Vergleich von anionenanalysenergebnissen aus der bestimmung mittels ionenchromatographie und klassischen analysenmethoden

(Comparison of analytical results for common anions obtained by ion chromatography and classical methods.)The concentrations of chloride, nitrate, and sulphate were determined in ground-water samples by ion chromatography and by the classical titrimetric, photometric, and gravimetric methods. The results were compared by an orthogonal regression procedure. The criteria established and the tests of the confidence intervals of the orthogonal regression function indicate that the results obtained by the different analytical methods are similar for chloride and sulphate but not for nitrate.     

Pentopyranosyl Oligonucleotide Systems. Communication No. 13

Among the members of a family of diastereoisomeric pentopyranosyl‐(4′→2′)‐oligonucleotide systems derived from D ‐ribose, D ‐xylose, L ‐lyxose, and L ‐arabinose, the α‐arabinopyranosyl system shows by far the strongest Watson? Crick base pairing. The system is, in fact, one of the strongest oligonucleotide‐type base‐pairing systems known. It undergoes efficient cross‐pairing with all the other members of the pentopyranosyl family, but not with RNA and DNA. The paper describes the synthesis and pairing of the properties of α‐L ‐arabinopyranosyl‐(4′→2′)‐oligonucleotides.     

Pyranosyl-RNA (‘p-RNA’): NMR and Molecular-Dynamics Study of the Duplex Formed by Self-pairing of Ribopyranosyl-(C-G-A-A-T-T-C-G)

The solution structure of the duplex formed by self-pairing of the p-RNA octamer β-D -ribopyranosyl-(2′→4′)-(CGAATTCG) was studied by NMR techniques and, independently, by molecular-dynamics calculations. The resonances of all non-exchanging protons, H-bearing C-atoms, P-atoms, and of most NH protons were assigned. Dihedral angle and distance constraints derived from coupling constants and NOESY spectra are consistent with a single dominant conformer and corroborate the main structural features predicted by qualitative conformational analysis. The duplex displays Watson-Crick pairing with antiparallel strand orientation. The dihedral angles β and ? in the phosphodiester linkages differ considerably from the idealized values. Model considerations indicate that these deviations from the idealized model allow better interstrand stacking and lessen unfavorable interactions in the backbone. The average base-pair axis forms an angle of ca. 40° with the backbone. The resulting interstrand π-π stacking between either two purines, or a purine and a pyrimidine, but not between two pyrimidines, constitutes a characteristic structural feature of the p-RNA duplex. A 1000-ps molecular-dynamics (MD) calculation with the AMBER force field resulted in an average structure of the same conformation type as derived by NMR. For the backbone torsion angle ?, dynamically averaged coupling constants from the MD calculation agree well with the experimental values, but for the angle β, a systematic difference of ca. 25° remains. The two base pairs at the ends of the duplex are calculated to be highly labile, which is consistent with the high exchange rate of the corresponding imino protons found by NMR.     

Coenzyme F430 from Methanogenic Bacteria: Complete Assignment of Configuration Based on an X-Ray Analysis of 12,13-Diepi-F430 Pentamethyl Ester and on NMR Spectroscopy

The structure of a derivative of coenzyme F430 from methanogenic bacteria, the bromide salt of 12,13-diepi-F430 pentamethyl ester ( 5 , X = Br), was determined by X-ray structure analysis. It reveals a more pronounced saddle-shaped out-of-plane deformation of the macrocycle than any hydroporphinoid Ni complex investigated so far. The crystal structure confirms the constitution proposed for coenzyme F430 ( 2 ) and shows that in the epimer 5 , the three stereogenic centers in ring D, C(17), C(18), and C(19), have the (17S)-, (18S)-, and (19R)-configuration, respectively. Deuteration and 2D-NMR studies independently demonstrate that native coenzyme F430 (2) has the same configuration in ring D as the epimer 5 . Therefore, our original tentative assignment of configuration at C(19) and C(18) [1] has to be reversed. This completes the assignment of configuration for all stereogenic centers in coenzyme F430, which has the structure shown in Formula 2 .     

