Coil to globule transition of pNIPAAm dopped with chiral amino acidic comonomers

By combined light scattering and circular dichroism measurements (CD), we have investigated the coil-toglobule
transition of the thermosensitive polymer poly(N-isopropylacrylamide) (pNIPAAm) copolymerized
with a 1/10 fraction of valine- or leucine-derived groups randomly positioned along the chains. The comonomers
provide the pNIPAAm chains with chirality, electric charge, and increased hydrophobicity. For valine-derived
copolymers, the coil-globule transition is basically unmodified with respect to pNIPAAm, whereas doping
with leucine-derived groups significantly lowers the transition temperature and makes the transition
discontinuous. We find the CD signal of the chiral comonomers to cleanly depend on the local chain density.
We interpret this behavior as an effect of the whole chain conformation on the conformations accessible to
the chiral groups.