Chinese Chemical Letters  2018, Vol. 29 Issue (11): 1560-1566   PDF    
Cucurbit[10]uril-based chemistry
Xiran Yang, Fengbo Liu, Zhiyong Zhao, Feng Liang, Haijun Zhang, Simin Liu    
The State Key Laboratory of Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
Abstract: Cucurbit[n]urils (CB[n]s, n=5-8, 10, 13-15) have been proven to be an interesting macrocyclic family with their distinctive molecular recognition properties and fascinating applications in fields such as catalysis, supramolecular materials, drug delivery and biological systems. With the biggest cavity in the CB[n]s family, CB[10] shows its unique molecular recognition properties that are rather distinctive from other CB[n]s. In this review, we summarize the progresses in CB[10]-based chemistry since CB[10] was first reported as CB[10]·CB[5] complex in 2001. Purification, molecular recognition and supramolecular assembly of CB[10] will be described and an outlook will be given for further exploration of CB[10]-based chemistry.
Keywords: Cucurbit[10]uril     Supramolecular chemistry     Host-guest interaction     Molecular recognition     Supramolecular assembly    
1. Introduction

Cucurbit[n]urils (CB[n]s, n = 5-8, 10) (Fig. 1) are a kind of macrocyclic hosts developing rapidly in recent years since CB[n] homologues were reported [1-5] and have played important roles in supramolecular host-guest chemistry [6-8]. Compared to other hosts such as crown ethers, cyclodextrins and calixarenes, CB[n]s are endowed with high selectivity [9] and affinity [10, 11] towards guests due to the combination of negatively-charged carbonyl rims and hydrophobic cavity. On one hand, the negatively-charged carbonyl rims provide binding sites for non-covalent interactions including ion-dipole interaction and hydrogen bonding interaction. On the other hand, the inner hydrophobic cavity surrounded by glycoluril units is favorable for holding hydrophobic groups/ neutral molecules. There are numerous reviews focusing on different aspects of the CB[n]s chemistry, such as synthesis [12], molecular recognition [8], supramolecular catalysis [10, 13], supramolecular polymers/materials [14-16], drug delivery and biomedical systems [17-22], and other potential applications in supramolecular chemistry [23, 24].

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Fig. 1. Structures of CB[n] (n = 5-8, 10) (left) and the space-filling model of CB[10] (right).

Although recent years CB[n]s (n = 13-15) have been reported with more number of glycoluril units than that of CB[10], their cavities are not bigger than that of CB[8] due to their "twisted" conformations [5]. So CB[10] still possesses the largest cavity with a Dnh symmetry in the CB[n]s family. The big cavity volume of ~ 870 Å3 and portal diameter of ~ 10 Å (Table 1) [4] impart CB[10] with outstanding properties of molecular recognition and practical applications. In this review, we summarize the recent progresses in CB[10]-based chemistry since CB[10] was first reported as CB[10]·CB[5] complex in 2001, mainly focusing on the purification, molecular recognition and supramolecular assembly of CB[10]. Finally, the prospects and challenges of CB[10] are extensively discussed.

Table 1
Dimensions parameters and solubility of CB[n] in water [7, 27].

2. Purification of CB[10]

In 2001, Day and coworkersreported the separation of CB[10]·CB[5] complex from CB[n]s mixture [3]. And in 2002, they reported the structure of CB[10]·CB[5] complex: gyroscane-liked supramolecular complex (Scheme 1a) [25]. In the complex, CB[5] is encapsulated inside CB[10] and one Cl- anion is located in the cavity of CB[5]. The two macrocyclic hosts are concentric but noncoaxial in the crystal. When potassium chloride was used in the synthesis of CB[n]s, Liu and coworkers found CB[10]·CB[5] inclusion complex is stabilized by potassium ion coordination [26]. Meanwhile, the structure of CB[10] in this inclusion complex is more severe distortion and one water molecule instead of one Cl- anion is encapsulated by CB[5]. Day and coworkers also found that CB[5] freely gets in or out of the cavity of CB[10], although the association constant of CB[10]·CB[5] complex is large (Ka > 106 L/mol), implying that CB[5] could be removed from the complex [25].

