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Photogeneration of quinone methides (QM) from 3- hydroxymethyl-2-naphthol (1) and 3-(2-hydroxy-2- adamantyl)-2-naphthol (2) was investigated using femtosecond and nanosecond transient absorption spectroscopy and supported with theoretical analysis of possible reaction pathways. Our results indicate that adamantylnaphthol 2 after UV excitation undergoes a non-adiabatic excited state intramolecular proton transfer coupled with dehydration via a conical intersection, delivering the corresponding QM (S0) within 1 ns. Surprisingly, in naphthol 1 on femtosecond time scale only formation of radical cation and solvated electrons was observed where the photoionization is a competing process not connected to the photochemical reaction of QM formation. Radical cations decay fast (45 ps) due to the back electron transfer. By quenching with ascorbate we identified phenoxyl radicals as a QM precursor, which are formed in a slower H-transfer reaction taking place on nanosecond time scale. Our combined experimental and theoretical investigation point to a polycyclic substituent effect, which profoundly changes the photochemical reaction pathway.

Muramyl dipeptide (MDP) is the smallest peptidoglycan fragment able to trigger the immune response. Structural modification of MDP can lead to the preparation of analogs with improved immunostimulant properties, including desmuramyl peptides (DMPs). The aim of this work was to prepare the desmuramyl peptide (L-Ala-D-Glu)-containing adamantyl-triazole moiety and its mannosylated derivative in order to study their immunomodulatory activities in vivo. The adjuvant activity of the prepared compounds was evaluated in a murine model using ovalbumin as an antigen, and compared to the reference adjuvant ManAdDMP. The results showed that the introduction of the lipophilic adamantyl-triazole moiety at the C-terminus of L-Ala-D-Glu contributes to the immunostimulant activity of DMP, and that mannosylation of DMP modified with adamantyl-triazole causes the amplification of its immunostimulant activity.

Liposomes are promising drug's delivery systems due to decreased toxicity of the liposome-encapsulated drug, but wider clinical application requires their more efficient tumor targeting with uptake, controlled drug release and higher shelf life. The unique metabolic characteristics of cancer cells based on higher demand for energy and therefore increased glucose utilization were exploited in the design of glucose modified liposomes (GML) with the aim to provide increased tumor targeting via glucose transporters and increased ability of drug delivery into tumor cells. Tumor accumulating potential of GML and non-glucose liposomes (NGL) were investigated on CT26 and LS174T tumor-bearing mice by simple and reliable radiotracer method using 177Lu as radioactive marker. Both liposomes, GML and NGL were radiolabeled in high radiolabeling yield, showing high in vitro stability in biological media, as the main prerequisite for the biodistribution studies. Tumors displayed significantly better accumulation of 177Lu-GML with the maximum uptake 6 h post-injection (5.8 ± 0.2%/g in LS174T tumor and 5.1 ± 0.5%/g in CT26 tumor), compared to negligible uptake of 177Lu-NGL (0.6 ± 0.1%/g in LS174T tumor and 0.9 ± 0.2%/g in CT26 tumor). Results of comparative biodistribution studies of 177Lu-NGL and 177Lu-GML indicate that increased accumulation of GML is enabled by glucose transporters and subsequent endocytosis, resulting in their prolonged retention in tumor tissues (up to 72 h). Direct radiolabeling of liposomes with 177Lu may be used not only for biodistribution studies using radiotracking, but also for cancer treatment. © 2021 Elsevier B.V.

We have synthesized a number of Quillaja saponaria Molina (QS) saponin analogues with a different C28 sugar unit, which features either 3, 4-diacetyl groups or a 3, 4-cyclic carbonate group at the reducing end fucoside to mimic the naturally occurring saponin adjuvant QS-7. Immunological evaluations of these analogues in BALB/c mice indicate that truncating the C28 oligosaccharide of the natural product to the tetrasaccharide (as in 5d (β)) could retain the adjuvant’s activity in enhancing IgG1 and IgG2a productions, albeit the activity is lower than that of QS-21. Further truncation or changing stereochemistry of glycosidic linkage between the tetrasaccharide and the triterpenoid quillaic acid (QA) core or within the tetrasaccharide eliminated the saponins’ adjuvant activity in terms of IgG production. On the other hand, increasing resemblance to QS-7 increased adjuvant activity and led to saponin 3’s similar IgG1 and IgG2a activities to QS-21’s, indicating that the unique adjuvant activities of QS saponins are determined by their specific structures.

