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See Yun-Bao Jiang et al., Chem. Commun., 2015, 51, 8017.

Showcasing research from Yun-Bao Jiang’s Laboratory/

Department of Chemistry, Xiamen University, Xiamen, China.

Chirality sensing using Ag+–thiol coordination polymers

An eff ective chirality sensing strategy has been developed by

using coordination polymers of Ag+ with an achiral thiol ligand

which is designed to be equipped with a binding site for the chiral

analyte. Chirality sensing for monosaccharides was shown to be

operative in the chosen thiol ligand p-mercaptophenylboronic

acid. The protocol also allows determination of the enantiomeric

excess.

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Cite this:Chem. Commun., 2015,

51, 8017

Chirality sensing using Ag+–thiol coordinationpolymers†

Qian Zhang, Yuan Hong, Na Chen, Dan-Dan Tao, Zhao Li and Yun-Bao Jiang*

We report here chirality sensing using achiral Ag+–thiol coordination

polymers as the framework which becomes CD active upon inter-

action with chiral species, taking p-mercaptophenylboronic acid as a

thiol ligand that bears a binding group for monosaccharides.

We recently showed that in aqueous Ag+ and cystein (Cys)solutions Ag+–Cys coordination polymers form at a pH aroundthe pI of Cys when Cys exists in the zwitterionic form,1 facilitatedby Ag+� � �Ag+ interactions (termed Argentophilic interactions2)and electrostatic interactions between the neighbouring aminoacid residues. We assume that within this coordination polymerthe chiral carbon centers are included in the interaction networkso that CD signals at 360 nm relating to the Ag+� � �Ag+ inter-actions were observed. In favor of this assumption was theobservation of an interesting pH switching behavior of both theabsorption and corresponding CD signals that turn off at highpH when the Cys residue exists in the anionic form so thatthe neighbouring Cys residues repulse thereby preventing theAg+� � �Ag+ interactions (Scheme 1a).1 The side chain interactionsseem to be also supported by the observation of CD signals of(1-naphthalene)acetamide attached to the amine group of theCys residue,3 presumably mediated by the inter-amide hydro-gen bonding identified in the columnar structure of benzene-1,3,5-tricarboxamides.4 Note that in these two cases, with Cysand its amide derivative of the same chirality of the Cys residue,the observed CD signal at 360 nm is of the same sign. It henceappears that the CD signals of those chiral Ag+–thiol coordina-tion polymers reflect the chirality of the chiral thiol ligand. Wetherefore envisaged that the achiral coodination polymersformed from the achiral thiol ligand in the presence of Ag+

could be a structural platform to respond to the chirality of thechiral species that interacts with the achiral ligand, better in amultivalent manner so that the chiral species is includedwithin the interaction network (Scheme 1b), not just attachingto the individual thiol ligand. As a proof-of-concept, we chosep-mercaptophenylboronic acid (MPBA) as the achiral thiolligand to examine if its coordination polymers of Ag+ couldprobe the chirality of saccharides that reversibly interact withthe boronic acid group in the chosen thiol ligand.5

Fig. 1a shows the absorption spectra of the Ag+–MPBA solutionof pH 10.0 carbonate buffer with 50% by volume methanol ofincreasing Ag+ concentration. The absorbance beyond 330 nmstarts to increase when Ag+ is introduced, suggesting the onset ofthe Ag+� � �Ag+ interaction.1 The plot of the absorbance at 385 nm,for example, as a function of Ag+ concentration indicates a 1 : 1

Scheme 1 Interactions in Ag+–Cys (a)1 and Ag+–MPBA (b) coordinationpolymers. In the case of (a) a2 4 a1 because of the repulsion between Cysresidues at high pH. Glucose has two cis-diol moieties which bind to twoboronic acid groups, thereby including the chiral saccharide moiety withinthe interaction network so that CD signals relating to Ag+� � �Ag+ interactionmight be observed to enable saccharide chirality sensing.

