The quantitative separation of leucin from valin

Abstract

Of the known amino-acids determined in semi-quantitative estimations of final proteolytic products, leucin and its relatives, isoleucin and valin, have proven unusually difficult to prepare pure in even approximately quantitative amounts. The separation of these substances, because of their close physical and chemical similarity, has offered almost insurmountable difficulties to previous investigators. The acids form isomorphous mixtures which are absolutely inseparable by crystallization; and their esters have so nearly the same boiling points that they cannot be fractionated by distillation. Because of these difficulties, most investigators have not attempted to separate the mixture, but have reported the entire mass as leucin. Fischer 1 states that all the figures reported from his laboratory for leucin in protein hydrolyses refer to this mixture. Ehrlich 2 has recently reported a method for separating the three substances, but it involves a long process, large losses, and the racemization of the isoleucin and valin.