Cyclization of the diastereoisomeric glycylleucylleucylglycylglycines, either through their p- nitrophenyl thiolesters or by means of dicyclohexyl carbodiimide, yields 39% of the DL and 12% of the ll cyclic pentapeptide. Correspondingly the dielectric increment of the open-chain dl pentapeptide is only 64% that of the ll, and similar differences have been found with several pairs of diastereoisomeric di-and-tri-peptides. A preliminary discussion of these ...