화학공학소재연구정보센터
Macromolecules, Vol.53, No.22, 9916-9928, 2020
Optically Active Polymers with Cationic Units Connected through Neutral Spacers: Helical Conformation and Chirality Transfer to External Molecules
Cationic chiral polymers consisting of a chiral "Arg monomeric unit" consisting of two L-arginine moieties joined together through an oxyhexane-1,6-diyloxy spacer unit and achiral "Sp monomeric unit" based on the spermine moiety where the two units are connected through a 1,4-dioxobutane-1,4-diyl spacer unit ("C2 monomeric unit") were prepared in the cationic form as a salt of p-toluenesulfonic acid. Polymer conformation could not be directly deduced from circular dichroism (CD) spectra based on the amide group bands (200-240 nm), which are generally used to quantify alpha-helical, beta-sheet, and random coil residues because of spectral overlapping with the p-toluenesulfonate bands. On the other hand, a helical conformation was proposed on the basis of the relation between CD intensity and Arg monomeric unit ratio of polymers, while the proposed conformation may not be highly ordered but rather fragmented and partial. The helix-forming propensity of a polymer consisting of a higher Arg monomeric unit ratio was supported by MD simulations in water and in vacuum. The copolymer consisting of 100% Arg monomeric units [poly(Arg100-C2)], the one consisting of 42% Arg monomeric units and 58% Sp monomeric units [poly(Arg50-C2-Sp50-C2)], and the one consisting of 20% Arg monomeric units and 80% Sp monomeric units [poly(Arg25-C2-Sp75-C2)] exhibited chirality transfer to methyl orange (MO), where transfer was most efficient with poly(Arg50-C2-Sp50-C2). Chirality transfer efficiency also remarkably varied depending on the standing time of polymer solution in H2O before it is mixed with MO, the time after mixing, and the concentration of the polymer. Theoretical calculations of absorbance and CD spectra of polymer-MO complexes suggested that the induced CD spectra mostly having bisignet spectral patterns are not based on exciton coupling between two MO molecules bound to the polymer chain but are composed of independent excitation bands. The design of chiral macromolecules introduced in this work based on a chain composed of charged chiral and achiral units separated by neutral achiral linker (spacer) units has not been studied from a view of chiral polymeric materials. Such a design of polymer chain may be applicable to a wider variety of chemical structures and will largely widen the scope of optically active polymers with helical conformation.