Separations of dipeptides by pH-zone-refining CCC. Hydrophobic (right) and hydrophilic (left) groups of dipeptides each consisting of two isomeric pairs were separated under the optimized conditions. Experimental conditions: Apparatus and column: see Fig. 10 caption; solvent systems: (hydrophobic group) methyl tert.-butyl ether–acetonitrile–water (4:1:5, v/v), 20 mM triethylamine and 10% DEHPA in the organic stationary phase and 20 mM HCl in the aqueous mobile phase, (hydrophilic group) methyl tert.-butyl ether–1-butanol–acetonitrile–water (2:2:1:5, v/v), 20 mM triethyl amine and 30% DEHPA in the organic stationary phase and 20 mM HCl in the aqueous mobile phase; flow rate: 3 ml/min; elution mode: head to tail; sample: dipeptides indicated in the chromatogram, total amount of 1 g for each group; detection: 280 nm; revolution: 800 rpm.