Threefold Symmetry (1)DOI: 10.1002/anie.200600776

C3 Symmetry: Molecular Design Inspired by NatureSusan E. Gibson* and M. Paola Castaldi

Keywords:asymmetric catalysis · biomimetic design · molecularrecognition · molecular symmetry · trivalent systems

The principles of symmetry have in-spired and directed the design of mole-cules for many years.[1] While twofoldrotational symmetry has been success-fully employed in a large number ofchiral ligands and catalysts,[2] there isstill comparatively little known aboutthe efficiency of systems of higher rota-tional symmetry in this and other areas.For this reason there is an ongoinginterest in the application of C3-sym-metric molecules in areas as diverse asasymmetric catalysis,[3] molecular recog-nition,[4] and materials science.[5]

In the area of asymmetric catalysis,for example, the current state of the artis typified by the use of a TiIV complex ofthe chiral C3-symmetric ligand 1 tocatalyze an enantioselective alkynyla-tion of aldehydes (up to 92% ee)(Figure 1).[6] However, a significant de-velopment in the use of C3 symmetry inasymmetric catalysis, which was inspiredby nature, was described recently byGade and co-workers.[7] Guided by theuse of tripodal N-donor ligands asmodels of the tris(histidine) bindingsites found in many zinc-containingenzymes,[8] Gade and co-workers con-sidered chiral tris(oxazolines) to begood candidates for mimics of zinc-dependent transesterases. Although theC3-symmetric dinuclear zinc complex 2,derived from ligand 3, showed onlymodest enantioselectivity in the kineticresolution of various phenyl ester de-rivatives of N-protected amino acids bytransesterification with methanol, the

principle of using C3-symmetric ligandsto mimic C3-symmetric active sitesfound in enzymes to inspire the designof new asymmetric catalysts was estab-lished and is predicted to lead to excitingdevelopments in the future.

In the area of molecular recognition,several key studies that feature archi-tectures constructed around C3-symmet-ric cores suggest that C3 symmetry hasan important role to play in this area.Moreover, a recent report by Guichard

and co-workers[9] that was inspired byC3 symmetry found in nature perhapspoints the way to exciting future devel-opments. For example, an early demon-stration of the power of C3 symmetry ina biological context was provided byWhitesides and co-workers, who showedthat tris(vancomycin carboxamide) (Fig-ure 2) binds a trivalent ligand derivedfrom d-Ala-d-Ala with exceptionallyhigh affinity: its binding constant is 25times higher than that for the biotin–avidin interaction, which is one of thestrongest known in biological systems.[10]

Whitesides and co-workers recognizedthat trivalent systems (and indeed poly-valent systems in general) are funda-mentally different from monovalent sys-tems in that dissociation of the complex,which occurs in stages, can be acceler-ated by addition of competing monova-lent ligand, thus adding an extra degreeof flexibility to potential applications ofsuch systems.

Precedent for a somewhat differentuse of C3 symmetry in a biological con-text was provided by the studies ofNishida et al. on carriers for the Lew-isX antigen.[11] Widespread interest inpolyvalent structures that carry humanoligosaccharide antigens led to the syn-thesis of 4 (Figure 3), in which threeLewisX antigen trisaccharides are at-tached to a C3-symmetric core. TheLewisX antigen is typically located oncell-membrane lipids and leads to asso-ciation in the presence of calciumions;[12] it is thus of interest to developprobes to investigate this recognitionphenomenon. Although other poly-valent systems including dimers, lipo-somes, gold nanoparticles, and self-as-sembling monolayers have previouslybeen used as multivalent probes in thisarea, it was reasoned that C3-symmetricprobes were attractive because they

Figure 1. a) The C3-symmetric tris(b-hydroxyamide) ligand 1. b) Synthesis of the dinuclearcomplex 2 from C3-symmetric tris(oxazoline)ligand 3.[6, 7] OTf= trifluoromethanesulfonate.

[*] Prof. S. E. Gibson, M. P. CastaldiDepartment of ChemistryImperial College LondonSouth Kensington CampusLondon SW72AY (UK)Fax: (+44)207-594-5804E-mail: s.gibson@imperial.ac.uk

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should produce significant multivalenteffects without generating the complexanalysis problems associated with non-symmetric or dendritic models. Thepreliminary analytical results reportedwere encouraging, and more detailedanalyses derived from this system areawaited with anticipation.