Department of Chemistry, College of Chemistry and Chemical Engineering,

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation,

and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM),

Xiamen University, Xiamen 361005, China. E-mail: ybjiang@xmu.edu.cn;

Fax: +86 592 218 6405; Tel: +86 592 218 8372

† Electronic supplementary information (ESI) available: Absorption spectraltitration traces, DLS, SEM images, Job plot, and saccharide concentration andee dependence of CD signals (Fig. S1–S6). See DOI: 10.1039/c5cc01221j

Received 9th February 2015,Accepted 5th March 2015

DOI: 10.1039/c5cc01221j

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8018 | Chem. Commun., 2015, 51, 8017--8019 This journal is©The Royal Society of Chemistry 2015

interaction stoichiometry between Ag+ and MPBA (Fig. 1b),suggesting the formation of either the 1 : 1 complex or (1 : 1)n

coordination polymers. The appearance of the absorptionrelevant for the Ag+� � �Ag+ interaction, dynamic light scattering(DLS) data (Fig. S1, ESI†) and fibril structures observed in SEMimages (Fig. S2, ESI†) demonstrate the formation of coordina-tion polymers with Ag+–MPBA repeating unit (Scheme 1b).Upon addition of glucose into the solution of Ag+–MPBA theabsorbance beyond 320 nm declined (Fig. S3, ESI†), which maysuggest a modulation of the Ag+� � �Ag+ interaction when glucosebinds to the boronic acid groups of the MPBA ligands. Similarprofiles were observed when other monosaccharides such asfructose (Fig. S3, ESI†) were allowed to interact with MPBAligands in the coordination polymers. It was however noted thatthe absorbance beyond 320 nm is higher than zero, whichmeans that the Ag+� � �Ag+ interaction remains in the saccharidebound Ag+–MPBA coordination polymers.

We next monitored the CD spectra of Ag+–MPBA in the pre-sence of chiral saccharides. Fig. 2 presents the traces of the CDspectra with increasing concentration of D- and L-glucose. Mirror-imaged CD profiles were indeed observed. This shows that theachiral Ag+–MPBA coordination polymers are able to sense thechirality of the binding saccharide. We next applied the coordina-tion polymers to other monosaccharides and found that the profileof the CD spectrum is indeed saccharide dependent in terms ofthe signal sign and intensity and wavelength (Fig. 3 and Fig. S4,ESI†), confirming the potential of this achiral coordination poly-meric system in saccharide chirality sensing. The extremely sensi-tive response of the CD signal toward glucose at the micromolarlevel (Fig. S4, ESI†) does suggest a chiral signal amplification inthis case that its sensitivity is much higher than that for fructosewhich has a much higher affinity towards monoboronic acidsuch as phenylboronic acid (4.4 � 103 M�1 for fructose versus1.1� 102 M�1 for glucose).6 The ‘‘boronic acid’’ in this coordinationpolymer thus acts as a compound containing multiple boronicacid groups that may afford multivalent interactions with asaccharide.5c We will show later that a chirality amplification isindeed observed in the case of glucose binding, see Fig. 4.

Glucose which is known to bind two boronic acid groups5

leads to strong CD signal upon binding to the coordinatinpolymers, presumably because of the multivalent interactionsin the glucose bound coordination polymers (Scheme 1b). TheJob plot indeed suggests a 1 : 2 stoichiometry in the interaction ofglucose with Ag+–MPBA coordination polymers (Fig. S5, ESI†).It is thus of interest to examine if the chirality of the guestmolecule is amplified. We monitored the CD spectrum as afunction of ee of glucose guest and confirmed that there occurredindeed chiral amplification from the observed ‘‘S’’- or ‘‘Z’’-shapedcurve4 (Fig. 4). This observation supports the multivalent inter-actions of glucose guest with Ag+–MPBA coordination polymersand may be of significance for understanding the mechansim forchiral amplification in supramolecular polymers.4 In aggreementwith this assumption is the fact that in the case of mannosewhich binds with one boronic acid group chiral amplification

Fig. 1 (a) Absorption spectra of MPBA in pH 10.0 carbonate buffer with50% by volume methanol in the presence of Ag+ and (b) absorbance at385 nm as a function of Ag+ concentration. [MPBA] = 5 � 10�5 M.

Fig. 2 CD spectra of Ag+–MPBA in aqueous-methanol (1/1, v/v) solutionsin the presence of (a) D- and (b) L-glucose of increasing concentrationfrom 0 (blue) via 0.1 (red) to 1 (pink) mM. Aqueous solution is buffered atpH 10.0 by carbonate. [Ag+] = [MPBA] = 1 � 10�4 M.

Fig. 3 CD spectra of Ag+–MPBA coordination polymers in carbonatebuffered pH 10.0 aqueous methanol solution (1/1, v/v) in the presenceof D-glucose (a, 2 mM), D-galactose (b, 10 mM), D-mannose (c, 2 mM),D-fructose (d, 2 mM) and D-xylose (e, 10 mM). [Ag+] = [MPBA] = 5� 10�5 M.CD signal intensity as a function of saccharide concentration, whichdependence is not only defined by the binding affinity of saccharide towardboronic acid, can be found in Fig. S4 (ESI†).

