TUTORIAL REVIEW
`
`www.rsc.org/csr | Chemical Society Reviews
`
`Molecular recognition of oxoanions based on guanidinium receptors
`
`Pascal Blondeau,a Margarita Segura,a Ruth Pe´rez-Ferna´ndez{a and Javier de Mendoza*ab
`
`Received 1st September 2006
`First published as an Advance Article on the web 9th November 2006
`DOI: 10.1039/b603089k
`
`Guanidinium is a versatile functional group with unique properties. In biological systems,
`hydrogen-bonding and electrostatic interactions involving the arginine side chains of proteins are
`critical to stabilise complexes between proteins and nucleic acids, carbohydrates or other proteins.
`Leading examples of artificial receptors for carboxylates, phosphates and other oxoanions, such
`as sulfate or nitrate are highlighted in this tutorial review, addressed to readers interested in
`biology, chemistry and supramolecular chemistry.
`
`1. Introduction
`
`Nature frequently uses guanidinium moieties to coordinate
`different anion groups. Present in the side chain of the amino
`acid arginine, the guanidinium group forms strong ion-pairs
`with oxoanions such as carboxylates or phosphates in enzymes
`and antibodies, and it also contributes to the stabilisation of
`protein tertiary structures via internal salt bridges, mainly with
`carboxylates.1 Not
`surprisingly, guanidinium-based com-
`pounds are found in many drugs and have been extensively
`used in molecular recognition studies, leading to the design
`and synthesis of various receptors for anions.2
`The capacity of the guanidinium group to bind oxoanions is
`due to its geometrical Y-shaped, planar orientation, which
`directs the hydrogen bonding, and to its high pKa value
`(around 12–13),3 which ensures protonation over a wide pH
`
`aInstitute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona,
`Spain
`bDepartment of Organic Chemistry, Universidad Auto´noma de Madrid,
`28049 Madrid, Spain
`{ Current address: Department of Chemistry, University of
`Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
`
`range. The positive charge is delocalized over the three
`nitrogen atoms, and four out of the five hydrogen bond
`donors present in the guanidinium group of arginine can
`complement bidentate oxoanion acceptors, along the two
`edges available (Fig. 1). This accounts for the geometrical
`versatility of the binding modes. From the energy point of
`view, binding to oxoanions results from both ion-pairing
`and hydrogen bonding, and this turns out to be a difficult
`challenge in highly polar solvents or in water. In fact, the
`binding energy arises from the difference of the energy released
`by the host–guest
`interactions and the energy penalty
`necessary to remove the solvation shell around the host, which
`is quite high in water.
`In proteins, the guanidinium–oxoanion interaction usually
`occurs inside hydrophobic pockets or in areas of low dielectric
`constant. On the contrary,
`in artificial synthetic systems
`designed to work in water or polar solvents, complexation
`takes place in an environment more exposed to solvation
`effects which compete with the donor and acceptor sites,
`causing a substantial decrease of the binding. This is usually
`overcome by increasing the number of charges or hydrogen
`bond donors or by the design of more sophisticated receptors
`
`Pascal Blondeau was born in Le
`Mans, France in 1978. He
`completed his MSc degree at
`University of Montpellier,
`France,
`in 2002 working on
`hybrid self-organized materials
`applied to transport bio-
`mimetism. He is currently doing
`his PhD under the supervision
`of Prof. Javier de Mendoza
`at ICIQ in Tarragona. His
`research interests involve the
`design and synthesis of chiral
`guanidinium receptors for mole-
`cular recognition of anions,
`enantioselective recognition of
`carboxylic acid derivatives as well as transport of amino acids.
`
`Pascal Blondeau
`
`Margarita Segura was born in Leiden, The Netherlands in 1969.
