Detection by circular dichroism of conformational transitions in pH and thermosensitive copolymers based on N-isopropylacrylamide and N-methacryloyl-L-leucine

Circular dichroism (CD) spectra in the region of 210–250 nm allow
visualization of intrachain phase transition of pH- and thermosensitive polyelectrolytes.
Indeed, in 0.001 M citrate and acetate buffers, at pH 4.0–5.5, aqueous solutions of a
poly(N-isopropylacrylamide-co-N-methacryloyl-L-leucine) (NIPAAm-MALEU) copolymer
containing 90.9 mol% of NIPAAm residues exhibit a well-defined sigmoidal increase
in the CD signal at 220 nm with increasing temperature. This phenomenon is suggestive
of a highly cooperative transition which occurs at lower temperatures compared to that
observed by cloud point measurements. The change in the CD signal is less sharp at
higher pH, indicating varying cooperativity with pH. For pH 6.0 and higher, no such
phenomena are observed.