Conformational aspects of polypeptide structure XVI. Rotatory constants, cotton effects, and ultraviolet absorption data for glutamate oligomers and co‐oligomers

We have reinvestigated the stereochemistry of a series of L ‐glutamate oligomers, preparing the peptides mainly via the azide route rather than the p ‐nitrophenyl ester method. Our results are in agreement with our previously published studies. Both investigations relied on derived rotatory dispersion constants b 0 and λ c , ultraviolet spectra in the 190 mμ range, and molar rotation data. Our measurements have now been extended to include the 233 mμ Cotton effect region. This paper also includes a study of the solvents, 2,2,2‐trifluoroethanol and hexafluoroacetone “trihydrate.” Both solvents are transparent in the far ultraviolet region. The former is a solvent supporting helical structures in glutamate oligomers above the heptamer. The latter solvent does not support secondary structure for any of the oligomers studied. We have treated our data for b 0 and Cotton effect rotations in terms of molar quantities. We use the additivity principle and plot molar b 0 or 233 mμ molar rotations against the number of residues in the oligomer chain. We are able to relate the changes in the slopes of the straight lines to the onset of helicity and the actual magnitudes of the slopes to the high polymer residue values. Co‐oligomers of glutamic‐aspartic acid form secondary structures of the glutamate type while glutamic acid‐glycine co‐oligomers show very little secondary structure. Finally, hexafluoroacetone “trihydrate” supports stable helices for several high polymers but denotes weak helices in polymer systems.