`She received her MSc degree in chemistry (1993) from the
`
`University of Granada, Spain
`and her PhD degree (1998)
`f r o m t h e U n i v e r s i d a d
`Auto´ noma (Madrid), super-
`vised by Prof. Javier de
`Mendoza, working on mole-
`cular recognition of oxoanions
`from phosphodiesters, uronic
`acids and dipeptides. After
`postdoctoral research at the
`University of Parma, Italy
`(1998–2000) worki ng on
`carbohydrate recognition in
`water in the group of Prof.
`Rocco Ungaro, she again joined
`de Mendoza’s group in Madrid
`as a postdoctoral researcher (2000–2003) leading a project on
`supramolecular donor–acceptor electroactive systems linked by
`multiple hydrogen-bonding. Since 2004 she has been the Group
`Coordinator at Prof. de Mendoza’s group at ICIQ in Tarragona.
`
`Margarita Segura
`
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`to a-helical stabilization and sometimes destabilization of
`peptides and proteins.6
`Guanidinium salt bridges play also important roles in
`enzyme active sites. Typical examples are carboxypeptidase
`A,7a creatine kinase,7b
`fumarate reductase,7c and malate
`dehydrogenase.7d
`Protein–protein hetero-dimerization processes are often
`mediated by salt bridges involving arginine on one molecule
`and phosphorylated amino acids on the other. For example,
`phosphorylation of the OH group of a serine residue in a
`receptor enables
`the simultaneous
`interaction with two
`adjacent arginine residues of another receptor. On the other
`hand, phosphorylation of serines (or threonines) adjacent to
`the arginines of the same molecule slows down the attraction
`between the receptors.8
`A related case is the involvement of the guanidinium group
`in cell–cell and cell–matrix adhesion motifs such as the tri-
`peptide sequence RGD (arginine-glycine-aspartate). Adhesive
`proteins like fibronectin, osteopontin, vitronectin and col-
`lagens display the RGD sequence at their cell recognition site
`in extracellular matrices,9 which is recognized by at least one
`member of the structural related integrins, a family of a,b
`hetero-dimeric transmembrane cellular receptors (Fig. 2).10 On
`the cytoplasmatic side of the plasma membrane, the receptors
`connect the extracellular matrix to the cytoskeleton.
`Thus, osteopontin (OPN), a multifunctional phosphorylated
`glycoprotein recognized as a key molecule in a multitude of
`biological processes such as bone mineralization or cancer
`metastasis, contains an integrin-binding RGD sequence. A
`significant regulation of OPN function is mediated through
`post-translational phosphorylation and glycosylation, a pro-
`cess that is essential for osteoclast attachment.11 Osteoclasts
`are cells that actively reabsorb old bones so that a new bone
`may be replaced. Osteoporosis (bone loss) occurs when
`osteoclasts reabsorb bone faster than the osteoblasts cells are
`producing it.
`
`Fig. 1 The guanidinium group of arginine and its two possible
`binding modes with carboxylates.
`
`where the access to the solvent is restricted. In this review,
`several examples on how this has been achieved in natural
`systems and in artificial guanidinium receptors are provided.
`
`2. Guanidinium–oxoanion ion pairs in proteins and
`nucleic acids
`
`2.1. Proteins
`
`the forefront of research
`Protein structure has been at
`studies with the goal of better understanding the function of
`these biomolecules in the chemistry, physiology and pathology
`of the cell. Proteins are remarkably flexible and susceptible
`to the influence of the environment. Both intramolecular
`and intermolecular interactions involving the protein and
`the solvent define the native conformation.4 To perform its
`function, a protein has to fold properly, a task where the
`various intra-protein or inter-protein interactions, as well
`as the interactions of
`the protein with metals or other
`molecules (such as co-factors, lipids or carbohydrates), are
`essential elements of control. An illustrative example of
`misfolding is the prion protein, which results in aggregated
`copies of the protein causing the ‘‘Mad Cow Disease’’ deadly
`condition.5 Hydrogen-bonded salt bridges, such as those
`involving guanidinium–carboxylate, are relevant contributors
`
`Ruth Pe´ rez was born in
`Madrid, Spain in 1975. She
`obtained her degree in chemis-
`try (organic chemistry) at
`the Universidad Auto´ noma
`(Madrid). She was awarded a
`European PhD at
`the same
`university in October 2005
`under the supervision of Prof.