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Scheme 1. Schematic illustration of the purification of CB[10] through competitive complexation with guest 1. Reproduced with permission [27]. Copyright 2009, The Royal Society of Chemistry.

By means of competitive complexation, pure CB[10] was isolated by the groups of Isaacs [4] and Day [28] from pure CB[10]·CB[5] complex. In 2005, synthetic melamine derivative (compound 1 in Scheme 1) was used as competitive guest to displace CB[5] from CB[10]·CB[5] complex [4], resulting in the formation of water-soluble inclusion complex CB[10]·12 and insoluble exclusion complex CB[5]·1. Binary inclusion complex CB[10] ·1 was given by washing CB[10]·12 with MeOH. Pure CB[10] was finally obtained after reacting CB[10]·1 with acetic anhydride followed by washing with DMSO, MeOH and water. CB[5] can also be displaced by 1, 12-diaminododecane (2) from the CB[10]·CB[5] complex upon refluxing CB[10]·CB[5] and 2 in 1 mol/L HCl/90% HCOOH (1:1), resulting in the formation of CB[10]·2 as insoluble complex. Pure CB[10] was available by repeatedly refluxing CB[10]·2 in 2 mol/L sodium hydroxide/methanol solution and the subsequent recrystallization in 9 mol/L HCl [28].

3. Molecular recognition of CB[10]

From Table 1 we can see that the cavity volume of CB[10] is the triple of that of CB[7] and almost the double of that of CB[8], implying that CB[10] can accommodate larger guest or multicomponent guests than CB[8] does. Besides CB[5], others compounds shown in Fig. 2 can also be encapsulated inside the cavity of CB[10] to form 1:1 or 2:1 inclusion complexes.

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Fig. 2. Chemical structures of guests for the binding with CB[10].

3.1. Towards cyclic host molecules

In 2005, Isaacs and coworkers reported the isolation of CB[10] and also preliminarily disclosed its unusual recognition properties [4]. They found that cationic calix[4]arene derivative (compound 3 in Fig. 2a) was encapsulated by CB[10] to form a 1:1 water-soluble complex CB[10]·3. The conformations of 3 in the complex were in dynamic equilibrium with 1, 3-alt-3 as the main conformer. Addition of 1-adamantanecarboxylic acid (compound 4 in Scheme 2) to the complex CB[10]·3 led to the formation of ternary complex CB[10]·3·4. Meanwhile, all conformers of 3 in the complex CB[10]·3 turned into corn conformation in the complex CB[10]·3·4. Of note, calix[4]arene 3 could not bind to 4 alone. The allosteric effects was further proved reversible upon addition of CB[7] as stronger competitive host for binding 4 (Scheme 2). This work suggested that CB[10] might find applications in control over biological allostery and constructions of molecular machines.

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Scheme 2. Schematic illustration of allosteric control over the conformations of CB[10]·3 with 4 and CB[7]. Reproduced with permission [4]. Copyright 2005, American Chemical Society.

In 2016, our group reported the encapsulation/assembly of a series of bipyridinium guests with CB[10] [29]. Among these guests, blue box (compound 5 in Fig. 2a), a known building block of supramolecular assembly pioneered by the Stoddart group [30, 31], formed stable 1:1 inclusion complex CB[10]·5 with CB[10] (Fig. 3) with an association constant more than 107 L/mol. X-ray crystal structure of CB[10]·5 reveals that two bipyridinium units of 5 were tilted inside the cavity of CB[10] with dihedral angle between the mean plane of 5 and the equatorial plane of CB[10] as ~ 59.6°. A rectangular channel through the two host molecules of ~ 3.0 × 4.6 Å dimensions suggested that smaller guest could go through the cavities of two hosts. It is worth to note that the encapsulation of 5 does not obviously influence its molecular recognition properties towards electron-rich guests, which might be explained by the dynamic binding and invariability of the rigid structure of 5. Our group also found that cryptand (compound 6 in Fig. 2a) can be encapsulated by the cavity of CB[10] [32].

