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Confinement of Nucleic Acid Bases and Related Compounds Using Tetra-p-sulfonatocalix[4]arene Peter J.




and Colin L.


School of Chemistry, Monash UniVersity, Melbourne, Victoria 3800, Australia, and School of Biomedical, Biomolecular and Chemical Sciences, The UniVersity of Western Australia, Crawley, Perth, WA 6009, Australia

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 5 1161-1167

ReceiVed December 20, 2005; ReVised Manuscript ReceiVed February 16, 2006

ABSTRACT: Supramolecular complexes of tetra-p-sulfonatocalix[4]arene and guanine, cytosine, benzimidazole, and 2-hydroxybenzimidazole have been isolated in the solid state, and their structures have been elucidated using single-crystal X-ray diffraction data, as have 2-hydroxybenzimidazole complexes and complexes with large cations tetraphenylphosphonium (Ph4P+) and triethylbenzylammonium (Et3BzN+). These large cations disrupt the usual bilayer arrangement of the sulfonated calixarenes, as can benzimadole itself, and a combination of guanine and cytosine, in the absence of the large cations. Some of the structures show a “molecular capsule” type arrangement with two calixarenes arranged with their cavities directed toward each other, effectively shrouding two nucleic acid bases. Introduction We recently reported the structures of supramolecular complexes of tetra-p-sulfonatocalix[4]arene and racemic (alanine, histidine, and phenylalanine) and chiral ((S)-alanine, (S)-serine, (S)-histidine, and (S)-tyrosine) amino acids.1,2 For alanine, histidine, and phenylalanine, racemic pairs of molecules are confined in “molecular capsules” made up of two tetra-psulfonated calix[4]arenes with their cavities directed toward each other in an overall bilayer arrangement. The (S)-amino acid isomers for alanine and histidine, however, form independent 1:1 complexes within the bilayer arrangement of the type that is prevalent for a range of inclusion complexes of tetra-psulfonatocalix[4]arene.2 For (S)-serine and (S)-tyrosine, pairs of chiral molecules are encapsulated within molecular capsules based on two tetra-p-sulfonatocalix[4]arenes.2 Water-soluble tetra-p-sulfonatocalix[4]arene is a versatile host molecule capable of forming host-guest complexes and renown for its bilayer arrangement.3 In addition to bilayers, tetra-p-sulfonatocalix[4]arene can form other arrays,4 some rather spectacular, including spherical arrays of 12 calixarenes and nanotubes.5 Organic or metal cations are widely employed in the formation of supramolecular arrays because they play an important role in the stability of the structures through charge balance, hydrogen bonding, bridging, and templating.6,7 In a continuation of our studies on confinement of biologically relevant molecules, we have looked at the host-guest behavior between tetra-p-sulfonatocalix[4]arene and nucleic acid bases and complexes involving some related molecules. The only nucleic acid base/tetra-p-sulfonatocalix[4]arene complex structure reported to date is the adenine complex reported by Atwood et al.8 2-Hydroxybenzimidazole has also been reported to complex with tetra-p-sulfonatocalix[4]arene, a pair of them in a “molecular capsule” with a large organic dication.9 In this paper, we present the isolation and structure elucidation of tetrap-sulfonatocalix[4]arene complexes of guanine, cytosine, and benzimidazole and new complexes of 2-hydroxybenzimidazole, Figure 1. * To whom correspondence should [email protected]. † Monash University. ‡ The University of Western Australia.




Figure 1. Nucleic acid bases and related compounds featuring in the present study and the tetra-p-sulfonatocalix[4]arene anion.

