Novel octacationic-resorcin[4]arenes featuring quaternary ammonium groups as multivalent biocides

Abstract

Novel resorcin[4]arene-based octacationic quaternary ammonium compounds (ResQAC^R) were obtained by linking tetraalkylammonium groups on the macrocycle wider rim to give multivalent multi-QACs. Their structures were elucidated through 1D and 2D NMR experiments, HR-MS analysis, and crystallographic studies. Additionally, the conformational dynamics of these octacationic-resorcinarenes were investigated using ¹H variable-temperature (VT) NMR experiments. The antimicrobial properties of ResQAC^R derivatives were studied by in vitro biological investigations. We identified that the ResQAC^butyl derivative shows an impressive bacteriostatic activity against S. aureus. A remarkable multivalent effect was observed for this bacteriostatic activity. Interestingly, cytotoxic studies indicate that ResQAC^butyl showed no adverse impact on cell viability of a human cell line, even at concentrations 30 times greater than the MIC for S. aureus and approximately 3 times higher than the MIC for E. coli. ResQAC^undecyl exhibits greater bacteriostatic activity against E. coli than against S. aureus, but it is cytotoxic at lower concentrations (IC₅₀ of 12.1 μM) than its MIC. ResQAC^phenyl shows scarce bacteriostatic activity against both S. aureus and E. coli, while no multivalent effect was observed. This is likely attributed to the conformational rigidity of the boat C_2v conformation of ResQAC^phenyl, which hinders optimal matching between the cationic chains and the negatively charged bacterial surface.

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Introduction

Staphylococcus aureus and Escherichia coli represent two of the eight bacteria considered significant for public health.¹ These pathogenic microorganisms are responsible for causing a considerable number of diseases.¹ It is crucial to control the environmental transmission of these organisms to safeguard public health. In recent decades, genetic recombination, widespread antibiotic usage, and other factors have led to bacterial resistance, raising the possibility that common infections will once again become potentially fatal. Very recently, analyses have been conducted¹ to study the correlations between antimicrobial consumption (AMC) and antimicrobial resistance (AMR) for specific combinations of bacteria and antibiotics. In detail, statistically significant decreasing trends in AMC and AMR were observed in food-producing animals and humans from 2014 to 2021.¹ Nevertheless, these measures necessitate reinforcement to maintain reductions in AMC. This emphasizes the importance of initiatives promoting human and animal health, such as vaccination and improved hygiene practices, alongside the use of disinfectants.

Among disinfectants, quaternary ammonium compounds (Fig. 1, QACs)² are a well-known class of cationic biocides, boasting a wide spectrum of antimicrobial activity.^3,4 QACs are commonly present in cleaning products, hand sanitizers, personal care products, various types of wipes, and diverse pesticidal products. They are frequently integrated into polymeric materials (poly-QAC in Fig. 1). Interestingly, QACs serve as antimicrobial treatments for biomedical instruments and high-touch surfaces in public areas.^2–4 In addition, the use of QACs as antimicrobial agents has experienced a notable increase in response to the COVID-19 pandemic.⁵

Fig. 1 Schematic representation of mono-QAC, bis-QAC, multi-QAC, and poly-QAC.³ A mono-QAC is constituted by a single tetraalkylammonium group. In the case of bis-QAC, multi-QAC, and poly-QAC, the segment connecting the charged nitrogen atoms (“head”) is named “spacer”. The alkyl chains extending from the head are called “tails”.

Over recent decades, numerous studies have highlighted the role of macrocycles as useful scaffolds for obtaining antibacterial derivatives (spacer in Fig. 2).^6–11 Interestingly, the synthetic versatility of pillararenes and calixarenes enabled the incorporation of multiple functionalities onto their rims.^6–15

Fig. 2 Resorcin[4]arene-based multi-QACs (ResQAC^R) investigated in this work.

Thus, in 2016, Cohen and coworkers¹² were able to obtain pillar[n]arenes decorated with 10 quaternary ammonium groups (five ammonium groups on each side of the macrocycle) which inhibited the formation of biofilms by two significant Gram-positive pathogens, Staphylococcus aureus and Enterococcus faecalis.

