Controlled nitrite anion encapsulation and release in the molecular cavity of decamethylcucurbit[5]uril: solution and solid state studies

Abstract

Nitrite anion encapsulation was realized using molecular cavitands of decamethylcucurbit[5]urils as molecular receptors. Single crystal structural analyses showed that the receptors formed sodium-capped molecular capsules that encapsulate nitrite anions inside the cavity. The inclusion entities were proved to be existing in both solution and the solid state. Moreover, the encapsulated nitrite anions could be released and decomposed in acidic solutions.

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Introduction

Anion encapsulation/inclusion is an important topic that has been a continuous research interest for its potential applications in anion extraction, transmembrane transportation, sensing, and so on.¹ Among various types of anion inclusions, encapsulation of nitrite anions has been far less studied, which is to our surprise given the importance of nitrites in biological systems.² Nitrite plays multiple roles in the public health field: it inhibits the growth of botulism-causing bacteria and prevents spoilage, especially in the meat curing industry.³ Nitrites comprise the largest storage pool for nitric oxide (NO, also known as free radicals), which is an important neurotransmitter that the body uses to control blood pressure, to kill tumor cells, and to heal wounds, as it is proved that nitric oxide produces as a result of nitrite anion reduction under physiological hypoxic conditions.⁴ However, the use of nitrite in cured food has caused considerable controversy due to the fact that it can be toxic to animals and humans at a high dosage, under the original intention of bacterial growth inhibition. A major concern for nitrites is the formation of hazardous carcinogenic and nitrosamines while overcooking preserved food.⁵ In response, practical measures have been taken to control the level of nitrite contents in preserved food, and gastric cancer mortality has declined significantly. Due to the controversial open issue and challenge on nitrite consumption, a building base of scientific evidence about nitrate and nitrite ions is crucial for knowledge accumulation related to nitrate/nitrite safety and human health.^4–6

As a matter of fact, the binding nature of nitrite anions (i.e. mode and properties) in artificial anion receptors has not been well-explored hitherto, with respect to the ever-increasing biologically related applications. The only artificial nitrite anion receptor documented so far is an anthracene based host containing two 1-methyl-1H-perimidine arms which, in DMSO solution, has shown selective nitrite reception utilizing aromatic C–H and aliphatic C–H as hydrogen bonding moieties.⁷ In this context, investigations targeting on the encapsulation of nitrite ions in aqueous solutions using molecular cavitands, i.e. cyclodextrins, calixarenes, pillararenes and cucurbiturils,⁸ would offer an alternative protocol to regulate the nitrite metabolism and to hamper the formation of carcinogenic nitrosamines in biological systems.

Among the numerous molecular cavitands, cucurbit[n]urils, especially decamethylcucurbit[5]uril (denoted as Me₁₀CB[5])⁹ have attracted our research interest as promising nitrite anion receptors, thanks to their excellent water solubility and, more importantly, sufficiently low toxicity that are key for their potential usage as additives in medicinal and pharmaceutical processes, as well as in food science under physiological conditions in humans or mammalians.¹⁰ Me₁₀CB[5] is one of the smallest members in the cucurbit[n]uril family, and similar to its structural analogue cucurbit[5]uril (CB[5]), it has a relatively small cyclic cavity structure with two identical portals decorated with carbonyl groups and permethyl-substituted equatorial outer faces. The molecular structure of Me₁₀CB[5] features ‘inner’ hydrophobic and ‘outer’ hydrophilic regions.¹¹ Of note, Me₁₀CB[5] has shown encapsulation capability towards various gases including Kr, Xe, N₂, O₂, Ar, N₂O, NO, CO, CO₂, and CH₄, as well as solvent molecules inside the cavity.¹² More recently, the anion encapsulation behavior of CB[5] has been reported by Liu et al., in which CB[5] exhibited selective encapsulation towards nitrate and chloride anions in the form of lanthanide-capped molecular capsules.¹³ In this article, we report the encapsulation behavior of Me₁₀CB[5], CB[5] and cucurbit[6]uril (CB[6]) towards nitrite anions in aqueous solutions and in the solid state. The host–guest interaction between cucurbiturils and nitrite is studied in detail by means of mass spectroscopy and single crystal X-ray crystallography. Research showed that Me₁₀CB[5] is a better nitrite anion receptor, it forms sodium-capped molecular capsules that encapsulate nitrite anions, and the inclusion entities exist both in solution and in the solid state. Moreover, UV-vis spectroscopy studies showed that the encapsulated nitrites can be released and decomposed in acidic solution. These results provide a promising alternative regulation method for nitrite related metabolism in biological systems.