Results and Discussion From the outset, we added large organic cations (tetraphenylphosphonium, Ph4P+, and triethylbenzylammonium, Et3BzN+) to the nucleic acid base/tetra-p-sulfonatocalix[4]arene mixtures to ascertain whether there is any disruption of the usual bilayer arrangement for complexes of containing tetra-p-sulfonatocalix[4]arene anions and to nucleic acid bases encapsulated by the tetra-p-sulfonatocalix[4]arene. This builds on our preliminary studies on Ph4P+/tetra-p-sulfonatocalix[4]arene complexes as a precedent for disrupting the bilayer arrangement.10 The above large cations have complementarity of charge and hydrophobicity with the calixarene, with the ability of the nucleic acid base to be accommodated by π-stacking (and other interactions) to the tetra-p-sulfonatocalix[4]arene periphery. For all the ensuing complexes, the nucleic acid bases were both exo and endo with respect to the cavity of the tetra-psulfonatocalix[4]arene anions. All complexes were prepared by the slow evaporation of equimolar mixtures of the “nucleic acid” base and calixarene and similarly for the addition of the large organic cations in acidified water (pH ≈ 2.5, 1 M HCl added). All compounds were characterized by single-crystal diffraction studies, which showed the calixarene in the usual cone

10.1021/cg0506673 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/13/2006

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Figure 3. Projections of complex 2 built up of guanine, cytosine, and tetra-p-sulfonatocalix[4]arene, 1:2:1. Figure 2. Projections of complex 1 involving guanine and tetra-psulfonatocalix[4]arene showing the bilayer arrangement, ratio 1:1.

conformation with the phenolic groups forming a torus of intramolecular hydrogen bonding. Na[{Guanine2+}⊂(p-O3S-calix[4]arene)+H+]‚7H2O (1). The complex crystallized in the space group P1h (No. 2) with a guanine dication perched above the cavity of a tetra-psulfonatocalix[4]arene anion in the hydrophilic area of the overall bilayer structure. The guanine dication is above and almost coplanar with the plane defined by the four SO3- groups of each calixarene, and adjacent guanine dications are well separated (closest contacts >5 Å), Figure 2. The guanine dications are involved in NH hydrogen bonds to oxygens on the sulfonato groups (N‚‚‚O, 2.28-2.86 Å), as well as to oxygens on water bound to sodium (O‚‚‚O, 2.830 Å) and waters of crystallization (O‚‚‚O, 2.522 Å). Sodium cations bridge two calixarenes (Na‚‚‚O (SO3), 2.386(3) and 2.471(3) Å), as well as binding to guanine (Na‚‚‚O, 2.373(3) Å) and three water molecules (Na‚‚‚O, 2.278(4), 2.333(3), and 2.361(4) Å). Tetra-p-sulfonatocalix[4]arene anions are well offset so as not to encapsulate one or two guanine dications in a molecular capsule arrangement, but nevertheless the overall interplay of calixarenes is through π-stack in a bilayer structure. Intramolecular hydrogen bonding of the phenolic groups in the calixarene is evident by the O‚‚‚O distances at 2.629-2.686 Å. (Guanine2+)(cytosine+)[{cytosine+}⊂(p-O3S-calix[4]arene)]‚ 8H2O (2). The complex crystallizes as a racemic twin in the space group Cc (No. 9) with one cytosine cation within a tetrap-sulfonatocalix[4]arene anion cavity and another cytosine along with a guanine dication located outside the cavity of the calixarene. None of the nucleic acid bases interact with each other, all being involved in extensive hydrogen bonding to water molecules, Figure 3. The endo cavity cytosine cation within the calixarene cavity is hydrogen-bonded to two sulfonato groups (N‚‚‚O, 2.712 and 2.996 Å) and to a water molecule (N‚‚‚O, 2.805 Å). The cytosine cations lie perpendicular to the base plane of phenolic oxygens of the tetra-p-sulfonatocalix[4]arene anion cavity with their polar groups radiating outward. The other cytosine cations π-stack with the aromatic rings on the tetra-p-sulfonatocalix[4]arene anions (N‚‚‚C, 3.390, to C‚‚‚C, 3.707 Å; centroid to centroid distance, 3.555 Å) with the cytosine NH2 aligned with one calixarene methylene bridge (N‚‚‚C, 3.501 Å) and the cytosine oxygen aligned with the other methylene bridge (O‚‚‚C, 3.476

Figure 4. Bilayer structure of complex 3 based on 2-hydroxybenzimidazole and tetra-p-sulfonatocalix[4]arene, ratio 3:2.