We demonstrate a proof of principle for a new approach in the development of a drug delivery system. A positively charged prodrug (phenol) can form a stable inclusion complex with CB[7], which enables more efficient delivery of the prodrug. After photochemical transformation (photoactivation) inside the complex, an active drug quinone methide (QM) is formed and released from the complex, since it is a neutral molecule and forms a less stable complex with CB[7].

ortho-, meta- and para-Hydroxymethylaniline methyl ethers 3-5-OMe and acetyl derivatives 3- 5-OAc were investigated as potential photocages for alcohols and carboxylic acids, respectively. The measurements of photohydrolysis efficiency showed that the decaging from ortho- and meta-derivatives takes place efficiently in aqueous solution, but not for the para-derivatives. Contrary to previous reports, we showthat the meta-derivatives are better photocages for alcohols, whereas ortho- derivatives are better protective groups for carboxylic acids. The observed differences were fully disclosed by mechanistic studies involving fluorescence measurements and laser flash photolysis (LFP). Photoheterolysis for the para-derivatives does not take place, whereas both meta- and ortho-derivatives undergo heterolysis and afford the corresponding carbocations 3-C and 4-C. The ortho-carbocation 4-o-C was detected byLFP in aqueous solution (λmax = 410 nm, τ ≈ 90 μs). Moreover, spectroscopic measurements for the meta-acetyl derivative 3-m-OAC indicated the formation of cation in the excited state. The application of ortho- aniline derivative as protective group was demonstrated by synthesizing several derivatives of carboxylic acids. In all cases, the photochemical deprotection was accomplished in high yields (>80%). This mechanistic study fully rationalized photochemistry of aniline photocages which is important for design of new photocages and has potential for synthetic, biological and medicinal applications.

We have developed synthetic pathways toward differently substituted hydroxyanthracenes (anthrols) with the aim to investigate their photochemical reactivity in dehydration reactions. Although the syntheses of anthracenes substituted at positions 9,10 are well known, reports for the synthesis of anthracenes with different substitution patterns are scarce. Herein we review known and report novel synthetic pathways toward anthrols with substituents at 1,2-, 2,3-, and 2,6- positions. We present two synthetic approaches: (i) building of the anthracene tricyclic fused ring system from the appropriate benzene derivatives, and (ii) reduction of the corresponding anthraquinones. Reduction of 2-hydroxyanthracene-1-carbaldehyde to the corresponding alcohol yields rather unexpected 1,1′-methylenedianthracen-2-ol, whose proposed mechanism of formation is supported by experimental observations and calculations.

We have derivatized Momordica saponins (MS) I and II through their coupling at C3 glucuronic acid site with dodecylamine. The derivatives show significantly different immunostimulant activity profiles from their respective natural parent saponins. In particular, adjuvant VSA-1 (5), the derivative of MS I, potentiates a significantly higher IgG2a responose than the corresponding natural product. Its IgG1 and IgG2a production is similar to that of GPI-0100, indicating a potential mixed and antigen-specific Th1/Th2 immune response, which is different from the Th2 immunity induced by the natural saponin MS I. In addition, toxicity evaluations show that adjuvant VSA-1 (5) is much less toxic than the widely used natural saponin mixture Quil A. These results prove that derivatizing Momordica saponins can be a viable way for easy access to structurally defined saponin immunostimulants with favorable adjvuant activity and low toxicity.