`J. de Mendoza and Prof. P.
`Prados working in the field of
`protein–ligand interactions,
`self-assembly and dynamic
`evolution based on guanidi-
`nium–oxoanion interactions.
`In November 2005 she joined
`Prof. Jeremy Sanders’ group at the University of Cambridge as a
`postdoctoral researcher and her current work concerns molecular
`recognition using a dynamic combinatorial approach.
`
`Ruth Pe´rez-Ferna´ndez
`
`Javier de Mendoza (Barcelona, Spain 1944) is Professor of
`Organic Chemistry at Universidad Auto´noma (Madrid) and
`
`the ICIQ
`Group Leader at
`(Tarragona). He obtained
`MSc and PhD degrees in
`Pharmacy (Barcelona, 1967
`and 1971). He has been
`Assistant Professor (1971–
`1975), Associate Professor
`( 1 9 7 5 – 1 9 8 0 ) a n d F u l l
`Professor at the Universities
`of Barcelona, Bilbao, Alcala´
`de Henares and Auto´ noma
`(Madrid). He pursued a post-
`doctoral stay (1971–1972) in
`Montpellier (France) under the
`supervision of Prof. Robert
`Jacquier and Dr Jose´ Elguero.
`He is Chevalier de l’Ordre du Me´rite of France since 1994 and he
`was awarded in 1999 with the Research National Prize and Medal
`of the Spanish Royal Chemical Society. Javier de Mendoza
`pioneered the introduction of Supramolecular Chemistry in Spain
`and his current research interests range from molecular recogni-
`tion to calixarene chemistry, self-assembly and catalysts design.
`
`Javier de Mendoza
`
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`(Fig. 3).12 Due to the diversity of binding modes in this
`system, the 39 arginine residues present in the four histone
`proteins forming the nucleosome core may be divided into
`three groups: a first group of 20 arginine residues involved
`in histone–histone interactions not contacting DNA, followed
`by 7 arginines which enter the minor groove of DNA and are
`essential for histone–DNA binding; and a final group of
`12 arginines which show direct guanidinium–phosphate salt
`bridge interaction. Methylation of arginine residues in the
`histone core leads to a conformational change allowing DNA
`transcription.13 In this way, the transcription of genes can be
`regulated.
`
`3. Guanidinium-based artificial receptors for
`oxoanions
`
`reported in the late 1970’s
`first
`Lehn and co-workers
`guanidinium-containing macrocycles for the recognition of
`32 in water.14 The weak association constants
`phosphate PO4
`(Ka 5 50 (1), 158 (2) and 251 (3) M21, pH titrations) can
`be explained in terms of the more delocalised charge of
`guanidinium over ammonium and accounts for the electro-
`static prevailing interaction.
`The guanidinium can be incorporated into a bicyclic
`framework (Fig. 4a) in order to improve its solubility in
`apolar solvents, where the hydrogen bonds are stronger, and to
`avoid the anti conformation, not suitable for hydrogen-
`bonding to oxoanions (Fig. 4b). As a result, the hydration
`of the cation is reduced and the conformational freedom
`restricted. Inserted into a decaline framework, the guanidi-
`nium cation becomes therefore an almost ideal complement
`for oxoanions, since both NH protons are docking sites for
`the two syn lone pairs of the oxoanion. The resulting ionic
`
`Fig. 2 Scheme of the binding of the RGD sequence to integrins in
`cell–cell and cell–matrix adhesion processes.
`
`2.2. Nucleic acids
`
`Proteins that interact with nucleic acids have a key role in
`biological processes. They are necessary for the control of the
`genetic information, replication, packaging and protection.