Å). The cytosine cation hydrogen bonds to sulfonato groups (N‚‚‚O, 2.777 and 2.880 Å) and water molecules (O‚‚‚O, 2.830 Å, and N‚‚‚O, 2.807 and 2.875 Å). The guanine dications also π-stack with the five-membered rings aligned with the sulfonato substituted end of the aromatic rings on a tetra-p-sulfonatocalix[4]arene anion (C‚‚‚C, 3.442, 3.625, and 3.377 Å, and N‚‚‚C, 3.451 and 3.452 Å; centroid to centroid distance, 3.422 Å). The guanine dications hydrogen bond to the sulfonato groups of neighboring calixarenes (N‚‚‚O, 2.627, 2.638, and 2.813 Å) and to water molecules (O‚‚‚O, 2.879 Å, and N‚‚‚O 2.633, 2.638, and 2.813 Å). The presence of both guanine and cytosine cations exo to the cavity of the tetra-p-sulfonatocalix[4]arene anions disrupts the usual bilayer structure formation, but the overall structure has the cavities of the calixarenes orientated in opposite directions, effectively canceling out their dipole moments, Figure 3. As expected the phenolic groups of the cone conformation calixarene are involved in intramolecular H-bonding (intramolecular O‚‚‚O distances for the phenolic groups in the calixarenes reflect hydrogen bonding (2.63-2.69 Å).

Confinement of Nucleic Acid Bases

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Figure 5. Projections of complex 4 based on benzimidazole and sodium tetra-p-sulfonatocalix[4]arene, ratio 1:1.

(2-HObenz+)[{(2-HObenz+)⊂(p-O3S-calix[4]arene)}2+5H+]‚ 21H2O (3). The complex crystallizes in the space group P1h (No. 2) with a pair of π-stacked 2-hydroxybenzimidazole cations (2HObenz) encapsulated by a pair of tetra-p-sulfonatocalix[4]arene anions approximating as a molecular capsule. Another 2-hydroxybenzimidazole cation is located in the hydrophobic layer outside of the calixarene cavity. The 2-hydroxybenzimidazole cations within the calixarene cavity hydrogen bond to sulfonato groups (N‚‚‚O, 2.772 and 2.894 Å) and form a π-stacked pair (closest C‚‚‚C, 3.57 to 3.74 Å, and C‚‚‚N, 3.60 Å) within a slightly offset bis-tetra-psulfonatocalix[4]arene molecular capsule (S‚‚‚S separations, 5.095 and 6.557 Å, and shortest O‚‚‚O separation, 3.689 Å, between the two calixarenes). The calixarene is hydrogen bonded to the 2-hydroxybenzimidazole cation and has the usual cone conformation optimizing a cyclic array of H-bonding (O‚‚‚O distances, 2.66 to 2.73 Å). The other 2-hydroxybenzimidazole cations are disordered over two overlapping positions, coplanar with and equidistant (ca. 3.4 Å) from two phenyl rings of alternate tetra-psulfonatocalix[4]arenes within the hydrophobic environment of the overall bilayer structure, Figure 4. Na[{Benzimidazole + }⊂(p-O 3 S-calix[4]arene)+2H + ]‚ 14H2O (4). The complex crystallizes in the space group P1h (No.