Experimental

Chemicals and materials

All chemicals and reagents were purchased from commercial suppliers and used without further purification except for decamethylcucurbit[5]uril (Me₁₀CB[5]) which was prepared and purified according to the procedure reported in the literature.⁹

Characterization

Elemental analyses of C, H, and N were carried out on an Elementar Vario EL III analyzer. Infrared (IR) spectra were recorded using KBr pellets on a PerkinElmer Spectrum One in the range 400–4000 cm⁻¹. Thermal gravimetric analyses (TGA) were performed under a flow of N₂ (100 mL min⁻¹) with a heating rate of 10 °C min⁻¹ using a TA SDT-600 thermogravimetric analyzer. Aluminum oxide crucibles were used for all samples, and the instrument was calibrated using indium as the standard. An empty crucible was used as the reference. A Miniflex600 powder X-ray diffractometer (PXRD) was employed for all measurements with experimental parameters as follows: room temperature, Cu-Kα radiation (λ = 1.54056 Å), 2θ range 5–30°, step size 0.02° (scanning rate with 2θ in 1° min⁻¹). Mass spectroscopy analysis was performed on a Bruker Impact II UHR–TOF mass spectroscope. A water solution containing 1 : 10 equivalents of Me₁₀CB[5] (or its analogues) and sodium nitrite, with a concentration of about 1 mM, was electrosprayed and the m/z of cationic species was recorded.

Crystal preparation of [Na₂(H₂O)₂(NO₂@Me₁₀CB[5])]·NO₂·14.5H₂O (compound 1)

Me₁₀CB[5] (198 mg, 0.2 mmol) was dissolved in 10.0 ml distilled water, and NaNO₂ (138 mg, 2.0 mmol) was then added. After sonication and magnetic stirring, the clear solution, with a pH value of 7.0, was allowed to evaporate slowly at room temperature. Colourless crystals of compound 1 were obtained after three days. The phase purity and stability of the product were identified by PXRD (Fig. S1†) and TGA (Fig. S2†). Anal. calcd for C₄₀H₈₃N₂₂Na₂O_30.5 (%) (M = 1406.10): C, 34.14; H, 5.90; N, 21.90. Found (%): C, 34.67; H, 5.42; N, 22.26. IR (KBr, cm⁻¹): ν = 3450 (br, νH₂O), 2995 (w, νCH₃), 1741 [vs, ν(C=O), red shift compared to ν(C=O) on pure Me₁₀CB[5] at 1752], 1635 (w, δH₂O), 1477 (vs, δCH₂), 1406 (s), 1373 (m, δCH₃), 1309 (s), 1267 (m, NO₂⁻), 1197 (m, asymmetric stretching of C–N), 1164 (w, symmetric stretching of C–N), 1072 (vs), 964 (w, stretching of C–C), 912 (m), 877 (s), 833 (s, NO₂⁻), 759 (s, out-of-plane deformation of the glycoluril ring), 721 (m), 699 (s, out-of-plane deformation of the glycoluril ring). Crystallographic data of 1: monoclinic, space group P2/n; a = 10.9837(1) Å, b = 14.5848(1) Å, c = 19.3219(3) Å, β = 99.214(1)°; V = 3055.32(6) ų, Z = 2, GOOF = 1.040. Total 29 606 reflections measured (7.64° ≤ 2θ ≤ 146.82°), 6097 unique (R_int = 0.0231, R_sigma = 0.0155) which were used in all calculations. The final R₁ was 0.0727 (I ≥ 2σ(I)) and wR₂ was 0.2061 (all data).