`Arginine is again essential for the interaction of proteins with
`DNA. In the nucleosome, in which the DNA winds around the
`arginine-rich histone, the amino acid side chains clearly show a
`direct
`interaction with the DNA phosphodiester chains
`
`Fig. 3 Stereoview of a double stranded DNA interacting with arginines of H2A histones along the major groove. (Reprinted with permission from
`Subirana et al.12 Biopolymers, 2003, 69, 432–439. Copyright (2003) Wiley Periodicals, Inc.)
`
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`gave Ka 5 7 6 106 M21.17 The crystal structure of an acetate
`salt confirmed the formation of
`two strong symmetric
`(N…O
`hydrogen bonds between the host and the guest
`2.850 A˚ ). This first binding study confirmed the good match
`of oxoanions by guanidinium receptors through ion pair and a
`linear array of hydrogen bonds in apolar solvents.
`for
`We developed receptor 5 (chloride as counterion)
`aromatic carboxylates, but the stability constant with TBA
`p-nitrobenzoate was much lower (Ka 5 1.6 6 103 M21, 1H
`NMR titrations in CDCl3).18 This example illustrates the
`competition with the initial counterion and the importance of
`the counterion in binding strength:
`in this case the tetra-
`phenylborate counterion results in significantly weaker bind-
`ing than chloride. Thus, poorly coordinating counterions such
`as hexafluorophosphate or tetraphenylborate are necessary if
`strong binding constants are desired.
`The strong deshielding of the NH signals in the 1H NMR
`spectrum of 5?p-nitrobenzoate indicates
`the presence of
`hydrogen bonds. Moreover, stacking interactions between
`the naphthoyl side arms and the p-nitrophenyl moiety are
`evidenced by the shifting of the aromatic signals. Despite their
`ionic character, hosts 4 and 5 are insoluble in water but soluble
`in chlorinated solvents. Thus,
`liquid–liquid extractions of
`water solutions of carboxylate salts give quantitatively the ion
`pair in the organic solvent, free from any competing ion.
`
`DD-AA (donor-donor–acceptor-acceptor) hydrogen-bonded
`complex is particularly stable and geometrically well defined.
`Due to the large pKa difference between guanidinium and
`carboxylic acids (ca. 9 pKa units in water) a trans-protonation
`that would destroy the salt bridge and give a less robust AD-
`DA hydrogen bond interaction15 is unlikely, although it could
`occur in non-polar solvents, where the differences in pKa are
`substantially reduced. Finally, C2 symmetry can be introduced
`into the molecule by two stereogenic centres at the vicinal
`atoms, allowing chiral recognition of the oxoanion guest. Such
`a chiral bicyclic guanidinium binding subunit can be con-
`veniently prepared in multigram quantities in nine steps from
`chiral amino acids (asparagine and methionine).16
`The association constant between bicyclic guanidinium
`derivatives and carboxylates are quite high in chloroform or
`apolar solvents. Thus, UV titrations between 4 (tetraphenyl-
`borate salt) and tetrabutylammonium (TBA) p-nitrobenzoate
`
`Hamilton and co-workers synthesised bis-acylguanidinium
`salt 6 as a receptor for phosphodiesters. The binding constant
`with TBA diphenylphosphate (Ka 5 4.6 6 104 M21, measured
`by UV in CH3CN), was one order of magnitude higher than
`with a simpler benzoylguanidinium tetraphenylborate.19 The
`carbonyl groups contribute to the binding in two ways: they
`increase the acidity of the guanidinium NHs (but not to such an
`extent that trans-protonation can occur) and they pre-organise
`the host by intramolecular hydrogen bonds (chelation effect).
`The combination of these two factors and the additional
`hydrogen bonding from the guanidinium groups allows strong
`complexation in more polar solvents, such as acetonitrile.
`Schmidtchen studied guanidinium–carboxylate interactions
`by isothermal titration calorimetry (ITC).20 The isotherm
`
`a) Chiral bicyclic guanidinium receptor. b) Anti and syn
`Fig. 4
`conformations of guanidinium group.