2) with a pair of π-stacked benzimidazole cations encapsulated by tetra-p-sulfonatocalix[4]arene anions, like that for 2-hydroxybenzimidazole (see above). The benzimidazole cations within the calixarene cavity hydrogen bond to sulfonato groups (N‚‚‚O, 2.765 and 2.814 Å) and form a π-stacked pair (closest C‚‚‚C, 3.47 to 3.65 Å) within an offset bis-tetra-p-sulfonatocalix[4]arene capsule, as for complex 3. Two sodium cations, bridged by two water molecules (Na‚‚‚O, 2.731 and 2.478 Å) span the hydrophilic layer and connect two adjacent tetra-p-sulfonatocalix[4]arene capsules via coordination to sulfonato groups (Na‚‚‚O, 2.385 Å). Three water molecules complete the six-coordination environment around each sodium cation (Na‚‚‚O, 2.359, 2.446, and 2.502 Å, Na‚‚‚Na, 3.656 Å), Figure 5. The molecular capsule has the endo cavity benzimidazole cation hydrogen bonded to each of the calixarenes (NH‚‚‚OS. 1.90 and 2.08 Å). The calix[4]arene has the usual cone conformation and associated phenol group H-bonding (O‚‚‚O distances, 2.67 to 2.88 Å). Na2(Benzimidazole+)3[{(benzimidazole+)⊂(p-O3S-calix[4]arene)}2+H+]‚30H2O (5). The complex crystallized in the space group C2/m (No. 12) with a pair of π-stacked benzimidazole cations encapsulated by two tetra-p-sulfonatocalix[4]arene

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Figure 6. Molecular projections of complex 5 based on benzimidazole and tetra-p-sulfonatocalix[4]arene, ratio 5:2.

Figure 8. Projections of the structure of complex 7, which comprises tetraphenylphosphonium cations, 2-hydroxybenzimidazole, and tetrap-sulfonatocalix[4]arene, ratio 4:1:2.

Figure 7. Projections of the extended structure in 6, also comprised of benzimadole and sodium tetra-p-sulfonatocalix[4]arene, ratio 2:1.

anions as molecular capsules linked through sodium cations. Benzimidazole cations are located outside of the capsules, as well as confined in the inside of the capsule, with the absence of the usual bilayer for tetra-p-sulfonatocalix[4]arene. The benzimidazole cations within the calixarene cavity are disordered equally over two positions as is one of the two types of benzimidazole cations located exo to the cavity of the tetra-p-sulfonatocalix[4]arene. A π-stacked pair of benzimidazole cations (short contact of the overlap N‚‚‚C/C‚‚‚C, 3.26-3.66 Å) is contained within a bis-tetra-p-sulfonatocalix[4]arene molecular capsule and a second π-stacked pair of

benzimidazole cations (overlap of N‚‚‚C/C‚‚‚C, 3.33-3.70 Å) is located parallel to and sandwiched between tetra-p-sulfonatocalix[4]arene phenyl rings (shortest contacts N‚‚‚C/C‚‚‚C, 3.313.37 Å). The third type of benzimidazole is located parallel to and sandwiched between tetra-p-sulfonatocalix[4]arene phenyl rings (shortest contacts C‚‚‚C/N‚‚‚C, 3.35-3.48 Å), Figure 6. Each benzimidazole cation within the capsule hydrogen bonds to a single sulfonato group on each of the capsules forming tetra-p-sulfonatocalix[4]arenes (N‚‚‚O, 2.831 and 2.738 Å). The benzimidazole cations π-stacked exo to the calixarene cavities hydrogen bond to a water molecule bound to the sodium cation (N‚‚‚O, 2.719 Å) and to an uncomplexed water molecule (N‚‚‚O, 2.763 Å). The third type of benzimidazole cation is not involved in any significant hydrogen bonding (N‚‚‚O distances, >3.0 Å). Sodium cations are bound to three tetra-p-sulfonatocalix[4]arene phenol oxygen centers (Na‚‚‚O, 2.95-3.00 Å), to a sulfonato oxygen of an adjacent tetra-p-sulfonatocalix[4]arene (Na‚‚‚O, 2.80 Å), and to two water molecules (Na‚‚‚O, 2.71 Å). This linking of calixarene cone base to cone rim causes the disruption of the usual hydrophilic/hydrophobic bilayer structure typical for complexes of tetra-p-sulfonatocalix[4]arene. The

Confinement of Nucleic Acid Bases

Figure 9. Projections of complex 8, which comprises triethylbenzylammonium cations, 2-hydroxybenzimidazole, and tetra-p-sulfonatocalix[4]arene, ratio 1:1:1.

