^79,81Br and ¹²⁷I NQR studies of hydrobromides and hydriodides of amino acids and dipeptides

Abstract

By 79,81Br and 127I NQR the hydrobromides of 1‐asparagine, 1‐asparatic acid, glycyl‐1‐alanine, and glycylglycine and the hydriodides of 1‐asparagine, 1‐glutamic acid, and glycylglycine were studied in the temperature range of 77 ≤ T/K ≤ 420 (or decomposition point). — A reversible phase transition is observed for glycyl‐1‐alanine hydrobromide at 203 ± 1 K. 79,81Br NQR frequencies are strongly influenced by N⊕—H…Br⊖ and O—H…Br⊖ hydrogen bonds. The 79Br frequencies can be grouped into two bands, one at 12.4 < v/MHz < 18.5, the other at 24.8 < v/MHz < 26.4. The compounds of the high frequency group are characterized by N⊕—H…Br⊖ and O—H…Br⊖ hydrogen bonds, whereas the bond scheme of the low frequency compounds is determined by N⊕—H…Br⊖ bonds only. This grouping of amino acid and dipeptide hydrobromides according to frequency band and hydrogen bond scheme can be applied to the hydriodides, too. The ones with a O—H…I⊖ hydrogen bond have nuclear quadrupole coupling constants of N⊕ > 120 MHz, the other compounds with N⊕—H…I⊖ hydrogen bonds only show nuclear quadrupole coupling constants of 70 < e2QzzQh−1/MHz < 120.

The Electric Field Gradient at the Site of the Halogen Ion and Structure of Amino Acid Hydrobromides and Hydroiodides

“…The bond distances and bond angles within the L-cysteine molecule of L-Cys • HBr • H 2 0 are comparable with the ones observed for other cysteine compounds such as L-Cys • HCl • H 2 0 [22,23], orthorhombic L-Cys [24,25], and monoclinic L-Cys [26]. Bond lengths and bond angles of the amino acid group are within the region found for many amino acid and peptide compounds [20,21].…”