`
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`binding curve between 7 (bromide) and tetraethylammonium
`acetate in acetonitrile (Ka 5 2.0 6 105 M21) revealed that the
`process was both entropically and enthalpically favourable
`for a 1 : 1 complex. Although thermodynamic parameters
`could be determined in both CH3CN and DMSO, the reaction
`in MeOH produced too little heat to allow quantification of the
`association constant. This result shows that the stabilization of
`the guanidinium–carboxylate is not only due to the strong
`electrostatic interactions (DHu) but also to a favourable release
`of solvent molecules (DSu), which strongly emphasises the
`importance of solvation in host–guest interactions, a factor
`often neglected in receptor design.
`The thermodynamic aspects of dicarboxylate recognition
`by artificial
`receptors with increasingly acidic hydrogen
`bond donor groups such as two ureas (8), thioureas (9), or
`guanidiniums (10 and 11) in polar solvents (from DMSO to
`water) were studied by Hamilton (Fig. 5).21
`As expected, association constants with carboxylate groups
`(12 and 13) increase with hydrogen acidity but are decreased in
`more polar solvents. While guanidinium–carboxylate associa-
`tion in DMSO is enthalpically driven, in more polar solvents
`such as methanol or water the association becomes an
`entropically driven process due to the liberation of solvent
`molecules upon binding.
`Anslyn and co-workers developed receptor 14, with three
`guanidinium moieties into a 1,3,5-triethyl-2,4,6-trimethylben-
`zene preorganized tripod platform,22 showing selective binding
`towards citrate 15 in pure water (Ka 5 6.9 6 103 M21, 1H
`
`NMR titrations). The host was able to complex citrate even
`from a crude extract of orange juice, which highlights its
`selectivity relative to other carboxylates. This receptor shows
`how the solvent competition can be overcome by accumulation
`of hydrogen bond donors (three guanidinium subunits) in a
`suitable fashion.
`
`The same principles inspired Schmuck’s 2-(guanidinio-
`carbonyl)-1H-pyrroles (Fig. 6), designed to complex carboxyl-
`ate groups in highly competitive media, such as water.23
`Whereas the simple guanidinium cation 16 (pKa 5 13) does not
`show any sign of complexation with carboxylates in aqueous
`DMSO,
`the increased acidity of
`the acylguanidinium 17
`(pKa 5 7–8), rises the binding affinity (Ka 5 50 M21). An
`additional hydrogen bond from the pyrrole NH (as in 18)
`increases the association significantly (Ka 5 130 M21) and the
`additional amide group (19) adds a further hydrogen donor
`well oriented to reach the anti oxygen lone pair (Ka 5 770 M21).
`The predicted geometries have been confirmed by X-ray
`
`Fig. 5 Urea- (8), thiourea- (9) and guanidinium-based (10–11) receptors and association data for dicarboxylates 12 and 13 by isothermal titration
`calorimetry.
`
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`Fig. 6 Guanidinium cations 16–20 and their carboxylate complexes.
`
`crystal structures. Even dipeptides are bound efficiently in
`water by a receptor such as 20 (Ka 5 54300 M21 for
`Val-Val).24 A similar scaffold has been used in a combinatorial
`approach showing the importance of additional interactions
`caused by the side arm to improve selectivity.
`
`4. Chiral guanidines for the enantioselective
`recognition of carboxylates
`
`Chiral discrimination of anions based on abiotic receptors is
`still an underdeveloped area of supramolecular chemistry.
`Enantiomerically pure compounds are usually obtained by
`asymmetric synthesis, crystallisation of diastereomeric salts,
`kinetic resolution of racemic mixtures or chiral chromato-
`graphy. An interesting alternative to these methods is the
`separation of enantiomers based on the complementarity of a
`receptor. Those processes based on the translocation of a guest
`between immiscible phases
`(chromatography,
`extraction,
`membrane transport) are particularly attractive. If the receptor
`is chiral, one of the enantiomers can be complexed preferen-
`tially and a kinetic resolution could be achieved. Moreover, the
`process needs only a catalytic amount of receptor since it can
`transfer several substrate molecules across the phases, without
`being removed from its own (stationary or liquid) phase.
