that the proposed method can be satisfactorily used for the quality control of these formulations.     

On the additivity of basis set effects in some simple fluorine containing systems

The reactions F + H₂ → HF + H, HF → H + F, F → F⁺ + e^? and F + e^? → F^? were used as simple test cases to assess the additivity of basis set effects on reaction energetics computed at the MP4 level. The 6-31G and 6-311G basis sets were augmented with 1, 2, and 3 sets of polarization functions, higher angular momentum polarization functions, and diffuse functions (27 basis sets from 6-31Gd, p) to 6-31 ++ G(3df, 3pd) and likewise for the 6-311G series). For both series substantial nonadditivity was found between diffuse functions on the heavy atom and multiple polarization functions (e.g., 6-31 + G(3d, 3p) vs. 6-31 + G(d, p) and 6-31G(3d, 3p)). For the 6-311G series there is an extra nonadditivity between d functions on hydrogen and multiple polarization functions. Provided that these interactions are taken into account, the remaining basis set effects are additive to within ±0.5 kcal/mol for the reactions considered. Large basis set MP4 calculations can also be estimated to within ±0.5 kcal/mol using MP2 calculations, est. E_MP4(6-31 ++ G(3df, 3pd)) ≈ E_MP4(6-31G(d, p)) + E_MP2(6-31 ++ G(3df, 3pd)) – E_MP2(6-31G(d, p)) or E_MP4(6-31 + G(d, p) + E_MP2(6-31 ++ G(3df, 3pd)) – E_MP2(6-31 + G(d, p)) and likewise for the 6-311G series.     

Surprising Homolytic Gas Phase Co−C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B₁₂ designed as potential ‘antivitamins B₁₂’. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co−C )-BDEs of 38.4 and 40.6 kcal mol⁻¹, respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol⁻¹ for the corresponding α-isomers. Surprisingly, the observed (Co−C )-BDEs of the Co_β-aryl-cobinamides were smaller than the (Co−C )-BDE of Co_β-methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co−C )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.