presence of benzimidazole interposed between the tetra-psulfonatocalix[4]arene highlights the disruption of the bilayer arrangement of calixarenes. The tetra-p-sulfonatocalix[4]arene molecular capsule has S‚‚‚S separations of 5.257 and 6.611 Å and shortest O‚‚‚O separation of 3.866 Å between the two calixarenes. There is hydrogen bonding between the endo cavity benzimidazole cations and the calixarene and π-stacking between the two benzimidazole cations. Intramolecular hydrogen bonding in the calixarene is reflected in the O‚‚‚O distances for the phenolic moieties of 2.641 and 2.655 Å. (Benzimidazole+)[{benzimidazole+}⊂(p-O3S-calix[4]arene)+2H+]‚5.5H2O (6). The synthesis of the complex differs from that of 5 in using 2 equiv of benzimidazole instead of 1 equiv. The complex crystallized in the space group P1h (No. 2) with one benzimidazole cation perched in a tetra-p-sulfonatocalix[4]arene anion and another benzimidazole cation outside of any tetra-p-sulfonatocalix[4]arene cavity. The overall arrangement deviates significantly from the usual bilayer structure for tetrap-sulfonatocalix[4]arene complexes, Figure 7. The endo cavity benzimidazole cations are disordered equally over two positions, and each is hydrogen bonded to one water molecule (N‚‚‚O, 2.276 and 2.808 Å) but not to the sulfonato groups of the calixarene. A benzimidazole phenyl hydrogen

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points toward the centroid of the tetra-p-sulfonatocalix[4]arene anion phenyl rings (C‚‚‚centroid, 3.480 Å). The second benzimidazole cation lies coplanar with and ca. 3.4 Å further away from the tetra-p-sulfonatocalix[4]arene anion. A phenyl ring of another tetra-p-sulfonatocalix[4]arene anion lies coplanar and ca. 3.5 Å away on the other side of this benzimidazole cation. The second benzimidazole cation hydrogen bonds to sulfonyl groups on two separate tetra-p-sulfonatocalix[4]arene anions (N‚‚‚O, 2.677 and 2.784 Å). The calix[4]arene has the usual cone conformation and associated intramolecular H-bonding for the phenolic groups (O‚‚‚O distances, 2.632-2.772 Å). (2-HObenz + )(Ph 4 P + ) 2 [{(Ph 4 P + )⊂(p-O 3 S-calix[4]arene)}2+3H+]‚27H2O (7). The complex crystallized in the space group P1h (No. 2) with a pair of phenyl-embraced tetraphenylphosphonium cations encapsulated by a pair of tetrap-sulfonatocalix[4]arene anions, each of the cations having a phenyl ring residing in the cavity of a calixarene. Another type of tetraphenylphosphonium cation is located outside of the tetrap-sulfonatocalix[4]arene cavities. The 2-hydroxybenzimidazole cations, which are disordered equally over two positions, are also located outside the calixarenes cavities, between (ca. 3.65 Å) phenyl rings of neighboring encapsulated tetraphenylphosphonium cations. Each hydrogen bonds to one water molecule (O‚‚‚O, 2.452 Å) and to a sulfonato group (N‚‚‚O, 2.616 Å), Figure 8. The first type of tetraphenylphosphonium cation has phenyl groups engaged in the multiple phenyl embraces established by Dance et al.11 The encapsulated dimer of {Ph4P+}2 has additional interactions with the calixarene cavity with (P‚‚‚P separation, 6.179 Å), and similar modes of interactions were reported earlier by our group.10 The second type of tetraphenylphosphonium cation located outside of calixarene cavities are well separated from the neighboring tetraphenylphosphonium cations (P‚‚‚P separations, 8.565 and 8.690 Å. While the tetra-p-sulfonatocalix[4]arenes are aligned with respect to their upper-rims, they are interposed by two embraced tetraphenylphosphonium cations and are thus too remote to be considered as part of a molecular capsule arrangement, unlike in the above cases where they are much closer and similarly aligned; S‚‚‚S separations are 12.542-13.753 Å, and shortest O‚‚‚O separation is 10.543 Å between the two calixarenes. The O‚‚‚O distances of the oxygens of the phenolic groups of the calixarene are in the regime for intramolecular hydrogen bonding, 2.599-3.054 Å. (Benzimidazole+)[{Et3BzN+}⊂(p-O3S-calix[4]arene)+2H+]‚ 10H2O (8). The complex crystallizes in the space group P21/n (No. 14) with the phenyl group of the triethylbenzylammonium cations in the cavity of tetra-p-sulfonatocalix[4]arene anions and the alkylammonium moiety in the hydrophilic layer of the overall bilayer structure. The 2-hydroxybenzimidazole cations are interposed by tetra-p-sulfonatocalix[4]arene anion and are almost coplanar with phenyl rings of the calixarenes (calixarene 1 has N‚‚‚C at 3.332 and 3.378 Å; calixarene 2 has N‚‚‚C at 3.246 and C‚‚‚C at 3.274 and 3.520 Å), Figure 9. 2-Hydroxybenzimidazole cations hydrogen bond to three water molecules (N‚‚‚O, 2.748 and 2.824; O‚‚‚O, 2.421 Å) but not to the sulfonate groups. The triethylbenzylammonium cations are perched above the tetra-p-sulfonatocalix[4]arene anion cavities with the phenyl ring directed into the cavities. The ethyl substitution prevents any hydrogen bonding to this cation. The tetra-p-sulfonatocalix[4]arene anions are significantly offset so that they are not collectively encapsulating triethylbenzylammonium cations. Nevertheless the bilayer arrangement