The high affinity and selectivity thatcan be achieved using trivalent com-plexes has been exploited by Anslyn andco-workers in the design of a verypromising assay for heparin.[13] Theconcentration of the clinical anticoagu-lant heparin is routinely monitored dur-ing and after surgery to prevent compli-cations such as hemorrhages, but cheap-er, more reliable, and more practicalmethods for analyzing heparin concen-trations than those currently availableare desirable. The large cavity receptor 5(Figure 4) was designed to envelop alarge surface of the oligosaccharide, thus

maximizing affinity and specific-ity by maximizing the number ofpossible interactions. Use of afluorescent scaffold, 1,3,5-tri-

phenylethynylbenzene, enabled Anslynand co-workers to generate calibrationcurves for heparin in serum at clinicallyrelevant dosing levels, thus demonstrat-ing that synthetic receptors of this typefunction successfully under physiologi-cal conditions and can be used to targetcomplex bioanalytes.

The examples of the uses ofC3 symmetry in a biological contextdescribed above are based on the in-ventive introduction of C3 symmetryinto molecular design to enhance orcreate desirable characteristics, for ex-ample, high affinity, good selectivity, andrelatively easy analysis. The report byGuichard and co-workers[9] describesthe design of a system that benefits fromall of these advantages but differs fromprevious studies insomuch as the inspi-ration for the use of C3 symmetry isderived from nature itself.

Signaling through receptors of thetumor necrosis factor receptor super-family relies strongly on the formationof C3-symmetric complexes.[14] Onemember of the family, the receptorCD40, interacts with its natural ligandCD40L by self-assembly of CD40Laround a threefold symmetry axis toform a noncovalent homotrimer thatbinds to three CD40 receptor molecules(Figure 5). The geometry of the result-ing 3:3 complex favors the formation ofa signaling complex, which ultimatelyleads to a range of regulatory effects.Moreover, CD40 antibodies with ago-nist activity were previously used toincrease immune response in infectiousdiseases, and in cancer immunothera-py.[15] It was thus postulated that thedevelopment of small-molecule CD40agonists that mimic the action of the 39-kDa natural ligand CD40L may lead toimportant therapeutic applications.

Guichard and co-workers designedlow-molecular-weight CD40L mimeticswith C3-symmetric architectures not on-ly to provide the correct geometry for

Figure 2. Structures of the trivalent derivatives of vancomycin (left) and of d-Ala-d-Ala (right).[10]

Figure 3. A C3-symmetric LewisX antigen trisacchar-ide.[11]

Figure 4. a) Heparin receptor 5. b) The major unit of heparin.[13]

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receptor binding and subsequent signal-ing but also to achieve tight bindingbetween the small low-surface-area li-gand and the receptor CD40.[9] A C3-symmetric d,l-a-hexapeptide and a b3-tripeptide were used as core structures

to distribute receptor-bindingelements with geometries anddistances that could matchthose of the homotrimer formof CD40L. The CD40-interact-ing region Lys143-Gly-Tyr-Tyr146 of CD40L was selectedas a CD40-binding motif andtethered by an aminohexanoicacid (Ahx) residue spacer tothe central core structures togive compounds 6 and 7 (Fig-ure 6).

A range of in vitro experi-ments revealed that these mol-ecules interact with CD40,compete with the binding ofCD40L to CD40, and repro-duce, to a certain extent, thefunctional properties of themuch larger natural ligandCD40L. This work not onlypaves the way to using rela-

tively small C3-symmetric CD40 ligandsto amplify immune responses in vivo,but also suggests that modulation of thefunctions of other members of the tumornecrosis factor receptor superfamilymay be successfully achieved by usingC3-symmetrical small molecules.

To conclude, C3-symmetrical func-tional molecules provide perhaps anoptimum balance between enhancedproperties, such as binding and selectiv-ity, and ease of synthesis and analysis.The work of Guichard and co-workers,inspired by natureAs exploitation ofC3 symmetry, has provided interestingand exciting results and suggests thatbiomimetic applications of C3 symmetryin the area of molecular recognition maylead to further significant advances inthe future. In parallel, recognition ofnatureAs use of C3 symmetry in enzymeshas inspired the first biomimetic C3-symmetric asymmetric catalyst, and theapproach of Gade and co-workers ispredicted to stimulate many new devel-opments in this area in the future.