Crystal preparation of {Na₂(H₂O)₁₀CB[6]}²⁺·3(H₂O)·2(NO₂⁻), (compound 2)

CB[6] (200 mg, ∼0.2 mmol) was dissolved in 10.0 ml distilled water, and NaNO₂ (138 mg, 2.0 mmol) was then added. After sonication and magnetic stirring, the clear up-layer solution was transferred to a small glass vial and was allowed to evaporate slowly at room temperature. Colorless crystals of compound 2 were obtained after two weeks. Anal. calcd (%) for C₃₆H₆₂N₂₆Na₂O₂₉ (M = 1369.01): C, 31.56; H, 4.53; N, 26.59. Found (%): C, 31.67; H, 4.98; N, 26.36. Crystallographic data of 2: orthorhombic, space group Pna2₁; a = 31.578(5) Å, b = 14.455(2) Å, c = 12.0570(17) Å, V = 5503.5(13) ų, Z = 4, GOOF = 1.0458. Total 29 673 reflections measured (5.60° ≤ 2θ ≤ 136.74°), 9189 unique (R_int = 0.0593, R_sigma = 0.0562) which were used in all calculations. The final R₁ was 0.1169 (all data) and wR₂ was 0.3177 (all data).

Crystallography

Crystals of compound 1 and 2 suitable for single crystal X-ray diffraction analysis were selected using a polarized light optical microscope. X-ray diffraction data were collected on an Agilent SuperNova diffractometer using Cu-Kα radiation (λ = 1.54184 Å). Structures were solved with the olex2.solve structure solution program¹⁴ using the charge flipping solution method and refined with the olex2.refine refinement package using the Gauss–Newton minimisation method.¹⁵ All non-hydrogen atoms except some disordered atoms were refined anisotropically. Hydrogen atoms on Me₁₀CB[5] were placed in geometrically calculated positions and included in the refinement process using a riding model.

Results and discussion

Mass spectroscopy

Mass spectroscopy has been used as a useful diagnostic technique for investigating molecular and supramolecular species in solution by giving indicative evidence of the existence and the stability of interactive entities. Electrospray of a water solution containing 1 : 10 equivalents of Me₁₀CB[5] and sodium nitrite resulted in a mass spectrum composed of a major mono-charged complex cationic peak, with an m/z value of 1062.3588, which corresponds to a Me₁₀CB[5] molecule binding with a NO₂⁻ anion and two Na⁺ cations. There was a minor peak with an m/z value of 1034.4035 that corresponds to Me₁₀CB[5] associating with a hydroxide anion and two Na⁺ cations (Fig. 1). This observation suggested that some supramolecular assemblies, such as {Na₂·NO₂·Me₁₀CB[5]}⁺ and {Na₂·OH·Me₁₀CB[5]}⁺, may possibly form in solution. However, it was still difficult to understand the exact structure of this cationic species in the solution, especially whether the nitrite anion was encapsulated inside the cavity of Me₁₀CB[5]. Moreover, the characteristics for bare Me₁₀CB[5] moiety-related species were absent in the mass spectrum, which implies the superior coordination and inclusion properties of Me₁₀CB[5] units. In case nitrite anions were trapped inside the cavity of Me₁₀CB[5] units, the contact of nitrite anions with other biological species in solution could be restricted. Hopefully, the effect of nitrites in biological and environmental systems can somehow be regulated using Me₁₀CB[5] as a regulator. For comparison, water solutions of sodium nitrite-CB[5] and sodium nitrite-CB[6] were also electrosprayed for mass spectra. The results showed that CB[5] and CB[6] also formed cationic nitrite containing species in solution, but these species were only identified as minor characteristics (see Table S1†). These experiments implied that Me₁₀CB[5] was a better nitrite receptor, preferable to form stable nitrite inclusion species, probably due to the size-matching effect and enhanced quadrupole–dipole interactions thanks to the equatorial methyl groups of Me₁₀CB[5].

Fig. 1 Mass spectrum resulting from the electrospray of a Me₁₀CB[5]–sodium–nitrite mixture in H₂O solution. The inset shows the amplified picture of the highest peak (red) in comparison with the simulated ones (green) with isotope peaks.

Crystal structural features

In order to interpret the detailed structural information of cationic species observed from the mass spectrum, we tried to crystallize the inclusion compound by means of the conventional solvent evaporation method using Me₁₀CB[5], CB[5] and CB[6] as nitrite receptors. Both aqueous solutions of Me₁₀CB[5]–NaNO₂ and CB[6]–NaNO₂ afforded fine single crystals, compounds 1 and 2, respectively, for crystal structural analyses.