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Table 1. Crystallographic Details for Compounds 1-8 compd formula FW (g mol-1) size (mm3) system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) vol (Å3) Z, dc (g dm-3) µ (cm-1) 2θmax (deg) R Rw reflns (total) reflns (unique) reflns (obsd) CCDC no.

1 C33H42N5O24S4Na1 1043.96 0.22 × 0.16 × 0.12 triclinic P1h (No. 2) 12.5543(1) 13.0502(1) 13.5866(2) 91.5715(7) 94.0897(6) 90.8543(4) 2219.15(3) 2, 1.562 3.1 56.58 0.0724 0.200 40020 10869 8683 283626

2 C28H55N11O27S4 1106.06 0.20 × 0.12 × 0.10 monoclinic Cc (No. 9) 19.0887(3) 15.2640(2) 18.1531(3) 90 107.3448(9) 90 5049.76(18) 4, 1.455 2.7 56.56 0.0455 0.112 34850 12441 10072 283627

3 C77H104N6O56S8 2611.96 0.20 × 0.14 × 0.11 triclinic P1h (No. 2) 13.7260(5) 13.8286(7) 15.7589(8) 70.434(2) 67.962(3) 81.603(3) 2611.96(9) 1, 1.441 4.2 56.68 0.147 0.356 26506 12507 6143 283628

4 C35H57N2O30S4Na1 1137.08 0.18 × 0.16 × 0.11 triclinic P1h (No. 2) 12.0518(3) 13.8172(4) 16.0624(5) 65.727(1) 70.861(1) 83.671(2) 2302.58(8) 2, 1.640 4.8 56.46 0.0880 0.146 38000 11118 7032 283629

compd formula FW (g mol-1) size (mm3) system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) vol (Å3) Z, dc (g dm-3) µ (cm-1) 2θmax (deg) R Rw reflns (total) reflns (unique) reflns (obsd) CCDC no.

5 C105H105N14O62S8 2857.54 0.18 × 0.15 × 0.11 monoclinic C2/m (No. 12) 19.685(1) 22.592(1) 15.512(1) 90 112.452(3) 90 6539.71(2) 2, 1.451 1.9 56.80 0.167 0.470 21908 8196 4593 283630

6 C84H94N8O43S8 2160.21 0.17 × 0.15 × 0.12 triclinic P1h (No. 2) 11.3374(4) 12.0011(4) 16.8372(4) 86.230(2) 88.574(2) 80.966(2) 2257.32(8) 1, 1.589 4.8 56.60 0.0941 0.242 23030 10892 6252 283631

