Tuning organic crystal chirality by the molar masses of tailored polymeric additives

We introduced a feasible way to fabricate enantiopure chiral crystal aggregates directly from their racemic solution and their chirality can be tuned by the molar masses of polymeric additives.

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Hierarchically ordered chiral crystals have attracted intense research efforts for their huge potential in optical devices, asymmetric catalysis and pharmaceutical crystal engineering. Major barriers to the application have been the use of costly enantiomerically pure building blocks and the difficulty in precise control of chirality transfer from molecular to macroscopic level.

Figure 1. The formation of hierarchically ordered chiral crystals from racemic solution and their chirality tuning by changing the molar masses of polymeric additives.

Herein, we describe a strategy that enables the preferred formation of one enantiomorph of p-phydroxyphenylglycine p-toluenesulfonate (pHpgpTs) directly from its racemic solution as well as their in-situ oriented attachment to fabricate macroscopic chiral crystal aggregates, aided by poly[p-methacrylamido-L-phenylalanine] (L-PMPA)s with various molar masses. When a L-PMPA with a medium molar mass is used, Plus (P)-type fan-shaped crystal aggregates of D-pHpgpTs are obtained. Changing the molar mass of the polymer can switch the chirality of the hierarchically ordered structures thus formed on both molecular and supramolecular levels (Figure 1). This unique and efficient chiral regulation method is realized by balancing the strength of stereoselective and nonstereoselective interactions between L-additive and D/L-pHpgpTs nucleus and crystals.

The application of this method has successfully been extended to other conglomerate forming racemates (i.e. Thr, aThr, Asp, and Asp2Cu) by using Poly(N6-methacryloyl-L-lysine) (L-PMAL) as the additive. It has considerably simplified the fabrication protocol of hierarchical chiral structures from racemic compounds. Low additive dosage and high purity of the crystalline product also make it suitable for large scale production. This work would inspire further research to increase our fundamental understanding in accurate chiral discrimination and cross-scale, multilevel transmission, and expand the scope of accessible building blocks. We envision the potential wide applications of this strategy in pharmaceutical crystal engineering, organic chiral micro-/nano-laser, and asymmetric catalysis.

More details of this work could be found  here in Nature Communications (https://www.nature.com/articles/s41467-021-27236-1).

Xichong Ye

Dr, Peking University