Consoli and colleagues^13,14 focused on calixarene scaffolds, designing and synthesizing novel water-soluble polycationic calix[4]arenes. Subsequently, the authors demonstrated the ability of these novel calix[4]arene-multi-QACs to work as antibacterial agents against Gram-positive and Gram-negative bacteria, both independently and in combination with appropriate antibiotics. The same research group reported a calixarene featuring long alkyl chains at the lower rim, which exhibited micellar behavior.¹⁴ These micelles inhibited the formation of biofilms by Gram-negative bacteria.

In the literature, a few rare examples have been reported where macrocycle-based antibacterial agents exhibited increased antibacterial efficacy by exploiting the multivalent effect.^15,16 Multivalency^17–19 refers to the phenomenon where multiple receptor/ligand interactions occur simultaneously, leading to enhanced overall affinity and selectivity of binding. Natural systems extensively utilize multivalency, such as in immunoglobulin function, virus particle adhesion to target cells, and regulation of cell–cell interactions. This concept has acquired increasing attention in the literature,^17–19 where multivalency was exploited in designing macrocyclic ligands^20–22 targeting pathologically relevant enzymes.

Resorcin[4]arene^23,24 (1a, Fig. 2) is a macrocycle bearing four resorcinol rings, and is obtained from resorcinol/aldehyde acid-catalysed condensation.^23,24 This macrocycle possesses eight functionalizable OH groups, all positioned on one side of the macrocycle (wider rim, see Fig. 2). Thus, the resorcin[4]arene scaffold can be considered as an ideal candidate to design multivalent quaternary ammonium compounds (multi-QACs, Fig. 1 and 2), where the valence indicates the number of ammonium units (heads in Fig. 1 and 2) linked to the macrocycle. To the best of our knowledge, no examples of polycationic ammonium resorcinarene compounds have been reported in the literature to data.

Inspired by these considerations, this study focused on the synthesis of novel multi-QAC resorcin[4]arenes (denoted as ResQAC^R in Fig. 2), featuring eight cationic ammonium groups on the wider rim (Fig. 2). In addition, conformational dynamics of these novel octacationic resorcinarenes will be investigated.

Finally, the antimicrobial effects of ResQAC^R against Staphylococcus aureus and Escherichia coli will be evaluated, alongside an assessment of their cytotoxic properties.

Results and discussion

Synthesis of ResQACs^R

Octacationic ResQACs^R (Fig. 2) were easily synthesized drawing inspiration from a method previously reported by Huang in 2011 for the preparation of cationic pillararenes.²⁵ Thus, resorcinarenes 5a–d, bearing bromoalkane groups, were initially synthesized as suitable precursors of ResQAC^R derivatives (Scheme 1). To obtain resorcinarenes 5a–d, we tested the direct macrocyclization of monomers 4a–b with appropriate aldehydes in the presence of BF₃·Et₂O in CH₂Cl₂ as solvent (Scheme 1). Precursors 5a–d were obtained in reasonable yields (25–35%). It is noteworthy that this represents the first example of direct macrocyclization of resorcinol monomers bearing alkyl bromide chains (4a–b) to yield the corresponding resorcin[4]arenes. Successively, 5a–d were treated with an excess of dimethylbutyl amine (30 equiv.) in ethanol solvent (Scheme 1). The mixture was stirred for 18 hours at 80 °C, and after usual work-up, ResQAC^R derivatives were obtained as bromide or iodide salts. Finally, halide to nitrate exchange was carried out by treating ResQAC^R(X⁻)₈ (X⁻ = Br⁻ or I⁻) with AgNO₃, leading to ResQAC^R(NO₃⁻)₈ in 90–95% yields (Scheme 1). The structures of ResQAC^R were confirmed using HR FT ICR mass spectra, 1D–2D NMR spectra, and X-ray studies (see the ESI†).

Scheme 1 Direct macrocyclization of resorcinols 4a,b bearing bromoalkane groups with aldehydes to obtain resorcin[4]arenes 5a–d as precursors for the synthesis of cationic resorcinarenes ResQAC^R.

