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 C14TAB is examined. The complexed mesophases poly-L-Glu/C14TAB and poly-d-Glu/C14TAB were characterized by circular dichroism (CD) in the temperature range 10-70 degrees 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/C14TAB and poly-D-Glu/C14TAB 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/C14TAB and poly-D-Glu/C14TAB possess a mainly alpha-helix structure and are exact mirror images to each other. The same mirror images and a mainly alpha-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 degrees C the alpha-helix of the poly-l-glutamic acid is gradually transformed to a mixture containing increased amounts of the 3(10)-helix while the alpha-helix structure of the poly-d-glutamic acid is constantly degenerated. In contrast the alpha-helices of the corresponding complexes poly-L-Glu/C14TAB and poly-d-Glu/C14TAB 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.