Hierarchical Self-Assembly of Lactams into Supramolecular CO-Spiked

These supramolecular objects shaped like spiked sea urchins further pack as a porous crystal whose interconnected channels occupy 24% of the whole ...
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DOI: 10.1021/cg1003935

Hierarchical Self-Assembly of Lactams into Supramolecular CO-Spiked “Sea Urchins” and Then into a Channeled Crystal

2010, Vol. 10 4357–4362

Pierre Baillargeon,† Daniel Fortin,‡ and Yves L. Dory*,† †

Laboratoire de synth ese supramol eculaire,, and ‡Laboratoire de cristallographie, D epartement de chimie, Institut de Pharmacologie, Universit e de Sherbrooke, 3001, 12e avenue nord, Sherbrooke Qc J1H 5N4, Canada Received March 24, 2010; Revised Manuscript Received July 22, 2010

ABSTRACT: Cobalt complexation of the two alkynes of a de novo designed dimeric lactam triggers a complete reorganization of the lactam ring through formation of a β-turn-like motif. Self-assembly with concomitant induced fit of six monomers yields a globular quaternary structure shaped like an ellipsoid. This final architecture generates a peculiar molecular electrostatic potential field, with both pole caps being negatively charged while the equatorial region is mostly positive. Further packing, at least partially controlled by these electrostatic effects, leads to a superquaternary structure, a porous crystal, in which the many intercommunicating negatively charged channels occupy as much as 24% of the whole crystal.

Introduction In self-assembly as observed in nature, an ordered structure emerges spontaneously from disorder.1 Scientists have long been interested in studying and reproducing this amazing phenomenon in their laboratories for various reasons and purposes. This kind of research can be purely driven by curiosity to grasp a deeper understanding of life, since all living organisms are in essence complex supramolecular associations of lifeless components.2 Moreover, copying this bottomup approach is also very appealing to material science engineers and researchers to craft novel materials of the future endowed with specific properties and functions.3 Synthetic or artificial protein-like self-assembly4 requires the design of a primary structure that can fold to yield successively secondary and then tertiary structures before self-organizing as quaternary and even superquaternary structures. In such a process the only human intervention remains the conception of a 2D structure encoded with the information necessary to predetermine the overall structure of the final supramolecule.5 Some coordination driven synthetic supramolecules already exist.6 Nevertheless, these objects take advantage of much stronger bonds than those usually found in natural biomolecules; they are consequently less flexible systems. On the other hand, some molecular capsules, held together with true weak interactions, can mimic viral protein capsids.7 However, all fully synthetic objects remain incredibly small in terms of sheer size and number of units by comparison with average natural architectures. Indeed, many natural globular proteins are symmetric oligomers constituted of as many as 60 units in icosahedral viral capsids.8 These objects can reach extremely large dimensions, with diameters of up to 350 nm in the case of the giant mimivirus capsid;9 even the dwarf satellite panicum mosaic virus capsid is 16 nm wide. Dimers, tetramers, hexamers, and octamers are commonly encountered clusters among which numerous metalloproteins can be found.10 Insulin, for instance, can exist either as a monomer (5.7 kDa) or as a hexamer in which two zinc atoms are coordinated, with its total weight being 34.6 kDa (with 6 H2O).11 *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