Single crystals of ResQAC^butyl as an iodide salt, suitable for X-ray diffraction, were obtained by slow evaporation of a H₂O/MeOH solution (Fig. 3). The X-ray structure of resorcinarene ResQAC^butyl (Fig. 3) was determined using synchrotron radiation with cryogenic techniques. The structure of the macrocycle shows a boat conformation with a pseudo C_2V symmetry (Fig. 3). The two opposite aryl rings, which define the sides of the boat, make almost orthogonal dihedral angles (Fig. 3d) with respect to the mean plane of the carbon bridging atoms (Fig. 3d). Angles less than 90° indicate that the aryl rings lean inwards towards the twofold axis of the molecule (Fig. 3c and d) and vice versa. In particular, one aryl ring of ResQAC^butyl is slightly inward oriented, and the other is slightly outward oriented (Fig. 3c and d), defining two almost parallel inclined sides of the boat (3.8°). This observation of a more open conformation of ResQAC^butyl than other resorcinarenes in the boat conformation^26,27 is consistent with the presence of the electrostatic repulsion between the positively charged quaternary alkylammonium side chains. Close inspection of the ¹H NMR spectrum of ResQAC^butyl in CD₃CN (Fig. 4a, 400 MHz, 298 K) clearly indicated that the macrocycle adopts, also in solution, a C_2v-boat conformation, stable on the NMR time scale at 298 K (400 MHz). In fact, four ⁺NCH₃ singlets were detected at 3.25, 3.24, 3.04, and 3.01 ppm (“f and g” in Fig. 4), which correlated in the 2D HSQC spectrum with carbon signals between 52.0 and 52.2 ppm. Furthermore, in accordance with the C_2v symmetry of ResQAC^butyl, the aromatic region of its ¹H NMR spectrum revealed the presence of four singlets at 7.35, 6.99, 6.69, and 6.15 ppm (“a and b” in Fig. 4), which are attributable to the aromatic protons.

Fig. 3 (a and b) X-ray structure of ResQAC^butyl (stick representation with van der Waals surfaces). (c) Simple stick representation of ResQAC^butyl. Hydrogen atoms are omitted for clarity. (d) Dihedral angles (°) between the corresponding aryl planes (A–D in Fig. 3c) of the ResQAC^butyl structure.

Fig. 4 (a) ¹H NMR spectrum and (b) 2D HSQC spectrum of ResQAC^butyl (600 MHz, CD₃CN, 298 K). (c) Chemical drawing of the boat (C_2v) and crown (C_4v) conformations of ResQAC^butyl. (d) ¹H VT NMR experiment (600 MHz) for ResQAC^butyl in DMSO-d₆.

In addition, the ¹H NMR spectrum of ResQAC^butyl at 298 K showed the presence of two sets of OCH₂CH₂N⁺ diastereotopic resonances between 3.0 and 5.0 ppm (c,c′ and d,d′ in Fig. 4). Subsequently, the conformational dynamics of ResQAC^butyl was studied by ¹H VT NMR.^27,28 At room temperature, this macrocycle is frozen in the C_2V boat conformation (Fig. 4c and d).

Upon heating, a broadening was observed for the aromatic signals “a,b” and the diastereotopic aliphatic “c,d” resonances (Fig. 4c and d), while their coalescence was observed at 373 K. This coalescence (Fig. 4d) clearly indicates conformational mobility of the C_2V boat conformation of ResQAC^butyl due to a boat(C_2V)–crown(C_4V)–boat(C_2V) conformational interconversion (Fig. 4c),^21,27 which becomes fast on the NMR time scale at 393 K (a ΔG^‡ value of 17.3 kcal mol⁻¹ was calculated for this process, see the ESI†). In fact, at 393 K, the ¹H NMR spectrum of ResQAC^butyl becomes simpler (Fig. 4d) and more compatible with an averaged crown-like structure with C_4v symmetry. In a similar way, the ¹H NMR spectrum at 298 K of ResQAC^undecyl reveals a stable boat C_2v conformation (with respect to the NMR time scale), while on heating, a broadening of its NMR signals was observed. Therefore, a boat–crown–boat conformational interconversion (Fig. 4c) occurs within the NMR time scale at higher temperature. A coalescence of NMR signals of ResQAC^undecyl was observed at 373 K, while at 393 K the ¹H NMR spectrum agreed with an averaged crown C_4V conformation. From these data, an energy barrier value of 17.2 kcal mol⁻¹ was calculated for the boat–crown–boat conformational interconversion of ResQAC^undecyl, a value very similar to that found for ResQAC^butyl.

