Comblike Ionic Complexes
of Poly(γ-glutamic
acid) and Alkanoylcholines Derived from Fatty Acids
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Abstract
A series of ionic complexes with almost stoichiometric
composition
have been “synthesized” from bacterial poly(γ-glutamic
acid) and alkanoylcholines derived from fatty acids with 12, 14, 16,
and 18 carbon atoms. The complexes were stable up to temperatures
close to 200 °C and were non-water-soluble but readily soluble
in organic solvents. In the solid state they adopted the biphasic
structure typical of comblike systems with the polypeptide chains
arranged in sheets and periodically separated by a paraffinic phase
which was partially crystallized for C14, C16, and C18. The window
width of the layered structure was estimated by SAXS to be within
the 3.6–4.5 nm range, and WAXS showed that the alkyl tails
were crystallized in a hexagonally packed lattice with a <i>d</i><sub>100</sub> spacing of 0.41 nm. These complexes displayed reversible
melting of the paraffinic phase in the 40–65 °C range
at temperatures with values increasing with the length of the alkanoyl
group. A slight expansion of the intersheet distance occurred at melting
followed by contraction upon heating at higher temperatures but without
significant alteration of the layered structure. The <sup>13</sup>C CP-MAS NMR analysis revealed the underlying polymethylene <i>anti</i>-to-<i>gauche</i> conformational conversion
that takes place at the thermal transition. An energy-based simulation
study of the crystallization process afforded a molecular picture
of the complex and evidenced the favoring effect of the choline structure
on the packing of the alkyl side chains