Prepn: Neuhaus, Korkes, Biochem. Prep. 6, 75 (1958); D. M. Theodoropoulos, J. Chem. Soc. 1960, 5257. Other methods of prepn and isoln from protein hydrolysates: Greenstein, Winitz, Chemistry of the Amino Acids (Wiley, New York, 1961), passim. Crystal structure: G. H. McCallum, Nature 184, 1863 (1959).