With these results in hand, we have investigated the conformational dynamics of ResQAC^phenyl. In this case, the boat conformation is blocked on the NMR time scale. In fact, on heating, the NMR signals of ResQAC^phenyl didn't show any evidence of broadening in the temperature range investigated (298–393 K, ESI†).

These results clearly indicate that the presence of phenyl groups at the methine bridges of ResQAC^phenyl increases the energy barrier to the boat–crown–boat conformational process with respect to the linear aliphatic chains (undecyl and butyl).

Antimicrobial activity of novel ResQACs^R

The antimicrobial activity of compounds ResQAC^butyl, ResQAC^undecyl, and ResQAC^phenyl was evaluated by comparative analysis of both the MIC and the MLD determined for the non-pathogenic Gram-negative strain Escherichia coli JM109 and Gram-positive Staphylococcus aureus (a pathogenic hospital isolate) (Table 1).

Table 1 Antimicrobial activity of the multivalent ResQAC^R compounds and reference compounds 1b, 2 and 3 towards Gram-positive Staphylococcus aureus (a pathogenic hospital isolate) and Gram-negative strain Escherichia coli JM109

	Valency^a (n)	Staphylococcus aureus			Escherichia coli JM109
		MIC^b^,^c μM (μg mL^−1)	r.p.	MLD^d μM (μg mL^−1)	MIC^b^,^c μM (μg mL^−1)	r.p.	MLD^d μM (μg mL^−1)
a n = number of ammonium units. b MIC (minimum inhibitory concentration). c r.p. = relative potency = MIC (derivative 2)/MIC (ResQAC^R). d MLD (minimum lethal dose). e R = resistance to the derivative.
ResQAC^butyl	8	0.89 (2)	182	>13 (>30)	≈13 (≈30)	4.6	>125 (>280)
ResQAC^undecyl	8	26 (70)	6.1	≈53 (≈140)	≈6.5 (≈17)	9.4	≈107 (≈280)
ResQAC^phenyl	8	≈43 (≈100)	3.8	>60 (>140)	≈43 (≈100)	1.4	>121 (>280)
1b	0	R > 187 (>280)	—	>187 (>280)	R > 187 (>280)	—	>187 (>280)
2	2	163 (80)	1	≈185 (≈140)	≈61 (≈30)	1	≈143 (≈70)
3	1	R > 954 (>280)	—	>954 (>280)	≈635 (≈140)	—	>954 (>280)

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All ResQAC^R compounds were tested as nitrate salts to avoid bromide and iodide counter-anions, thereby enhancing the safety of these compounds for both humans and the environment. In fact, recent reports^29–31 indicate that iodide and bromide salts of QACs typically threaten water sources with pollution. During disinfection, they can produce harmful by-products such as brominated disinfection by-products (Br-DBPs) and iodinated disinfection by-products (I-DBPs), which pose significant health risks.^29–31 All ResQAC^R compounds showed antimicrobial activity against both microbial strains, but ResQAC^butyl and ResQAC^undecyl were more effective, presenting lower MIC and MLD values (Table 1). The toxic activity was variable with respect to the bacterial group. ResQAC^butyl showed high bacteriostatic and bactericidal activity with very low MIC and MLD (0.89 and >13 μM, respectively) for the Gram-positive strain; on the other hand, ResQAC^undecyl was more bacteriostatic against E. coli than against S. aureus, but more bactericidal for the latter (lower MLD values). The less efficient ResQAC^phenyl showed similar bacteriostatic activity for the two bacteria (MIC = 43 μM), but a variable bactericidal effect. As reported in Table 1, resorcinarene 1b (Fig. 2) bearing eight neutral heptyl groups on the wider rim didn't show any bacteriostatic or bactericidal effects for the doses tested. With the aim of quantifying the multivalent effect, we chose the resorcinol diammonium derivative 2 and the dibutyl–dimethylammonium derivative 3 shown in Fig. 2 as the reference divalent and monovalent compounds, respectively. The monovalent derivative 3 exhibited very low activity against both E. coli and S. aureus (MIC = 635 and >954 μM, respectively, Table 1). The resorcinol diammonium derivative 2, which mimics a single monomeric unit of the multivalent ResQAC^R macrocycles, showed bacteriostatic activity, MIC = 163 and 61 μM, for S. aureus and E. coli, respectively (Table 1), which was significantly lower than that found for the ResQAC^butyl compound (0.89 and 13 μM).

