Chirality and helix stability of polyglutamic acid enantiomers

In this work the chirality and the relative thermal stability of ordered micellar aggregates of poly- l- and poly- d-glutamic acids with the cationic surfactant C 14TAB is examined. The complexed mesophases poly- l-Glu/C 14TAB and poly- d-Glu/C 14TAB were characterized by circular dichroism (CD) in the temperature range 10–70 °C for their chirality and thermal stability as well as by X-ray diffraction (XRD) for the micellar ordered structure. Low angle XRD analysis showed that both micellar aggregates poly- l-Glu/C 14TAB and poly- d-Glu/C 14TAB are hexagonally packed in a MCM-41 fashion with an intermicellar distance identical and equal to 3.55 ± 0.10 nm . The CD spectra indicated that both complexes poly- l-Glu/C 14TAB and poly- d-Glu/C 14TAB possess a mainly α-helix structure and are exact mirror images to each other. The same mirror images and a mainly α-helix configuration were also observed by CD for the free poly- l- and poly- d-glutamic acids at room temperature. As the temperature increases from 10 up to 70 °C the α-helix of the poly- l-glutamic acid is gradually transformed to a mixture containing increased amounts of the 3 10-helix while the α-helix structure of the poly- d-glutamic acid is constantly degenerated. In contrast the α-helices of the corresponding complexes poly- l-Glu/C 14TAB and poly- d-Glu/C 14TAB are degenerated upon heating without appreciable increase of the 3 10-helices as an intermediate configuration. This difference in helix conservation is attributed to increase protection of the l-enantiomers, compared to d-enantiomers, which might be related to the survival of l-aminoacids in the living world. Depiction of the chiral micellar aggregates developed in the systems P- l-Glu/C 14TAB and P- d-Glu/C 14TAB packed in hexagonal lattices of similar size 3.55 ± 0.10 nm ( a , b -axes).