Crystallographic data of various lower polypeptides

Synthesis and structures of new ternary aluminum chalcogenides: LiAlSe2, alpha-LiAlTe2, and beta-LiAlTe2

The synthesis and crystal structures of new ternary aluminum chalcogenides, LiAlSe2, alpha-LiAlTe2, and beta-LiAlTe2, are reported. These compounds are synthesized by solid-state reaction at 800 degrees C. The single-crystal X-ray structures of these compounds have been determined. LiAlSe2: a = 68228(9) A, b = 8266(1) A, c = 65236(7) A, Pna2(1) (No 33, Z = 4) alpha-LiAlTe2: a = 65317(4) A, c = 116904(9) A, I42d (No 122, Z = 4) beta-LiAlTe2: a = 44810(6) A, c = 7096(1) A, P3m1 (No 156, Z = 1). These ternary compounds are formed by fusion of AlQ4 (Q = Se, Te) tetrahedra. LiAlSe2 shows beta-NaFeO2 structure type, which can be viewed as a wurtzite superstructure. alpha-LiAlTe2 adopts chalcopyrite structure type. In LiAlSe2 and alpha-LiAlTe2, AlQ4 (Q = Se, Te) tetrahedra share four corners to build three-dimensional structures and Li atoms are located in the tetrahedral sites between the chalcogen layers. beta-LiAlTe2 has polar layers formed by three-corner shared AlTe4 tetrahedra, and Li cations are in the distorted antiprisms between the layers. 7Li MAS NMR studies show that chemical shifts of Li in these ternary chalcogenides are nearly identical regardless of different chemical environments.

Intramolecularly hydrogen-bonded peptide conformations

Over the past few years the possible occurrence of intramolecularly hydrogen-bonded structures in linear and cyclic peptides has attracted increasing attention. In this review emphasis is given to solid-state studies, particularly by X-ray diffraction and infrared absorption techniques. Conformational energy calculations are also considered. The discussion is focused both on model peptides and biological activity polypeptide molecules. The tetrapeptide system (Formula: see text), examined allows one to discuss the extended C5 structure and the various folded conformations, namely the C7 (gamma-turn), C8, C10 (beta-turn), C11, and C13 conformations. The four latter forms may include cis peptide configurations. The oxy-analogs to the C7, C10, and C13 conformations and structures containing bifurcated hydrogen bonds are also discussed. The last sections describe intramolecularly hydrogen-bonded peptide structures involving: (1) a side-chain group, (2) the N-protecting group (in synthetic model compounds), and (3) a beta-amino acid.