Thus, the relative antimicrobial potencies (r.p. in Table 1) of ResQAC^R derivatives were obtained by calculating the ratio of the MIC values (μM) of the resorcinol diammonium 2 to that of the multivalent ResQAC^R.³² In detail, a remarkable multivalent effect with an r.p. of 182 was observed for the ResQAC^butyl derivative (MIC = 0.89 μM) against S. aureus. According to this finding, each resorcinol diammonium monomer in ResQAC^butyl experiences an antimicrobial potency amplification of 45.5 times compared to the reference 2 (Fig. 5a and b). The data reported in Table 1 clearly show that S. aureus is more responsive to tetraalkylammonium multimerization than E. coli. In fact, multivalent ResQAC^R derivatives showed low r.p. values ranging from 1.4 to 9.4 against E. coli. Remarkably, the ResQAC^undecyl derivative demonstrated lower bacteriostatic activity against S. aureus (MIC = 26 μM) than ResQAC^butyl and exhibited six times greater efficiency than derivative 2 (r. p. = 6.1). Interestingly, ResQAC^phenyl showed a modest bacteriostatic activity towards both S. aureus and E. coli and very low multivalent effects (r.p. = 1.4 and 3.8). Thus, by close inspection of the data reported in Table 1, we can conclude that:

(a) the antibacterial activity of ResQAC^R derivatives is attributable to the interaction with the bacterial cell wall (Fig. 5a–c). As is known^33–37 for other quaternary ammonium compounds and polycationic macrocycle derivatives, ResQAC^R also can bind with the negatively charged bacterial surface by electrostatic and hydrophobic interactions (Fig. 5). Differently, no significant biocidal effect of compound 1b was observed against both E. coli and S. aureus.

(b) A significant bacteriostatic activity was observed for the ResQAC^butyl derivative (MIC = 0.89 μM) against S. aureus. In addition, ResQAC^butyl shows a remarkable multivalent effect against S. aureus.

(c) E. coli is more responsive to ResQAC^undecyl.

(d) As demonstrated previously by some of the authors,^20,21 the effectiveness of resorcinarene-based multivalent ligands relies on achieving a delicate balance between the preorganization of the macrocycle and its conformational flexibility.³⁸ In fact, the effectiveness of multi-QACs against bacteria relies on their ability to align all their ammonium groups with the negatively charged bacterial surface (Fig. 5). ResQAC^butyl and ResQAC^undecyl derivatives exhibit conformational adaptability, allowing them to fit and interact optimally with the bacterial surface (Fig. 5b). In contrast, the ResQAC^phenyl derivative features a rigid boat-shaped structure, with four alkylammonium groups positioned away from the surface (Fig. 5c) and four cationic groups facing towards it. Consequently, the decreased activity of ResQAC^phenyl is likely due to its weaker interaction with the bacterial wall, as it utilizes only four of its available eight cationic groups (Fig. 5c).³⁸

Fig. 5 Cartoon representations depicting the proposed model for the interaction between Gram-positive bacteria and (a) derivative 2; (b) ResQAC^butyl in the crown conformation; and (c) ResQAC^phenyl in the boat conformation.

Effect of ResQAC^R on cell viability of human HaCat cells

To evaluate the ability of ResQAC^R compounds to affect the cell viability of eukaryotic cells, a model of immortalized human keratinocytes (HaCat cells) was used. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed after 24 h of treatment with different concentrations of each compound. The percentage of cell viability measured at 100 μM and the calculated IC₅₀ values are listed in Table 2. The dose–response curves in the concentration range from 1 to 150 μM for ResQAC^butyl and compound 2 are presented in Fig. 6. From all these data, we can confirm that ResQAC^butyl has no negative effect on cell viability at concentrations reaching about 30 times the MIC for S. aureus and about 3 times the MIC for E. coli. It presents only a moderate toxicity at higher concentrations (IC₅₀ = 54.3 μM). On the other hand, compound 2 was less cytotoxic than ResQAC^butyl, with IC₅₀ almost twice that of ResQAC^butyl; however, at 100 μM, i.e. a concentration below the MIC for S. aureus and at about 1.6 times the MIC for E. coli, cell viability was reduced by more than 50%.