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Fig. 3. X-ray crystal structures of the inclusion complex CB[10]·5: (a) side view; (b) top view. Reproduced with permission [29]. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA.

3.2. Towards large-sized guests

With similar structure to other CB homologues, CB[10] can also bind metallic cations, organic cations and neutral molecules. Specifically, with extremely larger cavity than other homologues, CB[10] is capable of binding large-sized guests such as [1, 1'-binaphthalene]-2, 2'-diamine and tetralysine (compounds 7 and 8 in Fig. 2b). Interestingly, in 6 mol/L HCl 3, 5-dimethyl-1-adamantanamine (compound 9 in Fig. 2b) not only displaced CB[5] from CB[10]·5, but also formed precipitate with CB[10] [4], suggesting that extraction of CB[10] from the crude CB[n]s mixture might be possible.

In 2008, Isaacs and coworkers reported the host-guest interaction between porphyrins/metal-porphyrins (compounds 10 and 11 in Fig. 2b) and CB[10] [33, 34]. Upon mixing, 10 or 11 (ca. 15.3 Å) formed stable binary complexes with CB[10] in aqueous solutions, with flattening of the cavity of CB[10]. Ternary complexes were also constructed with CB[10], 11 and a series of derivatives of pyridine and quinoline, suggesting that the ternary complexation benefited from π-π stacking interactions rather than the metal-ligand coordination interactions. Furthermore, fundamental photophysical and electrochemical properties of 10 and 11 were retained even when they were trapped in the cavity of CB[10]. These might find applications in constructions of enzyme-mimetic catalysts, targeted phototherapeutic agents, light-harvesting materials or photovoltaic devices.

3.3. Towards (potential) drugs

Combined with large cavity, low toxicity of CB[n]s and obvious improvement in solubility and stability of drugs, CB[7] and CB[8] have shown satisfactory feasibility as drug container [17-19, 35]. For example, in 2005 Kim and coworkers reported CB[7] as drug carrier by the encapsulation of an anticancer drug-Oxaliplatin and showed a large enhancement in the stability of Oxaliplatin [36]. With a larger hydrophobic cavity, CB[10] holds potential applications as carrier of crucial drugs that CB[7] or CB[8] can not encapsulate.

For example, CB[10] was used by Day, Collins and coworkers as delivery vehicle of dinuclear platinum (Ⅱ) complex CT008 (compound 13 in Fig. 2c) and ruthenium (Ⅱ) complexes (compounds 14 and 15 in Fig. 2c) [28]. Upon complexation with CB[10], 13 underwent conformer folding similar to that in the case of CB[8] [37]. Furthermore, with deeper inclusion in hydrophobic cavity, 13 could be better protected by CB[10] from the attack of thiol nucleophiles than by CB[7] and CB[8]. On the contrary, 14 and 15 showed slow exchange kinetics on the 1H NMR time scale upon binding with CB[10] (Fig. 4). Specifically, it took approximately 24 h to give sharp resonances of complexed 14 on 1H NMR and even several days to release it. The CB[10]·14 complex was also confirmed to be stable (Ka was about 1.9 × 109 L/mol) even under high ionic strength, which was of significance for physiological systems. Therefore, CB[10] can be an ideal delivery vehicle for delivery and controlled release of potential anti-cancer drugs.

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Fig. 4. Molecular model of the inclusion complex CB[10]·14: a) side view; b) top view. Reproduced with permission [28]. Copyright 2010, The Royal Society of Chemistry.

Researches were also performed by the groups of Keene, Collins and Day to investigate effects of encapsulation by CB[10] on the antimicrobial activity and toxicity of 15 [38, 39]. It was revealed that the encapsulation resulted in no increase in the proteinbinding resistance and the antimicrobial activity of 15, whereas led to a two-fold decrease in its toxicity.

