CRYSTAL STRUCTURES OF BETA-POLY-L-GLUTAMIC ACID AND ITS ALKALINE EARTH SALTS

From an analysis of X-ray powder patterns and selected-area electron diffraction patterns obtained in an earlier study of chain-folded single crystals of poly-l-glutamic acid and of its alkaline earth salts, crystal structures are proposed for two modifications of β-poly-l-glutamic acid and for its calcium, strontium and barium salts. All are based on the antiparallel-chain pleated sheet proposed by Pauling & Corey (1951) and have monoclinic unit cells with space group P21. In the alkaline earth salts, the planes of the carboxylate ions lie parallel to the chain axis; one-half of these ions are each in contact with one cation and the other half each in contact with three cations. The cations are in all cases in contact with four carboxylate ions. On converting these salts in the solid state to β-poly-l-glutamic acid the carboxyl groups maintain the orientation of the original carboxylate ions but become intermeshed so that the intersheet spacing is reduced from 12.5 to 13 Å to about 8.8 Å. On the other hand, in β-poly-l-glutamic acid prepared directly by precipitation from aqueous solutions of the sodium salt at low pH, carboxyl groups lie in planes normal to the chain direction; the side chains then interpenetrate more deeply, giving rise to an even smaller intersheet spacing (7.8 Å). The proposed structures account satisfactorily for the diffraction maxima and their relative intensities, and are believed to represent principal features correctly. © 1969.