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does not occur, as the CD signal is linearly proportional to theee of mannose guest (Fig. S6, ESI†). We also applied the eedependence of CD signals at 330 nm (Fig. 4b) for glucose eemeasurements and found that the protocol allowed ee measure-ments with absolute errors within�5% (3 repeated experiments)for 5 samples of ee’s of 13, �17, 25, �26, 57% using both linearand nonlinear fittings. In the case of a sample of ee �65% higherrors (over 10%) were noted. Further optimizations may there-fore be needed for real sample ee measurements.

In conclusion, we introduced Ag+ coordination polymers ofan achiral thiol ligand bearing boronic acid group as a structuralplatform for saccharide chirality sensing. Upon interaction ofmonosaccharide with the boronic acid group of the thiol ligandin the coordination polymers which are intrinsically achiral, CDsignals are observed whose profile enables chirality sensing.In the case of multivalent interactions of saccharide with thepolymers the CD signal is strong and a chirality amplificationis observed. Whereas in the case of monovalent interaction theCD signal is weak and no chirality amplification is observed.

In the latter case it might be of help to introduce additionalinteraction motifs such as the N - B interaction,7 viaco-polymerization of another thiol ligand, for example,Me2N(CH2)nSH, to afford multivalent interactions and therebyenhance the CD response. This may lead to a general structuralframework for chirality sensing by using mixed thiol ligandsthat could afford multivalent interactions with the chiralguests. Use of coordination polymers invloving metal–metalinteraction can also be extended to other d10 or d8 transitionmetals such as Cu+, Au+, Pt2+ and Pd2+,2 and the correspondingligands that could be designed to bear binding group(s) for thechiral analytes. It is believed that efforts following this lineshall be of importance in promoting the developments ofchirality sensing using supramolecular sensors.8 This is nowunderway in our laboratory.

We acknowledge the support of this work by the MOST (grant2011CB910403), the NSF of China (grants 21275121, 21435003,91427304 and J1310024), and the Program for ChangjiangScholars and Innovative Research Team in University, admini-strated by the MOE of China (grant IRT13036).

Notes and references1 J.-S. Shen, D.-H. Li, M.-B. Zhang, J. Zhou, H. Zhang and Y.-B. Jiang,

Langmuir, 2010, 27, 481–486.2 H. Schmidbauer and A. Schier, Angew. Chem., Int. Ed., 2015, 54, 746–784.3 D.-H. Li, J.-S. Shen, N. Chen, Y.-B. Ruan and Y.-B. Jiang, Chem. Commun.,

2011, 47, 5900–5902.4 A. R. A. Palmans and E. W. Meijer, Angew. Chem., Int. Ed., 2007, 46,

8948–8968.5 (a) Y. Kubo, R. Nishiyabu and T. D. James, Chem. Commun., 2015, 51,

2005–2020; (b) Y.-J. Huang, W.-J. Ouyang, X. Wu, Z. Li, J. S. Fossey,T. D. James and Y.-B. Jiang, J. Am. Chem. Soc., 2013, 135, 1700–1703;(c) X. Wu, Z. Li, X.-X. Chen, J. S. Fossey, T. D. James and Y.-B. Jiang,Chem. Soc. Rev., 2013, 42, 8032–8048.

6 J. P. Lorand and J. O. Edwards, J. Org. Chem., 1959, 24, 769–774.7 T. D. James, K. R. A. S. Sandanayake, R. Iguchi and S. Shinkai, J. Am.

Chem. Soc., 1995, 117, 8982–8987.8 (a) X. Zhang, J. Yin and J. Yoon, Chem. Rev., 2014, 114, 4918–4959;

(b) H. H. Jo, C.-Y. Lin and E. V. Anslyn, Acc. Chem. Res., 2014, 47,2212–2221; (c) C. Wolf and K. W. Bentley, Chem. Soc. Rev., 2013, 42,5408–5424; (d) Z. Chen, Q. Wang, X. Wu, Z. Li and Y.-B. Jiang, Chem.Soc. Rev., 2015, DOI: 10.1039/c4cs00531g.

Fig. 4 (a) CD spectra of Ag+–MPBA in pH 10.0 carbonate buffer andmethanol mixtures (1 : 1, v/v) in the presence of glucose of varying ee and(b) CD signals as a function of ee. [Glucose] = 2 � 10�3 M, [MPBA] = [Ag+] =5 � 10�5 M.

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