Synthesis and Macromolecular Helicity Induction of a Stereoregular Polyacetylene Bearing a Carboxy Group with Natural Amino Acids in Water

Abstract

Since most important biological events occur in aqueous solution where biopolymers such as proteins and DNA form a characteristic helical conformation, significant attention has recently been paid to developing artificial polymers and oligomers that adopt a onehanded helical conformation in aqueous solution, particularly in light of emerging applications in the biomedical and materials science fields and also for mimicking protein folding. 1 However, most synthetic helical polymers and oligomers prepared so far are optically active due to their helicity in organic solvents. 2 We recently reported that macromolecular helicity can be induced on a stereoregular, cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1-H) by an optically active amine, 3 and this helicity can be "memorized" when the amine is replaced by various achiral amines in organic solvents. 4 The poly-1-H was prepared by the polymerization of (4-(triphenylmethoxycarbonyl)phenyl)acetylene with a rhodium (Rh) complex, ([Rh(nbd)-Cl] 2 : nbd ) norbornadiene), in tetrahydrofuran (THF), followed by hydrolysis of the ester groups. 3 We now show that this polymer can be directly prepared by the homogeneous, stereospecific polymerization of (4-carboxyphenyl)acetylene (1-H) in water in the absence of surfactants and any organic solvents, using watersoluble rhodium complexes, such as [Rh(cod) 2 ]BF 4 (cod ) cyclooctadiene), [Rh(nbd) 2 ]ClO 4 , and [Rh(nbd)(tos)-(H 2 O)] (tos ) p-toluenesulfonate) 5 in the presence of bases such as alkaline hydroxides and amines (Scheme 1). Furthermore, the macromolecular helicity can be induced on the polymer in water as a result of noncovalent, electrostatic interactions with natural, free amino acids; the complexes showed a characteristic induced circular dichroism (ICD) in the UV-vis region. Accordingly, the sequence of events including the synthesis of a polymer and the helicity induction on the polymer is entirely possible in water, which is an extremely important medium for organic reactions, polymerizations, and processes, because water is costeffective, safe, and environment-friendly; the use of wetorganic solvents or aqueous biphasic systems is still considered ecologically harmful. 6,7 Table 1 summarizes the polymerization results of 1-H with water-soluble Rh complexes. The Rh-based complexes effectively polymerize phenylacetylenes to afford high molecular weight, cis-transoidal polymers, and well-defined Rh-based initiators can induce living polymerization in organic solvents. 8,9 For these polymerizations, acetic acid is used to remove the active Rh moiety from the propagating polymer end to terminate the polymerization. 8,9 This indicates that acetylenes bearing a carboxy group may not be directly polymerized by the Rh-based complexes. 10,11 However, the use of bases brought about the stereospecific polymerization of 1-H in water. The polymerization was rapid and homogeneous in the presence of bases, producing yelloworange fibrous, high molecular weight polymers in high yields (Table 1), although the molecul...

Chiral and Chirality Discrimination on Helical Polyacetylenes

“…In the presence of bases such as NaOH and amines, (4-carboxyphenyl)-acetylene was rapidly and homogeneously polymerized with water-soluble rhodium complexes to yield high molecular weight poly-16 in high yields. 25 The obtained sodium salt of poly-16 also formed a predominantly one-handed helical structure with natural amino acids in water, and the complexes showed an ICD in the UV-visible region. However, this polymer is not sensitive to amino acids and other important chiral biomolecules in the fields of biology and medicine.…”

Detection and Amplification of Chirality by Helical Polymers

“…The complexes form supramolecular assemblies with controlled helicity through electrostatic and hydrogen-bonding interactions in water. [23,50] For instance, the assay of 19 of the common free l-amino acids produced the ICDs of 9 e with the same Cotton effect signs, demonstrating that the polyelectrolyte functions as the first powerful chirality-sensing probe in water. This concept, that is, the use of a chromophoric dynamic helical polyelectrolyte, is promising for the construction of a specific sensory system for a target biomolecule by tuning the functional group of the polyelectrolyte.…”

Atomic force microscopy observation of a helical poly(phenylacetylene) bearing aza‐18‐crown‐6 ether pendants induced by amino acids

“…We previously reported that macromolecular helicity could be induced on optically inactive, stereoregular poly(phenylacetylene)s bearing a variety of functional groups, such as carboxy, [23][24][25][26] boronate, 27,28 amino, 29,30 and phosphonate groups. 31 These poly(phenylacetylene)s form a predominantly one-handed helix upon complexation with optically active compounds capable of interacting with these functional groups and thus exhibit a characteristic induced circular dichroism (ICD) in the ultraviolet-visible region of the polymer backbone.…”