`In this context, a useful concept, developed for chiral
`chromatography, is the three-point binding rule, which states
`that a minimum of three simultaneous interactions between the
`chiral stationary phase and for instance one of the enantiomers
`are necessary to achieve enantioselection, with at least one of
`these interactions being stereochemically dependent.25 For
`anions, receptors based on ammonium groups, amides, ureas,
`thioureas and guanidinium moieties, as well as porphyrins,
`saphyrins, or metal-containing ligands have been employed.
`Only chiral guanidines aimed at the discrimination of the
`enantiomers of amino acids will be reviewed here.
`The first example of chiral recognition of a carboxylate by a
`guanidinium-based receptor was reported by de Mendoza in
`1989.18 Indeed, compound 5 was shown to extract enantio-
`meric salts of N-protected amino acids, such as tryptophan,
`
`with modest selectivities (up to 17% excess of N-Ac-L-Trp or
`N-Boc-L-Trp were extracted by (S,S)-5 from water to chloro-
`form). 1H NMR titrations of the triethylammonium salts of
`N-acetyltryptophan in CDCl3 gave Ka 5 1000 and 500 M21
`for the L- and D-enantiomers, respectively. For the non
`protected, strongly solvated zwitterionic amino acids, receptor
`(S,S)-21 was designed.26 The compound features non self-
`complementary binding sites for carboxylate (the guanidinium
`function) and ammonium (a crown ether moiety), preventing
`the receptor from internal collapse, and an aromatic planar
`surface (a naphthalene ring) as a third point for additional
`stacking interactions
`(Fig. 7a). Up to 40% of
`racemic
`tryptophan or phenylalanine were extracted by (S,S)-21 from
`saturated aqueous neutral solutions into dichloromethane,
`with a ca. 80% content of the L-enantiomer. Reciprocally,
`chiral host (R, R)-21 extracts mainly D-Trp.
`
`Further guanidinium receptors were then synthesised in
`order to optimise the binding and extraction properties, and
`were tested as membrane carriers (U-tube tests with dichloro-
`methane between two water phases).27 Interestingly, both
`single liquid–liquid extractions and U-tube transport experi-
`ments revealed that 22 and 23 transported Trp with degrees of
`selectivity comparable with 21. This suggests that the aromatic
`naphthoyl group does not play a significant role in the
`discrimination process. Even compound 24,
`lacking the
`potential p–p interaction, was enantioselective, although to
`a lesser extent. Another binding mode was then proposed,
`without participation of the naphthoyl arm, as the outcome of
`
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`Fig. 7 a) Three-point binding mode for receptor (S,S)-21 and L-Trp.26b b) Two-point binding mode for L-Trp.27 c) Two-point binding mode
`for D-Trp.27
`
`the apolar solvent, causing the overall energy to increase
`(Fig. 7b,c).27
`A series of receptors for N-protected amino acids, bearing
`guanidinium and carbamate moieties anchored to the curved
`and lipophilic surface of cholic acid (compounds 25–27) have
`been reported by Davis and co-workers.28 The chirality is
`provided by the steroidal framework, the guanidinium as well
`as the carbamate groups establishing the ion pair and
`hydrogen bonds with the substrate. All these hosts efficiently
`extract (52–87%) N-acyl a-amino acids from an aqueous phos-
`phate buffer solution (pH 7.4) into chloroform. Compound 25
`showed high enantioselectivity (up to 7 : 1, 1H NMR
`measurements) for several N-acyl a-amino acids although this
`selectivity decreased dramatically for the more hindered N-Boc
`derivatives. On the contrary, chiral discrimination increased
`(9 : 1) with derivatives 26 and 27, carrying the more acidic
`carbamoyl groups.