3.4. Towards fluorescent cyclometalated complexes

It was normal that fluorescence of dye molecules can be enhanced or quenched upon encapsulation by host molecules [40]. As one of intriguing hosts, CB[n]s also play crucial role in fabrications of luminescence-tunable materials and find applications in molecular imaging [41-43]. In particular, the capacious cavity of CB[10] make it possible to include bulkier dyes. For example, complexation of a series of iridium (Ⅲ) polypyridyl complexes (compounds 16-19 in Fig. 2d) and CB[10] was investigated by Wallace and coworkers [44, 45]. It was suggested that there was obvious enhancement in the luminescence of these cyclometalated complexes with blue shift (such as CB[10]·18 in Fig. 5). With the increasing availability of CB[10], the binding model of CB[10] and these complexes turned from 1:2 to 1:1 with binding constants more than 106 L/mol in MeCN medium [45]. These results might provide new approaches to intriguing fields of organometal-based materials.

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Fig. 5. a) Molecular representation of CB[10]·18 inclusion complex. b) buffer solutions (pH 4.7 at 22 ℃) of free 18 (left) and CB[10]·18 (right) at concentration of [18] =4 × 10-5 mol/L. Reproduced with permission [45]. Copyright 2013, The Royal Society of Chemistry.

4. CB[10]-mediated assembly

Inspired by natural oligomeric macromolecules, proteins and nucleic acids with precise folding of their conformations, chemists have been attracted to design and constructed non-natural oligomers and foldamers through non-covalent interactions.

In 2007, CB[10] was used by Isaacs and coworkers to fabricate refolding foldamers with triazene-arylene oligomers (compounds 20-24, Fig. 6a) and construct self-sorting systems on the base of competitive complexation [46]. There is no dominant well-defined conformation for oligomers which have 10 unique conformations in aqueous solution. However, the presence of individual CB homologues led to single conformation of oligomers. For instance, ternary complexes were formed between CB[7] and oligomers with the a, s, s, a-conformer in a host-gest ratio of 2:1. In the case for CB [8], oligomers exclusively adopted a, a, a, s-conformation in the cavity forming binary complexes with CB[8]. For CB[10], oligomers exclusively adopted the a, a, a, a-conformation in the cavity of CB[10] forming binary complexes (Fig. 6b). It was also demonstrated that oligomer 20 changed shapes in response to chemical stimuli (Scheme 3). In the process of the self-sorting systems, the conformation of 20 changed accordingly upon the addition of different hosts and competitive guests.

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Fig. 6. a) Structures of triazene-arylene oligomers; b) X-ray crystal structures of CB[10]·a, a, a, a-21. Reproduced with permission [46]. Copyright 2007, American Chemical Society.

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Scheme 3. Schematic illustration of the change of conformation of 20 (purple strand) in response to chemical stimuli. Reproduced with permission [46]. Copyright 2007, American Chemical Society.

In order to further explore the molecular recognition properties of CB[10], our group investigated the encapsulation/assembly of bipyridinium molecules and electron-rich compounds inside CB[10]·[29]. We found CB[10] can hold two methylviologen (MV2+, compound 26 in Scheme 4) molecules, which could not be observed in the case of CB[8] due to electrostatic repulsion between two 26. In the presence of CB[10], the weak chargetransfer interaction between 26 and electron-rich compound 2, 6- dihydroxynaphthalene (HN) was tremendously enhanced, forming "packed sandwich" assembly-CB[10]·262·HN. Mixing of bis(bipyridinium) (compound 27 in Scheme 4) and CB[10] only led to the formation of a 1:2 complex (CB[10]2·27). However, upon addition of electron-rich catechol, ternary complex CB[10]·27·catechol was formed with 27 switched from linear-shape to U-shape conformation (Scheme 4b). More interestingly, in combination of chargetransfer interactions and hydrophobic effects, "Russian doll" assembly composed of CB[10], 5 and 1, 5-bis[2-(2-hydroxyethoxy)ethoxy]naphthalene (BHEEN) was realized (Scheme 4c). Therefore, it will be great attractions to construct more interesting molecular machine with the large cavity of CB[10].