We have designed and synthesized a simple flexible dimeric lactam 112 (Scheme 1) whose complexation with Co2(CO)8 triggers a drastic conformational change to yield the rigid macrocycle 2 (846 g/mol) decorated with four cobalt atoms.13 2 self-assembles as a 5.1 KDa S6 symmetric hexamer 26 through van der Waals interactions, hydrogen bonds, and carbonyl-carbonyl weak interactions. This quaternary structure, shaped like an ellipsoid reminiscent of an M&M’s candy,14 further packs as a channeled crystal (Figure 1).15,16 Results and Discussion On its own, macrolactam 1 is unable to form an intramolecular hydrogen bond because the two rigid alkyne arms hold the two amide partners too far apart in a parallel or antiparallel (5.1 A˚) fashion (Figure 1a).12 1 self-assembles as a square grid (2,2)-net resembling a supramolecular wall built with six distinct conformers. On complexation with Co2(CO)8, the four sp carbons become sp3 hybridized; the macrocycle gains temporarily some flexibility until the two amides recognize each other and become locked into an intramolecular β-turnlike hydrogen bond. This hydrogen bond belongs to two rings with nine and eleven atoms instead of ten in a usual β-turn arrangement. Each of these rings harbors one of the two Co2(CO)6 moieties, which are held separated from each other at opposite ends of the molecule. As a result, 2 displays two distinctive regions: the ends that are covered with CO spikes, the cobalt ligands, and the middle part, whose surface is constituted by amides and methyl groups. The two amides define a plane angle R of 73°, which leads to a distance of 2.03 A˚ for the intramolecular hydrogen bond. The molecule is rather polar with a calculated dipole of 5.9 D. The β-turn like pattern and the overall 3D geometry of 2 can be compared with the secondary and the tertiary structure of a protein, respectively. Since there remain two orphan hydrogen bonding sites, a quaternary structure is expected. These unfulfilled sites are deeply buried, preventing dimers and even tetramer to assemble. In fact, the shape and the molecular electrostatic potential of the units are ideally preformed to induce self-assembly of 2 as a hexamer (Figure 1b). Published on Web 08/17/2010

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Figure 1. Graphical abstract of the whole process. (a) Variations of geometry in 1 and 2. (b) Relaxed stereoview of the hexameric S6 ellipsoid 26. H atoms are omitted, and Co2(CO)6 groups have been replaced by spheres for clarity. (c) Packing of several 26 sea urchin or chestnut units in the crystal.

Scheme 1. Synthesis of 2 and 4a

a

Crystals were obtained by slow vapor diffusion of hexane into solutions of 2 and 4 in methyl ethyl ketone and dichloromethane, respectively.

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Figure 2. X-ray crystal structures of dimers of 3 and 4 held together by hydrogen bonds (yellow dashed lines). Intramolecular bonds (red dashed lines) reminiscent of β-turns are into 11- and 9-membered rings (italics) for 3 and 4, respectively. H atoms are omitted and Co2(CO)6 groups have been replaced by spheres for clarity.

However, during the assembly process, 2 modifies its shape slightly and the internal hydrogen bond distance shrinks to 1.82 A˚, with concomitant opening of the plane angle of the amides (R = 88° by calculations; 83° in crystal) as well as consequential increase of the dipole to 7.1 D to meet induced fit requirements. The integrity of the resulting hexameric ellipsoid is maintained by a strong belt of twelve alternating intramolecular and intermolecular hydrogen bonds. These normally fragile bonds are efficiently shielded from all exterior solvents or other chemical species by CO cobalt ligands and methyl groups. Since the hydrogen bonding sites are located in the central region of each monomer, half of the Co2(CO)6 groups cram at both poles of the ellipsoid, whereas the remaining six fill the equatorial/tropical region, where all gem dimethyl groups can also be found. On the whole, the final object can be described as a supramolecular sea urchin or chestnut whose spikes are CO ligands (Figure 1c). The trimeric analog 3, which self-assembles as a supramolecular wall like 1,12 reacts with Co2(CO)8 too, producing 4 with three Co2(CO)6 groups (Scheme 1). Nevertheless, the larger and also more flexible 21-membered macrocycle 3 manages to absorb the sp-to-sp3 transitions that occur at six carbon centers to yield 4. As a result, there is no drastic structural shift during the complexation process, contrarily to the 1-to-2 metamorphosis. 4 self-assembles as dimers held rigid by means of two conventional intermolecular and two

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Figure 3. Comparison of crystal (green) and optimized (red) structures of 26 (a) and 42 (b).

