Sugar-induced conformational changes in boronic acid-appended poly(L- and D-lysine)s and sugar-controlled orientation of a cyanine dye on the polymers

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Abstract
Poly(L- and D-lysine)s have been modified with para- or meta-phenylboronic acid derivatives which act as sugar-binding sites (L-1 and D-1 for the para-isomer and L-2 for the meta-isomer). When monosaccharides were added to the L-1 solution, the helix content (monitored by CD spectroscopy) increased and the pH which gives the maximum helix content shifted to a lower pH region. The magnitude of the pH shift was correlated with the affinity of monosaccharides for phenylboronic acid. The pH dependence of the helix content in the presence of D- or L-fructose was similar although a slight difference was seen at the low pH region. This is a novel attempt towards the control of a polypeptide's conformation by sugars. A cyanine dye (4) can be orientated on 1 or 2 in the presence of saccharides. Judging from the CD spectra, the orientation in the presence of 1 is more ordered than that in the presence of 2. The main driving-force for the association of 4 with 1 or 2 is the electrostatic attraction between cationic 4 and the anionic centre developed on the polymer by complexation of the boronic acid residue with saccharides. The chiral orientation of 4 can be monitored by a CD spectroscopic method: it is mainly governed by the chirality of 1 but not by the chirality of saccharides. This is a novel system to control the dye orientation by the cooperative action of naturallyoriginated α-amino acids and saccharides.

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