Translation of Mutarotation into Induced Circular Dichroism Signals through Helix Inversion of Host Polymers

Abstract

One of the most exciting and challenging topics in molecular recognition is the development of artificial host molecules that recognize saccharides in aqueous protic media through the formation of hydrogen bonds. [1,2] We recently designed and synthesized water-soluble meta-ethynylpyridine polymers such as 1 (Scheme 1) [3] which spontaneously form helical structures in protic media through intramolecular solvophobic interactions. [3,4] This organization forces the pyridine N atoms to be positioned on the inside of the helix. In the resulting pore, saccharides interact with the polymer through the formation of hydrogen bonds between the OH groups of the saccharide and the pyridine N atoms of the polymer, even in 100 % protic media.[5] The association could be clearly detected by the appearance of induced circular dichroism (ICD) [6] signals in the absorptive region of the polymer. However, among the monosaccharides examined in this investigation, glucose exhibited weak ICD effects. This observation confused us because octyl b-d-glucopyranoside induced intense CD signals on association with 2, a lipophilic analogue of 1, in less polar solvents. [7] Detailed investigations on this phenomenon elucidated that helix inversion [8] occurs in polymer 1 as a result of anomerization of b-glucose to a-glucose and vice versa (Scheme 2). In other words, each anomer of glucose caused the polymer to bias a single-handed helix in the opposite sense. Therefore, the ICD effects should be virtually cancelled out by the existence of both the right-and left-handed helices from an equilibrium mixture of a/b-glucose (ca. 1/1). This finding means that real-time information concerning the mutarotation of glucose is translated into ICD signals of the polymer through recognition-induced chirality transfer from the small molecules to the supramolecular architectures.As mentioned above, a mixture of the water-soluble polymer 1 with d-glucose in MeOH/water (10:1) showed only a weak ICD band around 337 nm.[3] We also observed that the ICD effects could be reversed by changing the solvent composition. Figure 1 a shows the CD spectra for 1 with d-glucose in MeOH/water mixtures of various ratios after the solutions had been allowed to stand over 48 h to reach the equilibrium state. A weak negative ICD was observed around 337 nm in a MeOH/water mixture of 10:1. However, the ICD effects weakened when the fraction of water was increased, and almost disappeared in a MeOH/water mixture of 8:1. Further increasing the water fraction (up to MeOH/water 5:1) resulted in positive ICD signals appearing in the same wavelength region instead. In the case of 2 with octyl b-dglucopyranoside, the sign of the ICD effects remained unchanged regardless of the solvents used, although the ellipticity varied. There is a critical difference between dglucose and octyl b-d-glucopyranoside: the former undergoes mutarotation in solutions and the latter does not.[9] Therefore, we speculated that the curious ICD behaviors of 1 may result from the mutarotation of d-glucose, and...

Recent advances in arylene ethynylene folding systems: Toward functioning

“…It was thus suggested that the origin of the altered CD signals was the helix inversion of the host polymer ( Fig. 7), rather than partial dissociation of the complex [85]. It was then postulated that the different stereochemical configurations of C1 in glucoses, ␣ and ␤, led poly(m-ethynylpyridine) to adopt helical conformations of opposite handedness, which was illustrated by opposite signs and different magnitudes (i.e., −3 and +7 mdeg, respectively) of the initial CD signals.…”

Progress in biomimetic carbohydrate recognition

“…Moreover, carbohydrate recognition in organic media can be biomimetic in the sense of mimicking recognition at the membrane-cytosol interface, especially if extraction from water can be demonstrated. Accordingly, there has been considerable work in this area over the past 20 years, as recorded in a number of reviews [24,36,48,49] and recent articles [50][51][52][53][54][55][56][57][58][59]. For reasons of space, we will not attempt a further discussion of this work but will proceed to our major topic of carbohydrate recognition in water.…”

A Self‐Assembled Foldamer Capsule: Combining Single and Double Helical Segments in One Aromatic Amide Sequence

“…[11][12][13][14][15][16][17] These objects have until now been much smaller and far less complex than proteic receptors. Nevertheless, they share with them a vast potential for tunability: they are based on sequences whose monomers can be changed one at a time and introduced by using the same synthetic methods.…”