Use of diastereomeric interactions to probe the inplane attachment of water-soluble molecules to the polar head groups of Langmuir films of Cu-alpha-amino acid complexes
M. Berfeld et al., Use of diastereomeric interactions to probe the inplane attachment of water-soluble molecules to the polar head groups of Langmuir films of Cu-alpha-amino acid complexes, J PHYS CH B, 103(33), 1999, pp. 6891-6899
To provide more direct information on the role played by "tailor-made" auxi
liary molecules in the early stages of crystal nucleation, the interplay be
tween dusters of polar headgroups of monolayers of the copper complexes S-C
u-S' and S-Cu-R' and water-soluble copper complexes S'-Cu-S' and R'-Cu-R' w
ere investigated [where S represents enantiomerically pure (S)-palmitoyl-N-
epsilon-lysine, and S' and R' represent chiral resolved (S) and (R) forms o
f alanine, serine, or valine]. The different monolayers were formed by spre
ading the amphiphilic (R) or (S) alpha-amino acid on an aqueous solution of
copper acetate followed by injection of the water-soluble (S') or (R') alp
ha-amino acid into the subphase. The surface pressure-molecular area isothe
rms of the Langmuir monolayers of the two type of complexes (S-Cu-S' and S-
Cu-R') are different, the former being substantially more expanded. The pol
ar headgroups of the S-Cu-S' and of the S-Cu-R' monolayers transferred onto
a solid support assume a hans and cis configuration, respectively, accordi
ng to comparative X-ray photoelectron spectroscopy (XPS) studies with appro
priate cis and trans three-dimensional (3-D) alpha-amino acid Cu complexes.
A grazing incidence X-ray diffraction (GIXD) analysis demonstrated that th
e S-Cu-S' and S-Cu-R' monolayers have different 2-D crystal structures, in
keeping with the XPS results. A model is presented suggesting that the wate
r-soluble S'-Cu-S' copper complexes are enantioselectively bound to the per
iphery of the domains of the cis S-Cu-S' monolayers, but not to the domains
of the trans S-Cu-R' monolayers. By symmetry, the same principal holds for
the monolayers and water soluble copper complexes of alpha-amino acids of
the opposite handedness.