Recently, Professor Feng Chuanliang's research group at Shanghai Jiao Tong University reported a methanol-water-mediated dual assembly pathway strategy, achieving the simultaneous construction of both P- and M-helical structures in a single-component chiral system. In methanol, the conformation of LBpyF is constrained to a folded state due to the hydrogen bonds and C-H···π interactions present between LBpyF and methanol. Upon adding water to the LBpyF methanol solution, two self-assembly pathways occur simultaneously, leading to the formation of both P- and M-helical structures. The cooperative process is as follows: 1) Folded LBpyF self-assembles to form M-helices; 2) H₂O induces the unfolding of the folded LBpyF molecules, which subsequently self-assemble to form P-helices. In both M- and P-helices, the bipyridine, benzene ring, and amide units adopt different stacking patterns. Energy analysis indicates that a small amount of M-helix is the thermodynamically preferred product. This study provides a method for exploring the co-evolution of multiple chiral structures by controlling diverse assembly pathways.

In biological systems, the co-evolution of multiple chiral structures is crucial. One of the most intriguing features is that biomolecules can simultaneously evolve into different structures (e.g., the same peptide sequence can form β-sheets and α-helices) to maintain biological functions. To date, researchers have developed chiral composite materials consisting of multiple helical structures by integrating different molecular units into a single system. In these processes, different molecular modules can spontaneously assemble into their respective networks under thermodynamic control. However, these studies have primarily been limited to multi-component systems, and achieving the simultaneous construction of multiple chiral structures, particularly P- and M-helices, from a single-component system remains rare.

Enantiopure LBpyB/DBpyB was obtained by coupling L-/D-methylphenylalanine with 2,2'-bipyridine-5,5'-dicarboxylic acid (Fig. 1a). Enantiopure LBpyP in a CH₃OH/H₂O (1:9, v/v) mixed solution can simultaneously form both M- and P-helices. The coexistence of M- and P-helical structures was confirmed by characterization techniques such as scanning electron microscopy (SEM) and circular dichroism (CD) (Fig. 1b-d). Furthermore, 15 parallel experiments demonstrated the reproducibility and reliability of the assembly results for LBpyP in CH₃OH/H₂O (1:9, v/v), yielding a reproducible ratio of M- to P-fibers of approximately 18.2 : 81.6 (Fig. 1e). When the bipyridine unit in LBpyF was replaced with a biphenyl unit to obtain molecule LPPF, it assembled into a single-handed M-helix under identical conditions. This result indicates that the formation of M- and P-helices is closely related to the chemical structure of LBpyF. However, this phenomenon differs from many reported results regarding "molecular chirality determining supramolecular nanostructure chirality," which motivated the authors to explore the underlying mechanism for the simultaneous formation of P- and M-helices in the single-chiral LBpyF system.

Since the spontaneous formation of P- and M-helices is evidently a kinetically controlled self-assembly process, the authors investigated the effects of factors such as time, solvent, and temperature on the assembly process. Over time, the ratio of M- to P-fibers remained constant (Fig. 1f), indicating high stability of the system. Replacing CH₃OH with EtOH, n-PrOH, DMSO, or DMF resulted in the formation of only P-fibers (Fig. 1b). This suggests that CH₃OH is a key factor enabling LBpyF to self-assemble simultaneously into both P- and M-helices. Moreover, the percentage of M-fibers increased with the proportion of CH₃OH, indicating that M-fiber formation is promoted by CH₃OH. The ratio of M- to P-fibers gradually decreased with increasing temperature until the M-fibers completely disappeared at 333 K, demonstrating the thermal sensitivity of this coexistence.

Variable-temperature NMR, solvent-dependent ¹H NMR, 2D NMR, and density functional theory (DFT) calculations were employed to further study this coexistence phenomenon responsive to temperature and solvent changes. As temperature increased, the pyridine proton (Ha) shifted upfield. The disappearance of the M-helical structure at 333 K indicated the presence of hydrogen bonding interactions between the pyridine unit of LBpyP and the hydroxyl group of CD₃OD at 298 K. Additionally, the benzene ring proton (Hb) also shifted upfield with increasing temperature, suggesting the formation of noncovalent interactions between the benzene ring and CD₃OD (Fig. 2a). NOESY spectra showed a correlation signal between Hb and Hc in CD₃OD solvent at 298 K (Fig. 2c), which was not observed in DMSO-d₆ (Fig. 2d), indicating that the benzene ring folds towards the molecular center in CD₃OD. DFT calculations further confirmed the binding mode of LBpyP with CH₃OH (Fig. 2f).