When Me₁₀CB[5] was crystallized with sodium nitrite in water solution, colorless crystals suitable for single crystal X-ray diffraction analyses were successfully obtained. X-ray crystallography shows that there are two sodium cations, two nitrite anions, one Me₁₀CB[5] moiety, and 16.75 coordinated/uncoordinated water molecules in the basic building unit, suggesting a formula of [Na₂(H₂O)₂(NO₂@Me₁₀CB[5])]⁺·NO₂⁻·14.5H₂O (1). A prominent feature in the structure of compound 1 has been, as expected, the encapsulation of one nitrite anion inside the cavity of Me₁₀CB[5] (Fig. 2). The size of the nitrite ion is evaluated to be 1.77–1.81 Å, which is in reasonably good size matching with the cavity size of 2.0 Å of Me₁₀CB[5]. Moreover, both portals of the Me₁₀CB[5] are capped by sodium cations, giving rise to a molecular capsule. Each Na⁺ ion is situated at the centroid of the plane binding with five portal carbonyl oxygen atoms, which comprise the coordination sphere of a sodium cation together with the nitrite and one coordination water molecule. The Na–O distances range from 2.354 Å to 2.767 Å, and the Na–N bond is slightly longer (2.863 Å), which are comparable to those observed for sodium complexes reported in the literature.¹⁶ The encapsulated nitrite ion locates in two statistically equal positions and binds to either one of the two Na⁺ ions at the portals of a Me₁₀CB[5] unit (Fig. 2), suggesting that sodium cations assist in the encapsulation and stabilization of the nitrite ion. The molecular capsule observed in the solid state is in good agreement with the MS analysis, suggesting that the inclusion entities are also present in solution. The other crystallographically independent nitrite counter anion locates outside the cavity of Me₁₀CB[5], interacting with the Me₁₀CB[5] units and water molecules through weak hydrogen bond interactions (Fig. S3†).

Fig. 2 Top- (a) and side-view (b) of the nitrite anion encapsulated molecular capsule in compound 1.

X-ray crystallography analysis of compound 2, obtained from the CB[6]–NaNO₂ solution, shows a basic building unit containing one CB[6] molecule, two sodium ions, two nitrite ions and 13 coordinated/discrete water molecules, suggesting a formula of {Na₂(H₂O)₁₀CB[6]}²⁺·3(H₂O)·2(NO₂)⁻(see Fig. S4†). Each portal of CB[6] is half covered by one statistically disordered sodium cation. Different from compound 1, the nitrite ions are not encapsulated inside the cavity of CB[6], rather two water molecules are encapsulated, forming cationic species of {2Na⁺·2H₂O·CB[6]}²⁺, which is again in good agreement with the MS analysis (see Table S1†). This observation indicates that the cavity size and chemical environment of CB[6] are not suitable for encapsulating nitrite ions. In the crystal structure, nitrite ions stay outside the cavity of CB[6], working as counter ions and space fillers, binding with the outer face of CB[6] through weak noncovalent bonds (Fig. S5†).

Controlled nitrite encapsulation and release experiments

It was reported that the nitrite ion is unstable at pH < 4 (with a pK_a of HNO₂ about 3.4), which decomposes to nitric oxides and can be oxidized to nitrates under acidic conditions.¹⁷ In order to monitor the release of nitrite anions from the Me₁₀CB[5] receptor and the evolution of nitrite species in solution, semi-quantitative nitrite test strips sold under the trademark of “Quantofix” with a test range from 1 to 80 mg L⁻¹ NO₂⁻ were used to record the concentration of nitrites in solution. Crystals of compound 1 were dissolved in water by sonication. The above solution was divided into two batches: one was adjusted to a pH of 4.0 using hydrochloric acid (solution I, with part of its nitrite being in the form of nitrous acid), while the other solution remained neutral (solution II). These two solutions were tested by using nitrite test strips simultaneously at different time intervals. By means of dipping test strips into both solutions for 1 second and comparing carefully the color changes with the color scale, the initial NO₂⁻ concentration of both solutions was determined to be approximately 80 mg L⁻¹. Further semi-quantitative analysis of NO₂⁻ concentration in both solutions at different time intervals indicated that nitrites decomposed continuously in acidic solution I, in which the NO₂⁻ concentration was out of the measurement range of the nitrite test strip (lower than 1 mg L⁻¹) after 10 hours at room temperature, whereas the NO₂⁻ concentration remained unchanged throughout in the neutral solution II. Moreover, it was clear that nitric oxide fume was detected to release from solution I, as evidenced by the color change of the test strips that were hovered above the surface of the solution. In comparison, the color of the test strips put above the surface of solution II remained unchanged.

