Chemical properties of the natural neurotoxin of Lathyrus sativus 3-N-oxalyl-2,3-diamino-propanoic acid (beta-ODAP), its nontoxic 2-N-oxalyl isomer, and its hydrolysis product 2,3-diamino-propanoic acid (DAPRO) by 1H- and 13C-NMR spectroscopy.