conventional intramolecular hydrogen bonds (Figure 2).17 This description also fits free macrocycle 30 s appearance.12 High level B3LYP/SBKJC DFT calculations18 were performed on the S6 crystal hexamer in order to get better insight into the driving force at work during the association process besides the obvious hydrogen bonds. Since the calculated final geometry remained virtually identical to the crystal input geometry (confirmed for Ci symmetric dimer 4), the DFT method proved reliable and accurate (Figure 3). A special 3D molecular electrostatic potential (MEP) map19 was revealed with MacMolPlt.20 The equatorial region of the hexameric ellipsoid is surrounded by a mild positive electric field, whereas both polar areas are strongly negative electrostatic potential regions (Figure 4a). Owing to the S6 symmetry of the system, the gross dipole is obviously nil. Careful study of the global molecular electrostatic potential indicates that all monomers act in a synergistic way, owing to their relative positions in the hexamer. On its own, each monomer 2 exerts an expected electrostatic potential around its various groups (Figure 4b). The free amide oxygen generates a neighboring region prone to attract positively charged species or a free amide hydrogen atom from another monomer; the same effect is observed, although to a lesser extent, around the carbonyl ligands bound to the cobalt atoms. It is noteworthy that these carbonyl ligands affect the electric field with different intensities depending on their location on the cobalt atoms. A DFT theoretical survey of a model acetylenic dicobalt hexacarbonyl complex 5 clearly establishes which

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Figure 5. Positive (yellow) and negative (blue) isosurfaces (isovalues in italics) of the electrostatic potentials of 5 (a) and 512 (b) in which all heavy atoms occupy exactly the same positions as in 26.

Figure 4. Positive (yellow) and negative (blue) isosurfaces (isovalues in italics) of the electrostatic potentials of 26 (a) and 2 (b).

carbonyls are more influential (Figure 5a). The two carbonyls syn to the complexed dimethyl alkyne are much less potent field inducers than the four anti carbonyls. This trend is still present in 2 (Figure 4b). We proceeded to analyze the way the 12 Co2(CO)6 groups organize in the hexamer 26 and how they might interact with one another. Since all 12 amides are involved in hydrogen bonds, their contribution to the electrostatic potential is not significant (Figure 6). Therefore, we could remove them all from our calculations to end up with a cluster 512 made of 12 acetylene dicobalt hexacarbonyl complexes 5 whose geometries were frozen as in the original hexamer 26. We could then observe that the overall MEP maps of 26 and 512 can be superimposed (Figure 4a and 5b). The MEP map of 26 is definitely a result of the relative positions and orientations of the 12 Co2(CO)6 moieties. Each unit 2 sends one of its two Co2(CO)6 groups near one pole of the ellipsoid and the other

Co2(CO)6 group around the tropical region of the opposite ellipsoid hemisphere (Figure 1b). Each hemisphere contains six complexes 5 whose individual dipole contribution parallel to the ellipsoid axis (along the z direction) is either the major component or nearly so. The calculated dipole moment for one hemispheric hexameric cluster 56 is 15.5 D. The same description applies to the opposite hemispheric hexamer 56, and both hemisphere dipoles cancel each other out. Nevertheless, both polar regions are covered with strong negative MEP caps whose effect can extend far away in space and somehow direct further ellipsoid packing. A MEP negative polar cap is more accurately described as a 3-armed star such that the whole ellipsoid MEP map looks like a bis-conical staggered cogwheel (Figure 5a). The negative ridges and the positive valleys found on the surface of neighboring ellipsoids are mutually complementary. This particular feature can be held entirely responsible for the very precise packing of the ellipsoids 26, which defines the superquaternary structure as found in the crystal. Gas phase calculations allowed us to decompose the energy of the architecture into components from hydrogen bonds, van der Waals interactions, and Co2(CO)6 carbonyl-carbonyl interactions. Thus, the estimated total gain of energy that results from assembly of six units 2 into the hexamer 26

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Figure 6. Positive (yellow) and negative (blue) isosurfaces (isovalue of 0.07) of the electrostatic potential of the belt of hydrogen bonds as found in 26. Each unit (#1-6) is a pair of AcNHMe moieties corresponding to an original lactam 2.

Figure 8. Relaxed stereoviews of six hexamers 26 (CPK, surface) showing their packing and the oxygen-rich channels.