After decomposition, the end result of solution I was analyzed by the mass spectrum. There was one prominent peak, with an m/z value of 503.23, which was attributed to the doubly charged species of Me₁₀CB[5] + 2H₃O⁺. Other characteristic peaks with m/z values of 988.43, 993.38 and 1034.41 were assigned to the mono-charged species of Me₁₀CB[5] + H₃O⁺, Me₁₀CB[5] + Na⁺ and Me₁₀CB[5] + 2Na⁺ + OH⁻, respectively. The absence of ionic species related to nitrite anions indicated that nitrite anions were released from the cavity and decomposed completely in solution I.

The nitrite release and decomposition processes were also monitored by UV-vis spectra and the decomposition rate of nitrite anions under various conditions is plotted in Fig. 3. As shown in Fig. 3 top, the absorption characteristics of nitrite anions at 345 nm, 357 nm, 371 nm, and 385 nm decreased the range continuously over time and became invisible after 10 hours. This observation suggested the release and decomposition of nitrite anions in solution and NO₂⁻ anions being encapsulated in sodium-capped Me₁₀CB[5] hosts under acidic conditions. The guest release behavior of cucurbit[n]uril molecular capsules was studied previously by Kim et al., where the organic guest molecules could be released from sodium-capped CB[6] under acidic conditions.¹⁸ It was also suggested that the sodium ion ‘lids’ were first removed in strongly acidic solution due to the protonation of carbonyl groups at the portals, followed by the release of guest moieties encapsulated inside the cavity of CB[6]. The release/decomposition rate of nitrite anions was dependent on the initial concentration of compound 1. Moreover, the nitrite degradation rate in high concentration solutions was considerably higher than those in less concentrated systems within the first three hours (Fig. 3, bottom).

Fig. 3 UV-vis spectra of compound 1 in acidic aqueous solution with a pH of 4.0 at different time intervals (top), decomposition rates of nitrite anions at different concentrations (bottom), initial concentrations: compound 1 (888 mg L⁻¹ and 2700 mg L⁻¹, pH = 4.0).

Conclusions

In summary, a nitrite anion inclusion compound with a formula of [Na₂(H₂O)₂(NO₂@Me₁₀CB[5])]·NO₂·14.5H₂O has been prepared by using Me₁₀CB[5] macrocycles as anion receptors in the presence of sodium ions. X-ray crystallographic analysis indicated that nitrite anions are trapped inside the molecular capsules constructed by sodium ions (as ‘lids’) and Me₁₀CB[5] moieties (as ‘hosts’). Mass spectra and single crystal X-ray structural analyses showed that Me₁₀CB[5] is a better nitrite anion acceptor than its analogues, including CB[5] and CB[6], probably due to the size-matching effect and enhanced quadrupole–dipole interactions thanks to the presence of equatorial methyl groups of Me₁₀CB[5]. The release/decomposition of nitrites from the cavity of Me₁₀CB[5] in acidic solution has been confirmed semi-quantitatively by nitrite test strips and UV-vis spectra. Such inclusion properties of Me₁₀CB[5] might be applied to hamper the contact of nitrites with organic amines in aqueous solution, in turn, control the formation of hazardous nitrosamines. This study implies a potential use of Me₁₀CB[5] macrocycles in biosystems to regulate the metabolism of nitrite ions and accordingly cultivate pharmaceutical research interest.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We are grateful for financial support from the 973 Program (Grant No. 2014CB845605), the NSFC (Grant No. 21520102001, 21521061, 91622114, and 21331006), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB20000000), the State Key Laboratory of Structural Chemistry (Grant No. 20150023 and 20170032), the Fujian Educational Scientific Research Program for Middle and Young Aged Teachers (Grant No. JAT160572), the Fujian Universities and Colleges Engineering Research Center of Soft Plastic Packaging Technology for Food, and the New Century Excellent Talents in Fujian Province University.

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Footnote

† Electronic supplementary information (ESI) available: General characterization and additional structural figures. CCDC 1554723 for 1 and 1845091 for 2. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qi01168k

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