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Scheme 4. Schematic illustration of the formation of inclusion complexes between pyridinium guests and CB[10]. Reproduced with permission [29]. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA.

In 2016, CB[n]s (n = 7, 8, 10) were used by Li and coworkers to tune the pleating state of bipyridinium radical cation (BIPY+●)-incorporated polymers [47]. Upon reduction of the BIPY2+-based polymers (polymers 28-31 in Scheme 5) with sodium dithionite, pleated sheets (State A in Scheme 5) were formed driven by the rigidity and the inter-/intramolecular stacking of BIPY+● units [48]. However, upon the addition of CB[n]s (n = 7, 8, 10), the pleated polymers will be switched from state A to other states (states B-D in Scheme 5) with less pleating due to the weakened stacking interactions. Different binding models dominated for each state when each CB[n] (n = 7, 8, 10) with different size of cavity was introduced: forming state B in 1:1 complexation with CB[7]; forming state C in 1:2 complexation with CB[8] and state D in 1:3 complexation with CB[10] and BIPY+● units. Switching between different states could lead to the weakening or enhancement of the absorption of (BIPY+●)2. It was also demonstrated that these processes were reversible by the addition of strong competitive guests, including 1-adamantammonium chloride (compound 25 in Scheme 3), hexamethylenetetramine (HMTA, compound 33 in Scheme 5) and CB[5] for state B-C, respectively. These results show potential applications in the design of tunable photomaterials and molecular devices or machines.

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Scheme 5. Schematic illustration of the reversible switching of different pleating states of polymers 30 and 31 upon addition of CB[n]s (n = 7, 8, 10) and competitive guests. Reproduced with permission [47]. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA.

With CB[10] and a dumbbell-like guest 1, 1-ditetraphenylethenyl-4, 4'-bipyridine-1, 1'-diium dichloride (compound 34 in Scheme 6), Cao and coworkers prepared [2]rotaxane exhibiting a stepwise aggregation-induced emission enhancement (AIEE) effect in DMSO and THF/CHCl3 [49]. Upon mixing with CB[10] in DMSO at high temperature (95 ℃) for more than half a month, 34 threaded reversibly into the cavity of CB[10]. The assembly was further stabilized by ion-dipole interaction between N atoms on the 4, 4'-bipyridine-1, 1'-ium unit and the carbonyl rims of CB[10]. The resulting [2]rotaxane underwent increasing stacking accompanied by the restriction of intramolecular rotation (RIR) of tetraphenylethylene (TPE) on 34, leading to the stepwise AIEE effect with various sizes in different solvents (Scheme 6). By contrast, there were no effects on the fluorescence intensity of 34 in the presence of smaller CB[n]s (n = 6, 7, 8) by which 34 could not be encapsulated. These findings may have potential applications in luminescent materials with stimuli-responsiveness.

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Scheme 6. Schematic illustrations of a stepwise AIEE of CB[10]·34. Reproduced with permission [49]. Copyright 2017, American Chemical Society.

Tao and coworkers had been engaged in the fabrications of various CB[n]-based supramolecular architectures and functional materials in the presence of structure-directing agents, by taking advantage of coordination properties of CB[n]s towards metal cations [50, 51]. In 2017, Tao and coworkers demonstrated the supramolecular assemblies of CB[10] with [CdCl4]2- anions in 4 mol/L HCl [52]. Crystal structures showed perfectly-aligned 1D macrocycle nanotubes along the c-axis (Fig. 7). Each CB[10] molecule was surrounded by eight [CdCl4]2- anions through Cl…H interactions (Fig. 7c) and each [CdCl4]2- anion connected with four CB[10] molecules in the same way (Fig. 7d). However, no equivalent structure-directing effect was observed in HNO3 solution where Cd2+ cations did not form structure-directing [Cd(NO3)4]2- anions and instead CB[10]-based porous layer was formed. The CB[10]-based supramolecular assemblies may be promising as materials for sensing, absorption and separation technologies.

