Helicity-driven chiral self-sorting supramolecular polymerization with Ag+: right- and left-helical aggregates

The study of chiral self-sorting is extremely important for understanding biological systems and for developing applications for the biomedical field. In this study, we attempted unprecedented chiral self-sorting supramolecular polymerization accompanying helical inversion with Ag+ in one enantiomeric component. Bola-type terpyridine-based ligands (R-L1 and S-L1) comprising R- or S-alanine analogs were synthesized. First, R-L1 dissolved in DMSO/H2O (1 : 1, v/v) forms right-handed helical fibers (aggregate I) via supramolecular polymerization. However, after the addition of AgNO3 (0.2–1.1 equiv.) to the R-L1 ligand, in particular, it was found that aggregate II with left-handed helicity is generated from the [R-L1(AgNO3)2] complex through the [R-L1Ag]+ complex via the dissociation of aggregate I by a multistep with an off pathway, thus demonstrating interesting self-sorting properties driven by helicity and shape discrimination. In addition, the [R-L1(AgNO3)2] complex, which acted as a building block to generate aggregate III with a spherical structure, existed as a metastable product during the formation of aggregate II in the presence of 1.2–1.5 equiv. of AgNO3. Furthermore, the AFM and CD results of two samples prepared using aggregates I and III with different volume ratios were similar to those obtained upon the addition of AgNO3 to free R-L1. These findings suggest that homochiral self-sorting in a mixture system occurred by the generation of aggregate II composed of the [R-L1Ag]+ complex via the rearrangement of both, aggregates I and III. This is a unique example of helicity- and shape-driven chiral self-sorting supramolecular polymerization induced by Ag+ starting from one enantiomeric component. This research will improve understanding of homochirality in complex biological models and contribute to the development of new chiral materials and catalysts for asymmetric synthesis.


General characterization:
The 1 H and 13 C NMR spectra were taken on a Bruker DRX 300, and Bruker DRX 500. Mass spectroscopy samples were analyzed on a JEOL JMS-700 mass spectrometer. The high resolution mass spectra (HR-MS) were measured by electrospray ionization (ESI) with a micro TOF Focus spectrometer from SYNAPT G2 (Waters, U.K.). A UV-visible spectrophotometer (JASCO J-815) was used to obtain the absorption spectra. IR spectra were observed over the range 500-4000 cm -1 , with a Thermo scientific Nicolet iS 10 instrument. Powder X-ray pattern (PXRD) was recorded on a Rigaku model NANOPIX X-ray diffractometer with a Cu K α radiation source.

AFM observation:
Atomic force microscope (AFM) imaging was performed by using XE-100 and a PPP-NCHR 10 M cantilever (Park systems). The AFM samples were prepared by spin-coating (2000 rpm) onto freshly cleaved Muscovite Mica, and images were recorded with the AFM operating in noncontact mode in air at RT with resolution of 1024 × 1024 pixels, using moderate scan rates (0.3 Hz). AFM images were recorded for fibers obtained from different ratio of right-and left-handed helix at diverse Ag + equivalents. In each image, 50~100 fibers were selected from different regions of the mica and analyzed using XEI software developed by Park systems.

Circular dichroism (CD) and UV-vis studies:
The CD and UV-vis spectra were recorded on a Jasco J-815 CD spectrophotometer. The CD and UV-vis spectra were determined over the range of 200-500 nm using a quartz cell with 0.1 mm path length. Scans were taken at rate of 200 nm/min with a sampling interval of 0.5 nm and response time of 0.5 s. To elucidate the supramolecular polymerization process, we first prepared the sample by dissolving R-L 1 (7.2 mM) with or without AgNO 3 in H 2 O/DMSO (1:1 v/v). After adding the sample to the CD and UV cells, it was heated to 90 °C (1 °C/min) to form the monomeric species in CD and UV-vis spectroscopy. Then the sample was cooled to 20 °C (5 °C/min) in UV-vis spectroscopy. The time-dependent CD and UV-vis spectral changes were measured at 20 °C.

Calculation of thermodynamic parameter:
The thermodynamic parameters governing the supramolecular aggregation of R-L 1 were obtained by the global fitting of the melting curves. This global fitting is performed by using the equilibrium (EQ) model reported by ten Eikelder and coworkers. 1 The values for the elongation enthalpy (ΔH e ) and the entropy (ΔS e ), and elongation binding constant (K e ) used in the cooperative supramolecular polymerization models were determined by the global fitting of the heating curves, 2-4 which were obtained by plotting the degree of aggregation (α agg ) of R-L 1 (7.0 mM) without and with AgNO 3 (1.2 equiv.) at 326 nm against temperature with heating experiments. An elongation binding constant (K e ) for aggregation at 293 K was estimated according to equation 1, from which the enthalpy change (ΔH), and the entropy change (ΔS) were determined:

Synthesis of S-L 1
The synthesis of S-L 1 was performed as described in the synthesis of  [Note] The peaks at m/z 801.3854 correspond to [R-L 1 +Na] + .                  [Note] Since unaggregated species are visible in the AFM background, we did not include the background in the calculation of right-and left-handed fibers.           [Note] To elucidate the supramolecular polymerization process, we first prepared the sample by dissolving R-L 1 (7.       [Note] To elucidate the supramolecular polymerization process, we first prepared the sample by dissolving R-L 1 (7.7 mM) in H 2 O/DMSO (1:1 v/v). After adding the sample to the CD cell, it was heated to 90 °C (1 °C/min) to form the monomeric species in circular dichroism (CD) spectroscopy. Then the sample was cooled to 20 °C (5 °C/min) in CD spectroscopy. The time-dependent CD spectral changes were measured at 20 °C.   [Note] Temperature-dependent CD spectral changes were observed by heating with 1 °C/min.        [Note] In our repeated measurements of the cooling and heating curves for aggregate I (based on R-L 1 ), no evidences of the hysteresis were found (Fig. S47A). During the heating and cooling, the T e values are 327.2 and 325.7 K, respectively. While the T m values are 317.5 and 316.9 K, respectively.   Table S1. Photochemical, helical, and morphology propeties of suprmolecular polymers based on R-L 1 without and with AgNO 3 .