`The highly lipophilic receptor 28 was then synthesised in
`gram amounts for transport studies with N-acetylphenylal-
`anine either in U-tube bulk liquid membranes (dichloro-
`methane) or with hollow-fibre membrane contactors (2.5%
`octanol in hexane).29 High enantioselectivity and transport
`rates were observed in the U-tubes (27% of N-Ac-Phe
`transported in 24 h with 56% e.e.), as well as with the large
`scale hollow fibre system (ca. 70 equiv. of substrate trans-
`ported after 48 hours) although in this case the initial
`selectivity (ca. 30%) decreased over time.
`Guanidiniocarbonyl pyrrole systems have also been tested
`for enantioselection. Schmuck reported host 29 which was able
`to bind strongly carboxylates in water.23 Despite its flexible
`structure and the fact that it bears only one chiral centre, this
`
`molecular dynamics calculations with explicit solvent mole-
`cules. In this model, binding of D-Trp exposes a highly polar
`area of the receptor (around the crown ether nitrogen) to
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`receptor showed enantioselectivity towards N-acetylalanine
`(Ka 5 1610 and 910 M21 for N-Ac-L-Ala and N-Ac-D-Ala,
`respectively), a remarkable result considering the small size
`of alanine’s side chain. Curiously, other amino acids with
`bulkier
`side
`chains
`(such as N-acetylphenylalanine or
`N-acetyltryptophan) showed only slight differences in binding
`for both enantiomers.
`More recently, tris-cationic receptors based on the guanidi-
`niocarbonyl pyrrole scaffold were developed by combinatorial
`chemistry. One compound (30, R1 5 R2 5 Lys; R3 5 Phe)
`showed efficient binding to the sequence D-Glu-L-Lys-D-Ala-
`D-Ala-OH (31) with Ka . 104 M21 in buffered water.30 This
`peptide sequence is related to the bacterial peptidoglycan
`that is recognised by the vancomycin family of antibiotics,
`preventing formation of the cell wall.
`
`5. Phosphate, sulfate and nitrate recognition
`
`In addition to carboxylates or phosphodiesters, other oxo-
`anions such as phosphate, sulfate and nitrate are biologically
`relevant31 and chemically challenging to recognise, due to their
`22) and
`weak basicity. At neutral pH, phosphate (as HPO4
`sulfate present a tetrahedral binding mode with two negative
`charges, although nitrate has a trigonal planar binding motif
`with just one negative charge. Thus, two guanidines are
`required for phosphate and sulfate but only one is needed for
`nitrate, among other hydrogen bond donor atoms. The design
`of suitable linkers between the hydrogen donors with optimal
`orientation and maximum participation of host’s lone pairs
`constitutes the major issue in the field.
`
`5.1. Phosphates
`
`Anslyn and co-workers showed metallo-receptor 32 selective
`22) and
`binding for monoprotonated phosphate
`(HPO4
`22) over other anions (such as AcO2, NO3
`2,
`arsenate (HAsO4
`
`HCO32 or Cl2) at biological pH (Ka 5 104 M21 in 98 : 2
`water/methanol, UV/vis and ITC titrations).32 Hosts bearing
`only the Cu(II) centre were less effective (Ka 5 102 M21),
`highlighting the role of the cavity and the presence of the
`guanidinium groups. Thermodynamic data showed that
`22 with guanidinium derivative 32 was
`association of HPO4
`both enthalpically and entropically driven, whereas complexa-
`tion with an ammonium analogue was mainly governed
`by entropy. The different mode of binding was rationalized
`in terms of the different solvation energies of both binding
`groups.
`Ferrocenyl-based receptor 33 gives moderately strong
`
`complexes with pyrophosphate P2O742 (Ka 5 4600 M22,
`50% methanol–water) showing a 2 : 1 host–guest stoichio-
`metry.33 The presence of a redox active subunit (the ferrocene)
`allows its use as an electrochemical sensor for this biologically
`relevant anion. Another receptor binding pyrophosphate in a
`2 : 1 fashion (Ka 5 1.2 6 108 M22 and 1.0 6 104 M21 for 2 : 1
`and 1 : 1 complexes, respectively) is guanidinium 34, contain-
`ing a fluorescent pyrene subunit, which appeared to be highly
`42 over a variety of anions.34 Moreover, 1H
`selective for P2O7
`NMR suggests that the two hosts in the 2 : 1 complex are self-
`assembled through pyrene–pyrene stacking interactions.
