Precision neutron diffraction structure determination of protein and nucleic acid components. Part V. Crystal and molecular structure of the amino-acid L-arginine dihydrate

Abstract

L-Arginine dihydrate, C₆H₁₄N₄O₂,2H₂O, crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 5·638(6), b= 11·85(1), c= 15·68(1)Å, and Z= 4. The structure has been refined to R 0·034 (1364 reflections) by neutron diffraction techniques, and all hydrogen atoms located precisely. The arginine molecule is a zwitterion, with the positive charge residing in the guanidinium group. The neutral α-amino-group is not involved in the network of nine distinct N–H ⋯ O, N–H ⋯ N, and O–H ⋯ O hydrogen bonds present in the crystal structure. The barrier for internal rotation of the α-NH₂ group is estimated to be significantly less than the corresponding barrier for the hydrogen-bonded NH₃ ⁺ group in L-asparagine monohydrate. The N–H lengths in the guanidinium group are ca. 0·02 Å less than those found for comparably hydrogen-bonded ammonium groups in other aminoacids.