Environment-induced Sequential Interconversion of Amphoteric β-1,3-Glucans/Polythiophene Complexes: A Unique System Applicable to a Naked-eye Detectable Fluorogenic pH Probe

Abstract

The amphoteric curdlan and a cationic polythiophene can form two distinct complexes as a function of medium pH. On the basis of such an environment-induced sequential interconversion of the amphoteric curdlan/polythiophene complex, we have successfully demonstrated the application of the complex as a naked-eye-detectable multicolor fluorogenic pH probe.The development of fluorogenic sensor molecules has attracted much attention because they can allow for highly sensitive detection of analytes by simple and practical procedures. To develop such fluorescent sensors, conjugated polymers are appreciated by many researchers in this field as potential candidates due to the unique physical properties of these polymers.1 Such polymers have an extended π-conjugated structure, resulting in the delocalization of electrons between the repeating monomer units. Such an electronic structure provides the polymers with semiconductivity, optoelectronic properties, light absorption, emission ability, etc. As observed in their doping effect leading to semiconductive polymers, the electronic structure of the conjugated polymers is extremely sensitive even to very small environmental perturbations. This is a remarkable advantage in constructing highly sensitive fluorogenic sensor molecules composed of conjugated polymers. In addition, the electronic structure of the conjugated polymer is sensitive to its conformational change. On the basis of such useful physical properties, the development of the conjugatedpolymer-based molecular sensors is making remarkable progress.2 Recently, water-soluble conjugated polymers have been applied to the fluorogenic sensor toward biological analytes, 3 nucleic acids, 4 proteins, 5 ions, 6 pH, 7 etc. because they possess one of the highest sensitivities as probes for biologically important circumstances.In the last decade, our research group became interested in the unique function and broad application of β-1,3-glucans. 8These polysaccharides form a triple-stranded helical structure; the helix can be reversibly converted between a single-stranded random coil and a triple helix by changes in solvent, pH, temperature, etc. Utilizing such a structural interconversion, we have already reported that the β-1,3-glucans, schizophyllan and curdlan, act as unique one-dimensional helical host molecules toward various guest molecules with different features, including nucleic acids, 9 conductive polymers, 10,11 dyes, 12 and nanoparticles. 13 In addition, we established a method to introduce stimuli-responsive motifs into the side chain of β-1,3-glucans.14 Most recently, we reported the development of amphoteric curdlans (ACs, Figure 1). 15 In that study, we found that (1) ACs showed an isoelectric point (pI) and the net charge is controllable by pH change, (2) owing to such a pH responsiveness, ACs displayed unique protein-like pH-responsive structural conversion, and (3) pH-responsive catch-and-release of the ionic drug model molecules, such as cationic peptide and anionic DNA, was successfully carried out ...

Amylose‐grafted Curdlan: A New Class of Semi‐artificial Branched Polysaccharides for Hierarchical Polymeric Superstructures Created by the Action of “Orthogonal” Binding Sites

“…We have reportedt hat the triplehelix formation ability of curdlan derivatives possessing both amino groups and carboxylic acid groups on the C6 positions of glucoseu nits. [18] In this previous study,i tw as elucidated that the triple-helix formationa bility of the curdlan derivatives declinedw ith the reducingo ft he content of unmodified glucose units from 50 %t o3 0%.T hat is, the modificationo ft he 6-position of glucose units on curdlan tends to decline its triple-helix formation ability.Since amylose is bulky polysaccharide, it is concludedt hat the triple-helix formation ability of AGC is more sensitivet ot he density of amylose side chain. The ICD type absorption band ascribed to the non-specific interaction between PT-1 and CUR main chain was observed for PT-1/AGC 21 as as houlder at around 340 nm, whereas it was not observed for PT-1/AGC 43 and PT-1/AGC 58 .…”

“…In case of the complexation of PT‐1 with Glc 5 ‐CURs , a positive Cotton effect and an ICD type absorption band were observed at around 480 nm and 340 nm, respectively. The former one indicates the formation of co‐helical complex PT‐1 / Glc 5 ‐CURs and the latter one is attributed to non‐specific interaction between PT‐1 and CUR main chain of Glc 5 ‐CURs . The intensity of these CD bands was decreased with the increase in the modification rate on CUR main chain.…”

“…This result observed for the increase in the amylose side chain is rationalized by the following two possibilities; (1) the co‐helical complex formation ability of CUR main chain is weakened and (2) the helical structure of modified CUR main chain is relatively looser than the original triple helix. We have reported that the triple‐helix formation ability of curdlan derivatives possessing both amino groups and carboxylic acid groups on the C6 positions of glucose units . In this previous study, it was elucidated that the triple‐helix formation ability of the curdlan derivatives declined with the reducing of the content of unmodified glucose units from 50 % to 30 %.…”

“…In case of the complexation of PT-1 with Glc 5 -CURs,apositive Cotton effect and an ICD type absorption band were observed at around4 80 nm and 340 nm, respectively.T he former one indicates the formation of co-helical complex PT-1/Glc 5 -CURs and the latter one is attributed to non-specific interaction between PT-1 and CUR main chain of Glc 5 -CURs. [18] The intensity of these CD bands was decreased with the increase in the modification rate on CUR main chain. Thiss uggests that the modification of CUR main chain rather weakens not only the nonspecific interaction but also the specific co-helical complexa-tion ability between PT-1 and CUR main chain, under neutral pH conditions.…”

Synthesis of a one-handed helical polythiophene: a new approach using an axially chiral bithiophene with a fixed syn-conformation

“…Wennerström and Inganäs et al reported that polythiophenes bearing ionic pendant groups could form a one-handed helical conformation in aqueous solutions, where hydrogen-bonded ion-pair formations within the polymer chain played important roles in helix formation 24–27. Shinkai et al and Inganäs et al found that a helical conformation was induced in ionic polythiophenes upon complexation with guest molecules and polymers 28–35. Meijer et al and Koeckelberghs et al also reported nonionic helical polythiophenes that could fold only under poor solvent conditions containing water or methanol, probably due to hydrophobic interactions 36,37.…”