Crystalline phase separation of racemic and nonracemic zwitterionic α-amino acid amphiphiles in a phospholipid environment at the air/water interfce: A grazing-incidence X-ray diffraction study

A grazing-incidence X-ray-diffraction (GIXD) study of the self-assembly, on water, of nonracemic γ-stearyl glutamic acid (pure or as a mixture with racemic or (S)-1,2-dipalmitoyl-glycero-3-phosphoethanolamine (DPPE)) demonstrated a phase separation of the α-amino acid amphiphile into racemic and enantiomorphous two-dimensional crystallites within the phospholipid domains.

The packing arrangements of the two α-amino acid crystalline phases were identical to those found in the absence of DPPE and have been determined, at almost atomic resolution, by X-ray structure-factor calculations. By contrast, racemic and nonracemic Nε-stearoyllysine spontaneously segregated into two-dimensional enantiomorphous domains within the DPPE environment that induced a change in the tilt direction of the hydrocarbon chains of the α-amino acid molecules.

Phase separation of nonracemic amphiphiles, originating from preferred lateral homochiral or heterochiral intermolecular interactions, is in agreement with the formation of enantiomerically pure or enriched homochiral oligopeptides in overrepresented amounts in the polycondensation of activated nonracemic amphiphilic α-amino acids on plain water or within phospholipid monolayers.