ARTICLE Received 6 May 2014 | Accepted 2 Feb 2015 | Published 10 Mar 2015 DOI: 10.1038/ncomms7475 Chiral recognition and selection during the self-assembly process of protein-mimic macroanions Panchao Yin1,2, Zhi-Ming Zhang3, Hongjin Lv4, Tao Li5, Fadi Haso1,2, Lang Hu1,2, Baofang Zhang1,2, John Bacsa4, Yongge Wei6, Yanqing Gao3, Yu Hou4, Yang-Guang Li3, Craig L. Hill4, En-Bo Wang3 & Tianbo Liu1,2 The research on chiral recognition and chiral selection is not only fundamental in resolving the puzzle of homochirality, but also instructive in chiral separation and stereoselective catalysis. Here we report the chiral recognition and chiral selection during the self-assembly process of two enantiomeric wheel-shaped macroanions, [Fe28(m3-O)8(Tart)16 20 À (HCOO)24] (Tart ¼ D-orL-tartaric acid tetra-anion). The enantiomers are observed to remain self-sorted and self-assemble into their individual assemblies in their racemic mixture solution. The addition of chiral co-anions can selectively suppress the self-assembly process of the enantiomeric macroanions, which is further used to separate the two enantiomers from their mixtures on the basis of the size difference between the monomers and the assemblies. We believe that delicate long-range electrostatic interactions could be responsible for such high-level chiral recognition and selection. 1 Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USA. 2 Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, USA. 3 Key Laboratory of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun, Jilin 130024, China. 4 Department of Chemistry, Emory University, Atlanta, Georgia, 30322, USA. 5 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA. 6 Department of Chemistry, Tsinghua University, Beijing 100084, China. Correspondence and requests for materials should be addressed to E.W. (email: [email protected]) or to T.L. (email: [email protected]). NATURE COMMUNICATIONS | 6:6475 | DOI: 10.1038/ncomms7475 | www.nature.com/naturecommunications 1 & 2015 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7475 he homochirality of basic biological molecules such as relatively simple building blocks to organize themselves into amino acids and sugars, an intriguing puzzle in modern larger supramolecular structures, a phenomenon that has parallels Tchemistry, is still not clearly understood1–5. Current with the formation of viral capsids (both in morphology and research on the mechanism of homochirality mainly focuses on kinetics)37. Furthermore, two almost identical macroions remain chiral recognition and chiral selection through studies of chirality self-sorted and assemble into homogeneous ‘blackberry’ transfer and amplification induced by chemical reactions, force structures in solutions containing both, demonstrating a high fields3,6–12, physical properties of solid samples (including level of self-recognition mimicking biosystems38,39. 13–15 16 crystallization) , amino-acid clustering in gas phase , Here two inorganic chiral macroions, [Fe28(m3-O)8(Tart)16 17–20 20 À molecular interaction on surfaces and supramolecular (HCOO)24] (Tart ¼ D-(S,S)-(-)-tartaric acid tetra-anion, 21–30 assemblies of chiral molecules in bulk and solutions . A few D-Fe28; Tart ¼ L-(R,R)-( þ )-tartaric acid tetra-anion, L-Fe28), sketchy theories have been developed to explain how are used as simple models, for the first time, to study the role of homochirality originated, was amplified in macromolecules and one type of universal physical interaction, electrostatic interac- supramolecular assemblies and was transmitted to other tion, in chiral recognition and selection behaviour of biomole- biomolecules/assemblies during the evolution of life2,31. This cules in solution. The kinetics study of the self-assembly indicates process certainly involves recognition and competition between that the two enantiomers recognize each other and form their the enantiomers during their organization into supramolecular individual ‘blackberry’ structures. The addition of chiral co- structures32,33. The limitation of stereoselective synthetic anions is critical to selectively suppress the self-assembly process technology and the rare examples of the natural of one of the enantiomers. This study provides a new way to biomacromolecule enantiomers make it difficult to study chiral experimentally achieve chiral separation. recognition/selection. Through manipulating intermolecular hydrogen bonding and aromatic stacking interactions, Aida and Results Meijer et al. observed the self-sorted behaviour in the solution of Molecular structures of the chiral macroanions. The nano- 20 À organic enantiomers during their formation of one-dimensional scaled anionic ferric wheel [Fe28(u3-O)8(Tart)16(HCOO)24] 27–29 supramolecular structure . However, the investigation of consists of four {Fe7} subunits connected by four Tart linkers physical interactions (for example, electrostatic interaction) and (Fig. 1 and Supplementary Figs 