On the influence of charged side chains on the folding-unfolding equilibrium of beta-peptides: a molecular dynamics simulation study

The influence of charged side chains on the folding-unfolding equilibrium of beta-peptides was investigated by means of molecular dynamics simulations. Four different peptides containing only negatively charged side chains, positively charged side chains, both types of charged side chains (with the ability to form stabilizing salt bridges) or no charged side chains were studied under various conditions (different simulation temperatures, starting structures and solvent environment). The NMR solution structure in methanol of one of the peptides (A) has already been published; the synthesis and NMR analysis of another peptide (B) is described here. The other peptides (C and D) studied herein have hitherto not been synthesized. All four peptides A-D are expected to adopt a left-handed 3(14)-helix in solution as well as in the simulations. The resulting ensembles of structures were analyzed in terms of conformational space sampled by the peptides, folding behavior, structural properties such as hydrogen bonding, side chain-side chain and side chain-backbone interactions and in terms of the level of agreement with the NMR data available for two of the peptides. It was found that the presence of charged side chains significantly slows down the folding process in methanol solution due to the stabilization of intermediate conformers with side chain-backbone interactions. In water, where the solvent competes with the solute-solute polar interactions, the folding process to the 3(14)-helix is faster in the simulations.     

NMR‐Solution Structures of Fluoro‐Substituted β‐Peptides: A 314‐Helix and a Hairpin Turn. The First Case of a 90° OC?C?F Dihedral Angle in an α‐Fluoro‐Amide Group

To further study the preference of the antiperiplanar (ap) conformation in α‐fluoro‐amide groups, two β‐peptides, 1 and 2 , containing a (2‐F)‐β³hAla and a (2‐F)‐β²hPhe residue, have been synthesized. Their NMR‐solution structures in CD₃OH were determined and compared with those of non‐F‐substituted analogs, 3 and 4a . While we have found in a previous investigation (Helv. Chim. Acta 2005 , 88, 266) that a stereospecifically introduced F‐substituent in the central position of a β‐heptapeptide is capable of ‘breaking’ the 3₁₄‐helical structure by enforcing the F? C? C?O ap‐conformation, we could now demonstrate that the same procedure leads to a structure with the unfavorable ca. 90° F? C? C?O dihedral angle, enforced by the 3₁₄‐helical folding in a β‐tridecapeptide (cf. 1 ; Fig. 4). This is interpreted as a consequence of cooperative folding in the longer β‐peptide. A F‐substituent placed in the turn section of a β‐peptidic hairpin turn was shown to be in an ap‐arrangement with respect to the neighboring C?O bond (cf. 2 ; Fig. 7). Analysis of the non‐F‐substituted β‐tetrapeptides (with helix‐preventing configurations of the two central β²/β³‐amino acid residues) provides unusually tight hairpin structural clusters (cf. 3 and 4a ; Figs. 8 and 9). The skeleton of the β‐tetrapeptide H‐(R)β³hVal‐(R)β²hVal‐(R)β³hAla‐(S)β³hPhe‐OH ( 4a ) is proposed as a novel, very simple backbone structure for mimicking α‐peptidic hairpin turns.     

Pleated Sheets and Turns of β-Peptides with Proteinogenic Side Chains

Components of a toolbox with predictable secondary structural elements: β-peptides. The β-peptide shown here with proteinogenic side chains adopts a parallel pleated sheet structure in the solid state upon incorporation of suitably configured β-amino acids. When a β-dipeptide turn segment is incorporated in the center, a hairpin is formed in solution.     

Mixed β2/β3‐Hexapeptides and β2/β3‐Nonapeptides Folding to (P)‐Helices with Alternating Twelve‐ and Ten‐Membered Hydrogen‐Bonded Rings

The structural properties of four mixed β‐peptides with alternating β²/β³‐ or β³/β²‐sequences have been analyzed by two‐dimensional homonuclear ¹H‐NMR‐ and CD spectroscopic measurements. All four β‐peptides fold into (P)‐helices with twelve‐ and ten‐membered H‐bonded rings (Figs. 3–6). CD Spectra (Fig. 2) of the mixed β³/β²‐hexapeptide 4a and β³/β²‐nonapeptide 5a , indicating that peptides of this type also adopt the 12/10‐helical conformation, were confirmed by NMR structural analysis. For the deprotected β³/β²‐nonapeptide 5d , NOEs not consistent with the 10/12 helix have been observed, showing that the stability of the helix decreases upon N‐terminal deprotection. From the NMR structures obtained, an idealized helical‐wheel representation was generated (Fig. 7), which will be used for the design of further 12/10 or 10/12 helices.