`
`Polyanionic messenger inositol-1,4,5-triphosphate (36) was
`recognized by receptor 35 bearing up to six guanidinium
`subunits on top of a pre-organized 2,4,6-triethylbenzene
`platform.35 Steric gearing causes the guanidinium groups to
`converge toward the cavity. As a result, a cleft-like cavity is
`formed. Since 35 has no chromophore, the binding constant
`(Ka 5 2.2 6 104 M21 in a buffered solution, 1.0 6 108 M21 in
`MeOH) was measured by competition with a fluorescent guest
`(5-carboxyfluorescein), which is released in the presence of the
`preferred guest 36.
`
`Schmidtchen designed a urethane-linked bis-guanidinium
`receptor 37 for the binding of ditopic tetrahedral anions.36 A
`binding constant of 106 M21 in water for p-nitrophenyl
`phosphate and cytidine-59-phosphate was determined by 1H
`NMR. Such a high value in a polar solvent was explained by
`the simultaneous complexation of both guanidinium groups to
`the tetrahedral guest.
`Recently, binding studies between 38 and 4 and phosphates
`of different sizes were measured by both 1H NMR and ITC in
`acetonitrile.37 For 38, 1H NMR gave a curve fitting for a 1 : 1
`stoichiometry whereas ITC predicted a 1 : 2 host–guest binding
`model and revealed that the binding was not caused by a large
`enthalpic contribution but to a strong entropic factor instead.
`
`This journal is ß The Royal Society of Chemistry 2007
`
`Chem.Soc.Rev., 2007, 36, 198–210 | 205
`
`Published on 09 November 2006. Downloaded by University of Oxford on 8/23/2022 2:32:36 PM.
`
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`Calorimetry indeed prevents misleading conclusions from
`NMR in cases where rapid interconverting species are in
`equilibrium. Thus,
`introduction of several hydrogen bond
`donors in the receptor scaffold counteracts rather than
`enhances the enthalpic stabilization of the host–guest complex.
`
`Therefore, no detailed studies comparing binding constants
`with sulfates and other anions are available. However, despite
`both phosphate and sulfate being tetrahedral, the latter is less
`basic, thus the affinity for guanidinium receptors is shifted
`towards phosphate.
`In 1996 de Mendoza and co-workers reported on chiral
`bicyclic bis-guanidinium (40 and 41) and tetrakis-guanidinium
`(42) salts whose sulfate counterions, unlike the corresponding
`chloride salts, required hydrogen donors from two different
`molecules to balance the charges and to fully wrap around the
`anion, since the spacer CH2SCH2 is simply too short to use
`guanidines from the same chain (Fig. 8).39 Therefore, two
`subunits are forced to self-assemble orthogonally around the
`tetrahedral anion in a double-helical
`structure
`(sulfate
`helicates). 1H NMR spectra showed large downfield shifts of
`guanidinium NH’s as dimers or tetramers complexed sulfate
`anion. Moreover, ROESY spectra confirmed intermolecular
`contacts due to the folded conformation.
`A recent computational study concluded that for simple
`sulfate–guanidinium interactions several minima of similar
`
`Macrocycle 39, based on the chiral bicyclic guanidinium
`subunit, has been designed by de Mendoza and co-workers to
`afford six strong hydrogen bonds oriented towards its cavity to
`facilitate wrapping around tetrahedral oxoanions.38 Although
`diphenylphosphate was readily extracted from water,
`the
`binding constant could not be measured from the tetra-
`phenylborate salt (Ka . 105 M21) by NMR in CDCl3.
`H