Infrared studies of synthetic alamethicin fragments and model peptides containing alpha-aminoisobutyric acid (Aib) have been carried out in solution. Tripeptides and larger fragments exhibit a strong tendency to form beta turns, stabilized by 4 leads to 1 10-atom hydrogen bonds. Dipeptides show less well-defined structures, though C5 and C7 conformations are detectable. Conformational restrictions imposed by Aib residues result in these peptides populating a limited range of states. Integrated intensities of the hydrogen-bonded N-H stretching band can be used to quantitate the number of intramolecular hydrogen bonds. Predictions made from infrared data are in excellent agreement with nuclear magnetic resonance and X-ray diffraction studies. Assignments of the urethane and tertiary amide carbonyl groups in the free state have been made in model peptides. Shifts to lower frequency on hydrogen bonding are observed for the carbonyl groups. The 1--6 segment of alamethicin is shown to adopt a 3(10) helical structure stabilized by four intramolecular hydrogen bonds. The fragments Boc-Leu-Aib-Pro-Val-Aib-OMe (12--16) and Boc-Gly-Leu-Aig-Pro-Val-Aib-OMe (11--16) possess structures involving 4 leads to 1 and 5 leads to 1 hydrogen bonds. Supporting evidence for these structures is obtained from proton nuclear magnetic resonance studies.
Reaction systems with a constant 2:1 Na2edt:M(II,III) mole ratio (edt = ethane-1,2-dithiolate) have been shown to yield the following complexes, isolated as quaternary ammonium salts: [Ti(edt )3]2™ (1), [V2(edt)4]2" (2), [Cr(edt)2]2" (3), [Mn(edt)2]2' (4), [Mn2(edt)4]2-(5), [Fe2(edt)4]2" (6), [Co(edt)2]2" (7), [Co(edt)2]" (8), [Ni2(edt)3]2" (9), [Cu3(edt)3]3-(10), [Zn(edt)2]2" (11), and [Cd(edt)2]2" (12). The following compounds have been structurally characterized by single-crystal X-ray diffraction (space group; crystal parameters; Z; unique data (/ > (2-3)[
Structures of lithium, sodium, magnesium, and calcium complexes of 7V,7V-dimethylformamide (DMF) have been investigated by X-ray crystallography. Complexes with the formulas LiChOMF-'/^O, and Mg(C104)2-6DMF crystallized in space groups P2{/c, P2/c, Pi, and P2,/a, respectively, with the following cell dimensions:
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