7 C159H188N2O60S8P4 3467.63 0.18 × 0.14 × 0.11 triclinic P1h (No. 2) 14.1890(2) 14.6510(2) 21.9725(4) 88.1723(9) 71.9093(9) 71.7921(9) 4113.75(14) 1, 1.400 4.6 56.66 0.0718 0.196 69793 19935 12789 283632

8 C48H70N3O27S4 1249.35 0.18 × 0.15 × 0.15 monoclinic P21/n (No. 14) 14.0301(3) 29.4166(4) 14.9317(3) 90 105.1657(7) 90 5949.51(2) 4, 1395 2.5 56.64 0.125 0.355 47518 13873 7059 283633

prevails. The calix[4]arene has the cone conformation with phenolic O‚‚‚O distances of 2.677-2.822 Å. Conclusion The current series of tetra(p-sulfonato)calix[4]arene complexes clearly indicate a high degree of ambivalence of tetra(p-sulfonato)calix[4]arene toward the nucleic acid base type molecules. The adeninium complex reported by Atwood et al. could be described as the usual bilayer tetra(p-sulfonato)calix[4]arene structure with the cation guest residing in the hydrophilic layer of water and sulfonato groups.8 The guanine complex, 1, exhibits the same overall bilayer structure, and along with a sodium cation, the guanine cation resides in the hydrophilic layer. The guanine cations are orientated in a perpendicular direction with respect to adeninium in the adeninium structure and do not participate in nucleic acid base to nucleic acid base hydrogen bonding. The benzimidazole complex, 4, also has a bilayer structure with the guest cation residing in the hydrophilic layer. Complexes 3 and 5, containing 2-hydroxybenzimidazole and benzimidazole cations, respectively, also exhibit bilayer structures, but as well as having the cations residing in the tetra(psulfonato)calix[4]arene cavities in the hydrophilic layer, there are also nucleic acid base cations residing in this layer. In complexes 2, 6, and 7, the presence of nulceic acid base cations

outside of the tetra(p-sulfonato)calix[4]arene cavities results in disruption of the bilayer arrangement. In complexes 1 and 8, the presence of different organic cations results in the nucleic acid base cations being exo to the tetrap-sulfonato)calix[4]arene cavities. Overall the results show a versatility of the sulfonated calixarene to build up structures containing nucleic acid bases with greater structural diversity. Moreover there is the possibility of binding large organic cations in the cavity of the calixarene, coupled with intercation π-stacking. The scene is then set for preparing complexes with even more components spatially arranged around the sulfonated calixarene with higher complexity. Experimental Section All reagents were either commercially available products of high purity and were not further purified or, in the case tetrasodium tetrap-sulfonatocalix[4]arene, prepared via the sulfuric acid reaction with tetra(tert-butyl)calix[4]arene.13 Crystals of the complexes discussed were grown by evaporation of aqueous acid (pH ≈ 2.5, 1 M HCl added) solutions of tetrasodium tetra-p-sulfonatocalix[4]arene, nucleic acid base or other guest, and any added salt as indicated. The complexes tended to lose water of crystallization rapidly, and the presence of precipitated starting materials made bulk microanalyses unreliable. Crystallography. All crystals were mounted onto a glass capillary under paraffin oil. X-ray data were collected at 123(1) K on an EnrafNonius KappaCCD single-crystal diffractometer with Mo KR radiation (λ ) 0.710 73 Å). The structures were solved by direct methods with

Confinement of Nucleic Acid Bases SHELX-9712 and refined by full matrix least-squares on F2 (I > 4σ(I)) using the X-Seed12 interface for SHELX-97. Data were corrected for Lorentzian and polarization effects but not absorption. The nonhydrogen atoms were refined anisotropically, and hydrogen atoms were included at geometrically estimated positions but not refined unless otherwise indicated. Crystallographic data are summarized in Table 1. Crystallographic data (excluding structure factors) for the structures reported have been deposited with the Cambridge Crystallographic Data Centre with reference numbers CCDC 283626-283633. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax (+44) 1223-336-033; e-mail [email protected]).

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