Upon adding D₂O to CD₃OD (CD₃OD/D₂O, 10:1, v/v), the proton signals of the pyridine and benzene ring units showed trends similar to those induced by temperature changes (Fig. 2b). Furthermore, NOESY spectra revealed the disappearance of correlation signals between the benzene ring and the bipyridine unit (Fig. 2e). This result indicates that, compared to LBpyP, D₂O can form stronger hydrogen bonds with CD₃OD, leading to the dissociation of the LBpyP-CH₃OH complex and subsequent unfolding of the benzene ring unit. Molecular dynamics (MD) simulations were further used to investigate the unfolding process when CH₃OH detaches from LBpyP. In the folded system, the folded molecules rapidly converted to an unfolded conformation after ~5 ns, indicating that the unfolding process occurs spontaneously when the LBpyP-CH₃OH complex dissociates (Fig. 2o). Moreover, after 50 ns of MD simulation, the quantities of folded, unfolded, and other molecular species in both systems remained nearly consistent (Fig. 2p, 2q), suggesting that the folded system can achieve a conformational distribution comparable to that of the unfolded system. Snapshots at 50 ns showed that all molecules self-assembled into continuous aggregates (Fig. 2j, 2m), consistent with the formation of nanofibers in experiments. Additionally, hydrogen bonds between amides and π-π stacking between bipyridine units resulted in highly similar stacking patterns in both systems (Fig. 2k, 2n), indicating that systems originating from either folded or unfolded molecules can self-assemble into consistent structures.

NMR results showed that 10% H₂O is sufficient to trigger the dissociation of the LBpyP-CH₃OH complex (Fig. 2b). UV spectroscopy revealed that after adding 10 μl H₂O to 100 μl CH₃OH, the peak intensity at 299 nm increased within 17 minutes and slightly shifted to 297 nm (Fig. 3a), also indicating completion of the dissociation. Further addition of 890 μl H₂O to this system resulted in the formation of only P-helices (Fig. 3b). Adding 900 μl H₂O all at once to the system allowed the simultaneous formation of both P- and M-helices, likely because H₂O-induced partial dissociation provided an opportunity for CH₃OH-bound LBpyP (folded conformation) to assemble into M-helices (Fig. 3e). Changes in CD signals indicated that pre-dissociation significantly influences the self-assembly process (Fig. 3c). Furthermore, adding ethyl acetate (EA), p-xylene (PX), or toluene (Tol) to the LBpyP CH₃OH solution all resulted in the formation of M-helices; completing dissociation with H₂O before adding EA/PX/Tol led to the formation of 100% M-helices (Fig. 3e). This further demonstrates that CH₃OH-bound LBpyP tends to form M-helices, whereas LBpyP not bound to CH₃OH self-assembles to form P-helices.

Selected area electron diffraction (SAED) and AFM-IR techniques were used to study the stacking differences between P- and M-fibers. The lattice distance in M-helices is 0.33 nm, corresponding to the intermolecular distance of π-π interactions between bipyridine units. Additionally, there is an angle of ~60° between the Bragg reflection direction and the long axis of the helical fiber, indicating an inclined relationship between the molecular packing direction and the fiber elongation direction (Fig. 4a, b, c). In contrast, the lattice distance in P-helices is 0.36 nm, and the angle between the Bragg reflection direction and the helical fiber's long axis is ~65° (Fig. 4a, d, e), significantly different from the stacking pattern of bipyridine units in M-helices. A 5 cm⁻¹ shift in AFM-IR results indicates slight differences in hydrogen bond distances and angles between P- and M-fibers (Fig. 4f, g). The AFM-IR of M-helices formed from CH₃OH/EA matched the AFM-IR of point B in Fig. 5f, indicating they share the same stacking pattern (Fig. 4h). Solid-state NMR showed that the different stacking patterns in M- and P-fibers place the carbon atoms in the benzene ring in different electronic environments (Fig. 4i). These results demonstrate that conformational differences in the benzene ring are successfully transferred to the supramolecular scale, further influencing the final self-assembled aggregates.

Finally, the authors investigated the energy difference between P- and M-helices. Differential scanning calorimetry (DSC) results showed that M-fibers have higher melting points and enthalpy changes, at 454 K and 13.78 J·g⁻¹ respectively (Fig. 5a), indicating that M-fibers are more stable self-assembly products compared to P-fibers. Additionally, CD spectroscopy was used to estimate the thermodynamic parameters of M- and P-fibers during thermal dissociation. The CD signals at 327 nm for both M- and P-fibers showed non-sigmoidal thermal dissociation curves at different concentrations (Fig. 5b, c), indicating a nucleation-elongation self-assembly process. The Gibbs free energies (ΔG°) for M-fibers and P-fibers were -33.03 kJ·mol⁻¹ and -31.01 kJ·mol⁻¹, respectively, further indicating that M-fibers are thermodynamically more stable than P-fibers (Fig. 5d, e), consistent with DSC results. However, an annealing process caused all M-fibers to convert to P-fibers. These results further emphasize the importance of molecular conformation in the self-assembly process.

In summary, this work reports a methanol-water-mediated dual assembly pathway strategy, achieving the simultaneous construction of P- and M-helical structures in a single-component chiral system. This work provides a method for manipulating multiple evolutionary processes in single-component systems by controlling assembly pathways, promoting the development of supramolecular self-assembly in creating diverse chiral composite systems. The related work was published in Angew. Chem. Int. Ed., with Professor Chuanliang Feng from the School of Materials Science and Engineering at Shanghai Jiao Tong University as the corresponding author. Laiben Gao and Kaikai Yang are co-first authors.

Reference:
Simultaneous Fabrication of P and M Helices in One-component Chiral System by Methanol-Water Mediated Dual Assembly Pathway

Laiben Gao¹⁺, Kaikai Yang¹⁺, Chao Xing¹, Biyan Lin¹, Changli Zhao¹, Xiaoqiu Dou¹, and Chuanliang Feng¹*

Link： https://doi.org/10.1002/anie.202417876