L -Arginine L -aspartate

C 6 H 15 N 4 O 2 + .C 4 H 6 NO 4 - , monoclinic, P2,,a = 5.511 (3), b = 8.438 (4), c = 15.265 (9) A , β = 97.9 (I) ° , D m = 1.467 (8) (flotation), D c = 1.452 Mg m -3 , Z = 2. The structure has been refined to a final R value of 0.044 for 1226 independent counter-measured reflections. The conformation of the arginine molecule is different from those previously observed, whereas the conformation of the aspartate ion is similar to that found in L-aspartic acid, DL-aspartic acid and L-lysine L-aspartate. The unlike molecules aggregate into separate alternating layers and the a-amino and acarboxylate groups in the arginine layer are periodically brought into close proximity in a 'headto-tail' arrangement. There exist a specific ion-pair interaction involving electrostatic attraction and two nearly parallel N-H...O hydrogen bonds between the guanidyl group and the a-carboxylate group of the aspartate ion.