Surface pressure/area (TVA) isotherms and infrared reflection-absorption spectra (IRRAS) of racemic and enantiomeric 2-hydroxyhexadecanoic acid (HHDA) Langmuir films showed that the presence ofbivalent cations in the aqueous subphase gives rise to considerable compression (Ca2+ > Pb2+ -Zn2+) and to increased chiral discrimination (Ca2+ Pb2+ -Zn2+) in HHDA monolayers. Although both in the presence ofPb2+ and Zn2+ cations the TVA isotherms ofthe L-enantiomer exhibit the more condensed characteristics, the IRRAS measurements show that Pb2+ cations induce, against expectation, heterochiral discrimination in compressed HHDA films, while the presence of Zn2+ cations leads to preferred homochiral interactions. In the latter two instances, the conformational order of the alkyl chain is nearly independent of the compressional status of the monolayer; i.e., the optimum order induced by the presence of Pb2+ and Zn2+ cations is already attained at very low surface pressures around 1 mN m-l. The different influence of bivalent cations on chiral recognition is assumed to be caused by different complexes formed by the respective cation and the functional group of the film-forming substances. Hence, in addition to van der Waals and electrostatic interactions as well as hydrogen-bond formation, potential complex formation between bivalent cations and the head group appears to play an important role for heterochiral and homochiral discrimination in chiral Langmuir films.
Comparison between the macroscopic order (inferred from surface
pressure/area isotherms and
fluorescence microscopy) and the molecular order [determined by
infrared reflection−absorption spectroscopy
(IRRAS)] for N-acylamino acid monolayers shows that
hydrogen bond formation via the NH, COOH, and
p-hydroxyphenyl groups, respectively, may lead to pronounced
chiral discrimination. On a pure water
surface, N-hexadecanoylalanine films exhibit preferential
homochiral interactions, which may be
strengthened by Pb2+ in the aqueous subphase, while
Zn2+ disturbs this structure and suppresses
the
chiral discrimination. In N-octadecanoyltyrosine and
N-octadecanoyltyrosine methyl ester monolayers,
both homo- and heterochiral discrimination may be observed, depending
on the available area per molecule,
where the free fatty acid carboxyl group gives rise to additional film
compression and, thus, to a higher
conformational order for the alkyl chains. However, in the
presence of both PbCl2 and ZnCl2 in the
aqueous
subphase, the pronounced chiral discrimination effect disappears.
Analysis of the methylene scissoring
δ(CH2) bands shows that the different effects of
Pb2+ and Zn2+ cannot be explained on the
basis of different
subcell structures (in both cases an orthorhombic structure is
prevailing), but on the basis of different
complex formations between the bivalent cations and the carboxylic acid
headgroup as inferred from the
separation Δ between the antisymmetric and the symmetric carboxylate
vibrations, Δ = νa(COO) −
νs(COO). The results summarized in the present paper suggest that
theories like Andelman's tripode theory
should be expanded to account for potential metal complex
formations.
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