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Fig. 7. Crystal structures of CB[10-CdCl4]2- assemblies: a) and b) along the c-axis; c) detailed structure of CB[10] surrounded by eight [CdCl4]2- anions; d) detailed structure of [CdCl4]2- anion with four CB[10]. Reproduced with permission [52]. Copyright 2017, CSIRO Publishing.

Porous materials have been widely studied and used in heterogeneous catalysis, gas adsorption, drug delivery and energy materials, exhibiting high surface areas, great stability, excellent sorption ability and high catalytic activity. Among various kinds of porous materials, supramolecular organic frameworks (SOFs) have attracted considerable interest due to their enhanced capacity of encapsulation and dynamic reversibility [15]. In 2017, smart SOFs were constructed by Ni and coworkers with CB[10] and applied to selective isolation of metal cations [53]. In 6 mol/L HCl solution, CB[10] quantitatively formed precipitates of CB[10]-Mn+ with metal cations (alkali A+, alkaline earth AE+ and lanthanides Ln3+) and single crystals of CB[10]-Mn+ complexes were successfully collected sequentially. As demonstrated, the CB[10]-Ba2+-based 2D coordination network assembled mainly through the outersurface interaction of CB[10] with Ba2+ cations (Fig. 8a). Each CB[10] molecule coordinated with four Ba2+ cations and each Ba2+ cation coordinated with eight oxygen atoms (Fig. 8c). Specifically, two Ba2+ cations coordinated directly to the portal carbonyl oxygen atoms of two adjacent CB[10] in the triangular structure composed of three CB[10]. Additionally, in HNO3 solution was successfully constructed the free CB[10]-based SOFs bearing porous layers with square holes. Similar to work from Tao and coworkers [52], CB[10]- Mn+ complexes could be reversibly precipitated in 6 mol/L HCl whereas dissolved in 3 mol/L HNO3, providing approach to control over the porous size. Practically, these CB[10]-based SOFs exhibited sequence selectivity isolation of specific metal cations and show potential inmcooln/LstrHuCclting metal-selective materials. Very recently, the same group developed a simple way for the construction of a novel CB[10-Cd4Cl16]8--based porous supramolecular framework by mixing CB[10] and CdCl2 in HCl solution [54]. Further investigation proved that various organic dyes can be effectively adsorbed by this porous materialwith high quantum yield and great thermal stability.

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Fig. 8. Crystal structures of a) the CB[10]-Ba2+-based 2D network; b) the CB[10]-Gd3+-based 2D network; c) and d) detailed structure of three CB[10] molecules in a triangular branch with four metal cations. Reproduced with permission [53]. Copyright 2017, American Chemical Society.

5. Conclusions and outlooks

With the biggest cavity in the CB[n]s family, CB[10] has shown its unique molecular recognition properties. Polynary inclusion complexes with sophisticated structures can be constructed with CB[10] and many intriguing guests. Large-sized guests, such as porphyrins, calix[4]arene, azacryptands, cyclophanes (including blue box) and cyclometalated complexes, can be encapsulated in the capacious cavity of CB[10]. Those findings suggest that CB[10] can be used as an advanced building component in the areas such as supramolecular functional materials, catalysis, drug delivery and biological systems.

So far, there are only ~ 17 research articles involving CB[10], which is much less than those involving CB[7] and CB[8] discovered at the same period as CB[10]. The low yield of CB[10]·CB[5] and difficult isolation of pure CB[10] are great challenges for the development of CB[10]. Very recently our group developed a facile and highly efficient way to isolate CB[10] from CB[n] mixtures [55]. We believe that this will be great promotion for the further explorations of CB[10] chemistry.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 21472143, 21604066 and 21372183), Thousand Youth Talents Program of China (No. D1118031), and Program for Innovative Teams of Outstanding Young and Middle-aged Researchers in the Higher Education Institutions of Hubei Province (No. T201602).

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