Fig. 6 Effect of ResQAC^butyl and compound 2 on the viability of human keratinocytes (HaCat) after 24 h of treatment with concentrations ranging from 1 μM to 150 μM. Mean values (± standard deviation) of biological triplicate are reported. *p < 0.0001 vs. vehicle-treated cells.

Table 2 Residual cell viability measured in HaCat cells treated for 24 h with each compound at 100 μM. Calculated mean IC₅₀ values are also reported

	Residual cell viability at 100 (μM)	IC50 (μM)
ResQAC^butyl	4.1%	54.3
ResQAC^undecyl	4.7%	12.1
ResQAC^phenyl	49.8%	99.8
1b	99.4%	>300
2	45.3%	97.2
3	100%	>1 mM

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Finally, viability assays clearly indicated that ResQAC^undecyl was cytotoxic at concentrations (IC₅₀ = 12.1 μM) lower than its MIC for S. aureus, while ResQAC^phenyl had a moderate ability to reduce cell viability (IC₅₀ = 99.8 μM), whereas compounds 1b and 3 had no effect on the viability of HaCat cells.

Conclusions

We reported the synthesis of novel resorcin[4]arene multi-QACs (ResQAC^R) bearing eight tetraalkylammonium groups on the wider rim. This study marks the first example, in the resorcin[4]arene literature, of direct macrocyclization of resorcinol monomers functionalized with bromoalkane groups. In this regard, the information described here could pave the way for the easy synthesis of new resorcin[4]arene hosts.

In the solid state, ResQAC^butyl adopts a boat conformation with a pseudo C_2V symmetry (Fig. 3) to minimize the electrostatic repulsion between the positively charged quaternary alkylammonium side chains. For the first time, the conformational dynamics properties of resorcin[4]arenes bearing cationic groups on the wider rim have been investigated. Dynamic NMR investigations indicate that cationic chains do not inhibit the boat-to-crown conformational process, whereas the groups on the CH-bridges play a crucial role in this process. In fact, ResQAC^butyl and ResQAC^undecyl derivatives exhibit a boat–crown–boat conformational interconversion. Conversely, for the ResQAC^phenyl derivative, conformational boat-to-crown interconversion is blocked within the NMR time scale.

Biological studies unequivocally underscore the antibacterial abilities of ResQAC^butyl and ResQAC^undecyl derivatives. ResQAC^butyl demonstrates a notable bacteriostatic activity against S. aureus with a remarkable multivalent effect. Interestingly, studies on the viability of HaCat cells indicate that ResQAC^butyl exhibits no detrimental effect on cell viability, even at a concentration higher than the MIC for S. aureus and E. coli. ResQAC^phenyl show a modest bacteriostatic activity against both S. aureus and E. coli. Probably, this is due to the conformational rigidity of the resorcinarene framework in ResQAC^phenyl, which does not facilitate optimal matching between the ammonium groups and the negatively charged bacterial surface. These results corroborate²¹ the significance of the conformational flexibility of macrocyclic scaffolds in relation to their interaction with biological targets.²¹

Author contributions

V. F., V. I. and P. D. S. synthesized and characterized all compounds. L. D. S. and G. V. performed all antimicrobial tests. G. P. and I. C. performed all cell viability tests. S. G. and N. H. were responsible for the X-ray crystal structure analyses. P. D. S. performed the FT ICR MS analysis. V. I., G. V. and C. T. supervised the project. C. G. secured funding and overall supervised the project. The manuscript was written by C. G., P. N., and C. T. and edited by all co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

V. F. and C. T. thank the Programma Operativo Nazionale, Ricerca e Innovazione 2014–2020 (CCI 2014IT16M2OP005), Fondo Sociale Europeo, Azione I.1 “Dottorati Innovativi con caratterizzazione Industriale – CUP D43D20002240006”, for the financial support. C. Gaeta thanks “NEXTGENERATIONEU (NGEU)” funded by the Ministry of University and Research (MUR) (PRIN: PROGETTI DI RICERCA DI RILEVANTE INTERESSE NAZIONALE – Bando 2022, Project code: 2022FSC2FA) for the project: New stimuli-responsive pharmacological chaperones based on prismarenes (PH-PRISM).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2336815. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo00728j

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