As part of an investigation on the coordination ability of peptides, structural analyses of the solid di-, tri, and tetrapeptides glycyl-glycine (GG), glycyl-glycyl-glycine (GGG), glycyl-glycyl-glycyl-glycine (GGGG), and their protonated hydrochlorides glycyl-glycine.HCl (GGH), glycyl-glycyl-glycine.HCl (GGGH), and glycyl-glycyl-glycyl-glycine.HCl (GGGGH) have been carried out. The quantum chemical calculations (Hartree-Fock/6-31++G**) and linear-dichroic infrared (IR-LD) spectroscopy predict a near to linear structure of the pure ligands, but the experimental IR-LD data are in accordance with a cross-linked disposition of amide fragments in the protonated forms.

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The hydrogensquarate [LeuNH(2)] (HSq) of L-leucinamide has been synthesized and its structure has been determined by single crystal X-ray diffraction. A three dimensional network is formed by hydrogen bonds with participation of the O=C-NH(2) function, the hydrogensquarate ion and the N(+)H(3) group [NH(2)...O=C((Sq)) (2.840 and 2.749 A), ((Sq))OH...O=C(NH(2)) (2.618 A), NH(3) (+)...O=C((Sq)) (3.246, 2.804 and 2.823 A)], respectively. A theoretical approximation of the electronic structure was carried out by means of ab initio UMP2 and MP2 level of theory at the 6-311++G** basis set. The IR-spectroscopic assignment in the solid-phase was obtained by linear-polarized IR-spectroscopy of oriented samples as colloid suspensions in a nematic host and application of the reducing-difference procedure for the interpretation of polarized IR-spectra.

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