Figure 7. (a) Interconnected empty channels within a unit cell. (b) Photo of a crystal (length unit: mm).

amounts to 106 kcal.mol-1 (E.26 - 6  E.2): most of this energy (89 kcal 3 mol-1) originates from hydrogen bonds (around 15 kcal 3 mol-1 per intermolecular hydrogen bond), a little (10 kcal 3 mol-1) comes from carbonyl-carbonyl interactions,21 and the remaining (7 kcal 3 mol-1) should result mainly from van der Waals forces. The large energy values associated with hydrogen bonding presumably come from cooperativity effects.22 These figures suggest that the shape of the architecture is driven by hydrogen bonds, whereas all other weaker forces accommodate the hydrogen bonding geometrical requirements. Thus, peptide hydrogen bonds can

override other forces and be used to induce the formation of desired quaternary structures. To summarize, the peculiar MEP map created by 26 arises though synergetic mixing of the individual contributions. In this new arrangement the twelve acetylenic dicobalt hexacarbonyl moieties are held in precise positions, allowing most of the carbonyl ligands to build up electron density at both poles of the ellipsoid through carbonyl-carbonyl interactions. Finally, a superquaternary assemblage can be observed within the crystal lattice. Packing of the ellipsoids follows the expected trend for such spheroids: a hexagonal lattice that leads to the densest ellipsoid packing with an observed empty volume of 24% and close to theory.14 The many intercommunicating channels are all under the influence of the electric field generated by the ellipsoids (Figures 7a and 8). Despite its porosity, the crystal architecture proved very strong; the acquisition of the X-ray data could even be carried out on previously dried out crystals which do not require mother liquor for stability. The crystal unit cell is surprisingly large and compares easily with small proteins’.15 Its large Rfactor (0.200) may result from inclusion of some disordered molecules (fluid material like solvent or atmospheric gases) inside the channels, although the low density of the crystal suggests that there are not many such molecules trapped inside.23 The density of crystal 2 is indeed remarkably low (1.36, calculated and experimentally measured by the flotation method)24 by comparison with that of 4 (1.68) and other crystals from compounds containing similar proportions of C, O, N, H, and Co elements (average density of 1.69 out of 12 selected representative crystals).25 The rhombohedral shape

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of the crystal is a faithful macroscopic copy of the nanoscopic unit cell (Figure 7b). For each hexamer, the 12 amides and hydrogen bonds draw a wavy circle to which an arbitrary direction can be associated based on an identical orientation for all constitutive atoms OdC-N-H. This direction is absolutely the same for all hexamers in the crystal, as if they could influence each other (Figure 1c). As a result of this arrangement, the crystal is remarkably chiral and anisotropic, although it is constituted from lactams and even S6 hexamers originally devoid of chirality. Conclusions The self-assembly of the simple metalloprotein model 2 into a S6 symmetric globular protein 26 shows that the entropically disfavored formation of large multimers can be overcome by a strict control over hydrogen bonds. For example, partial internalization of amides can be used as a tool to prevent dimers or other small oligomers from forming by means of steric repulsions. By default, larger clusters, such as hexamers, become accessible if a favorable alignment is possible for CO and NH partners. This trick was used to force hexameric aggregation of 2, whereas 4 forms dimers due to the absence of such detrimental steric effects. Fulfillment of most, if not all, hydrogen bonding capacities appears to be a way to overcome other less favorable interactions. In this way, most CO ligands could gather on both poles of the ellipsoid, leading to an intriguing MEP map that can be held responsible for the properties of the finally obtained material. It can be safely stated that the hexameric ellipsoid 26 is utterly different from its constituent 2, in the same way that an enzymatic active site is created at the interface of merging monomers that would otherwise remain inactive when kept separated. Rational design of self-assembled catalytic sites can be imagined following this precursory work. Acknowledgment. We thank RQCHP (Reseau Quebecois de Calcul de Haute Performance) for providing computa tional facilities and time and CEMOPUS (Centre d’Etudes des Materiaux Optiques et Photoniques de l’Universite de Sherbrooke), CSACS (Centre for Self-Assembled Chemical Structures), CQMF (Centre Quebecois sur les Materiaux Fonctionnels), and NSERC (Natural Sciences and Engineering Research Council of Canada) for financial support. Supporting Information Available: Experimental procedures and spectral characterization data, crystallographic information files (CIF) for compounds 2 and 4, theoretical calculation details, movie files (avi) of crystal hexamer 26, and 3D MEP map of 56. This material is available free of charge via the Internet at http://pubs.acs.org.

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