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 °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 nm3.55±0.10 nm. The CD spectra indicated that both complexes poly-l-Glu/C14TAB and poly-d-Glu/C14TAB 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 310-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/C14TAB and poly-d-Glu/C14TAB are degenerated upon heating without appreciable increase of the 310-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.

Graphical abstractDepiction of the chiral micellar aggregates developed in the systems P-l-Glu/C14TAB and P-d-Glu/C14TAB packed in hexagonal lattices of similar size 3.55±0.10 nm3.55±0.10 nm (a,ba,b-axes).Figure optionsDownload full-size imageDownload as PowerPoint slide