1–12 and Supplementary spherical supramolecular assemblies (good analogues of virus Tables 1–5)40. If the building units, the Tart ligands, are treated capsid) in chiral recognition and selection is still rare. as ‘amino acids’, the Fe28 cluster can be considered as a protein- Macroions are soluble ions that are a few nanometres in size; mimic molecule involving ‘polymerization’ of 16 such ‘amino examples include various polyoxometalate molecular clusters, acids’ via the connections to Fe3 þ . Therefore, it is metal–organic nanocages, dendrimers and some biomacromole- straightforward to understand chiral recognition and chiral cules. They exhibit solution behaviour distinct from that selection among biomacromolecules from the corresponding 34–36 associated with simple ions and large colloids . When studies of Fe28 chiral clusters. Small-angle X-ray scattering, zeta- bearing moderate charges, these macroions can strongly attract potential measurements and structural analysis confirm that Fe28 each other via counterion-mediated interactions and sometimes clusters preserve their molecular structures in aqueous solution also hydrogen bonding, forming stable, robust supramolecular and stay as macroanions with associated counterions around their assemblies in polar solvents, that is, single-layered, hollow, surface without involving any hydrophobic or intermolecular spherical ‘blackberry’ structures36. This self-assembly allows chemical interactions (Supplementary Figs 13–17). Mirror 0.95 nm L-Fe28 D-Fe28 Fe O C H 2.65 nm Figure 1 | Molecular structures of enantiomers of Fe28 clusters. (a) Ball–stick representation of molecular structures of enantiomers of Fe28. (b,c) Side and top view of the molecular structures of L-Fe28 (molecular units highlighted in blue are tartaric acid fragments). (d) Ball–stick representation of {Fe7} subunit of the cluster. Colour code of the fragment: blue and teal, two coordination modes of Tart; bright green and turquoise, two coordination modes of methanoate group. 2 NATURE COMMUNICATIONS | 6:6475 | DOI: 10.1038/ncomms7475 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7475 ARTICLE 1,000 900 D28 800 L28 700 Racemic mixture 600 500 400 300 200 Scattered intensity (kcps) 100 0 0246 81012 14 16 Time (days) L-Fe28 D-Fe28 Recemic mixture solution 400 350 300 D28_0.5 mg ml–1 –1 250 L28_0.5 mg ml Mixture(D28(0.25 mg ml–1)+ 200 L28(0.25 mg ml–1)) –1 150 D28_0.25 mg ml L28_0.25 mg ml–1 100 Intensity sum of D28_0.25 mg ml–1 & Scattered intensity (kcps) 50 L28_0.25 mg ml–1 0 010203040 Time (h) Figure 2 | Self-recognition behaviour among the enantiomers of Fe28 clusters. (a) SLS results of Ba-D-Fe28, Ba-L-Fe28 and their racemic mixture -1 solutions. (b) TEM image of the assembly in the aged solution of Ba-D-Fe28 (0.5 mg ml ). Scale bar, 50 nm. (c) Graphical representation of the chiral recognition behaviour. (d) SLS monitoring of different solutions for 31 h. Chiral recognition between macroanionic enantiomers. With form during the self-assembly process (Fig. 2b and the aim of using simple macroions to understand some funda- Supplementary Figs 19 and 20). mental processes and features in biological systems, we study the The two individual enantiomer solutions (0.5 mg ml À 1) and À 1 racemic mixture solutions of the Fe28 enantiomers to determine their mixtures (D/L ¼ 1:1; 0.25 mg ml for each) were monitored whether they form homogeneous or heterogeneous ‘blackberry’ by SLS for the first 31 h after preparation to check whether the lag structures without any disturbance. With Ba2 þ being counter- phase period, known as the dimer/oligomer formation stage, is À 1 À 1 37 ions, Ba-D-Fe28 (0.5 mg ml ), Ba-L-Fe28 (0.5 mg ml ) and responsible for the self-recognition behaviour . The first two À 1 their racemic mixture (D/L ¼ 1:1, 0.25 mg ml for each) in water, solutions self-assemble at almost the same rate and much faster as monitored by the static light scattering (SLS) technique, all than that of the mixed solution, confirming that the self- show a slow self-assembly process and reach equilibrium in B10 recognition starts at the early stage of the assembly (Fig. 2d). The À 1 days (Fig. 2a). The two enantiomers self-assemble at the same scattered intensity from the D/L mixed solution (0.25 mg ml speed, as confirmed by time-resolved SLS measurements. Since each, blue dots in Fig. 2d) also grows slower than the sum of the the self-assembly rate is proportional to the macroionic con- scattered intensities of two individual enantiomer solutions at centration (Supplementary Fig. 18 and Supplementary Note 1), 0.25 mg ml À 1 (black dots in Fig. 2d), indicating that the presence the time-resolved SLS curve for the mixed solution should be of the other type of monomer seems to negatively impact the similar to the two individual enantiomeric solutions if no assembly process, possibly due to the ineffective collisions among recognition exists between the two enantiomers. However, the the macroions.