Published online: June 23, 2006

[1] See, for example: R. Noyori, Asymmet-ric Catalysis in Organic Synthesis, Wiley,New York, 1994.

[2] See, for example: W. Tang, X. Zhang,Chem. Rev. 2003, 103, 3029 – 3070.

[3] See, for example: a) G. Bringmann, R.-M. Pfeifer, C. Rummey, K. Hartner, M.Breuning, J. Org. Chem. 2003, 68, 6859 –6863; b) S. Bellemin-Laponnaz, L. H.Gade, Angew. Chem. 2002, 114, 3623 –3625; Angew. Chem. Int. Ed. 2002, 41,3473 – 3475.

[4] See, for example: a) J. Chin, C. Wals-dorff, B. Stranix, J. Oh, H. J. Chung, S.-M. Park, K. Kim, Angew. Chem. 1999,111, 2923 – 2926; Angew. Chem. Int. Ed.1999, 38, 2756 – 2759; b) S.-G. Kim, K.-H. Kim, Y. K. Kim, S. K. Shin, K. H.Ahn, J. Am. Chem. Soc. 2003, 125,13819 – 13824.

[5] See, for example: a) J. van Gestel,A. R. A. Palmans, B. Titulaer,J. A. J. M. Vekemans, E. W. Meijer, J.Am. Chem. Soc. 2005, 127, 5490 – 5494;b) M. L. Bushey, T.-Q. Nguyen, W.Zhang, D. Horoszewski, C. Nuckolls,Angew. Chem. 2004, 116, 5562 – 5570;Angew. Chem. Int. Ed. 2004, 43, 5446 –5453.

[6] T. Fang, D.-M. Du, S.-F. Lu, J. Xu, Org.Lett. 2005, 7, 2081 – 2084.

[7] C. Dro, S. Bellemin-Laponnaz, R. Wel-ter, L. H. Gade, Angew. Chem. 2004,116, 4579 – 4582; Angew. Chem. Int. Ed.2004, 43, 4479 – 4482.

[8] G. Parkin, Chem. Rev. 2004, 104, 699 –767.

[9] S. Fournel, S. Wieckowski, W. Sun, N.Troouche, H. Dumortier, A. Bianco, O.Chaloin, M. Habib, J.-C. Peter, P.Schneider, B. Vray, R. E. Toes, R. Off-ringa, C. J. M. Melief, J. Hoebeke, G.Guichard, Nat. Chem. Biol. 2005, 7,377 – 382.

[10] J. Rao, J. Lahiri, L. Isaacs, R. M. Weis,G. M. Whitesides, Science 1998, 280,708 – 711.

[11] Y. Nishida, T. Tsurumi, K. Sasaki, K.Watanabe, H. Dohi, K. Kobayashi, Org.Lett. 2003, 5, 3775 – 3778.

[12] S. Hakomori, Cancer Res. 1996, 56,5309 – 5318.

[13] A. T. Wright, Z. Zhong, E. V. Anslyn,Angew. Chem. 2005, 117, 5825 – 5828;Angew. Chem. Int. Ed. 2005, 44, 5679 –5682.

[14] J.-L. Bodmer, P. Schneider, J. Tschopp,Trends Biochem. Sci. 2002, 27, 19 – 26.

[15] See, for example: a) L. Diehl, A. T.den Boer, S. P. Schoenberger, E. I. H.van der Voort, T. N. M. Schumacher,C. J. M. Melief, R. Offringa, R. E. M.Toes, Nat. Med. 1999, 5, 774 – 779; b) D.Chaussabel, F. Jacobs, J. de Jonge, M.de Veerman, Y. Carlier, K. Thielemans,M. Goldman, B. Vray, Infect. Immun.1999, 67, 1929 – 1934.

[16] J. Singh, E. Garber, H. V. Vlijmen, M.Karpusas, Y. M. Hsu, Z. Zheng, J. H.Naismith, D. Thomas, Protein Sci. 1998,7, 1124 – 1135.

Figure 5. Model of the 3:3 complex between CD40 (surfacerepresentation) and CD40L (ribbon) viewed down the C3 axis, anda magnified view of the polar CD40-binding surface and the “hot-spot” region Lys143–Tyr146 of CD40L identified as the CD40-binding motif.[9,16]

Figure 6. Synthetic C3-symmetric CD40L mim-etics.

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