Formation of quinone methides (QMs) by photoelimination of an ammonium salt from cresol derivatives was investigated by femtosecond transient absorption spectroscopy (fs-TA) and computationally by TD-DFT using the PCM(water)/(TD-)B3LYP/6-311++G(d, p) level of theory. The photoelimination takes place in an adiabatic ultrafast reaction on the S1 potential energy surface delivering the corresponding QMs(S1), which were detected by fs-TA. Computations predicted a high energy cation intermediate in the pathway between the Franck-Condon state of a monoammonium salt and the corresponding QM(S1) that was not detected by fs-TA. On the other hand, elimination from a disalt in H2O takes place in one step, giving directly the QM(S1). The combined experimental and theoretical investigation fully disclosed the formation of QMs by the deamination reaction mechanism, which is important in the application of cresols or similar molecules in biological systems.

We have derivatized Momordica saponins (MS) I and II through their coupling at C3 glucuronic acid site with dodecylamine. The derivatives show significantly different immunostimulant activity profiles from their respective natural parent saponins. In particular, adjuvant VSA-1 (5), the derivative of MS I, potentiates a significantly higher IgG2a responose than the corresponding natural product. Its IgG1 and IgG2a production is similar to that of GPI- 0100, indicating a potential mixed and antigen- specific Th1/Th2 immune response, which is different from the Th2 immunity induced by the natural saponin MS I. In addition, toxicity evaluations show that adjuvant VSA-1 (5) is much less toxic than the widely used natural saponin mixture Quil A. These results prove that derivatizing Momordica saponins can be a viable way for easy access to structurally defined saponin immunostimulants with favorable adjvuant activity and low toxicity.

We have synthesized a QS-17/18 analogue (7) and evaluated its adjuvant activity in the formulation with rHagB antigen. Compound 7 and QS-21 analogues 5 and 6 are presumably the major components of GPI-0100, a widely used complex mixture of semisynthetic derivatives of Quillaja saponaria (QS) Molina saponins. The QS-17/18 analogue 7 shows an adjuvant activity profile similar to that of GPI-0100, potentiating mixed Th-1/Th-2 immune responses, which is different from those of QS-21 analogues 5 and 6 that probably only induce a Th2-like immunity. The combination of QS-17/18 and QS-21 analogues does not show a synergistic effect. These results suggest that QS-17/18 analogue 7 might be the active component of GPI-0100 responsible for its immunostimulant property. Therefore, compound 7 can not only be a structurally defined alternative to GPI-0100 but also provide a valuable clue for rational design of new QS-based vaccine adjuvants with better adjuvant properties.

There is increasing interest in developing new vaccines against cancers, infectious diseases and degenerative disorders. Vaccine adjuvant is a substance formulated as part of a vaccine boost and/or modulates the antigen-specific immune responses. Recently, QS-21 was approved as a component of the GlaxoSmithKline’s AS01b adjuvant formulation for the human vaccine Shingrix. QS-21 is a mixture of two isomeric saponins extracted from the bark of Quillaja saponaria Molina. It has been studied in over one hundred clinical trials of vaccines against cancers and infectious diseases. However, it has its own drawbacks including chemical instability, dose-limiting toxicity, limited supply, and laborious and low- yielding purification. Significant efforts have been devoted to developing synthetic analogs of QS-21 to overcome the limitations of the natural product. We have designed and synthesized two analogs (3 and 4) of QS-7, a natural saponin isolated from Quillaja saponaria Molina tree bark. The only structural difference between compounds 3 and 4 is that 3 is acetylated at the 3- and 4-O positions of the quillaic acid C28 fucosyl unit, whereas 4 is not. Interestingly, the immunostimulant profiles of the two compounds are significantly different. Compound 3 potentiates a mixed Th1/Th2 (Th, T helper cells) immune response against the tested antigens, whereas compound 4 showed induction of Th2-biased immunity only. These results suggest that the 3- and 4-O acetyl groups of the fucosyl unit may play an important role in tuning the adjuvanticity of the QS-7 analogs. Compound 3 can serve as a structurally defined synthetic adjuvant when a mixed Th1/Th2 immune response is desired and also as a molecular probe for mechanistic studies.

New bifunctional quinone methide (QM) precursors, bisphenols 2a–2e, and monofunctional QM precursor 7 were synthesized. Upon treatment with fluoride, desilylation triggers formation of reactive intermediates, QMs, which was demonstrated by trapping QM with azide or methanol. The ability of QMs to alkylate and cross-link DNA was assayed by investigation of the effects of QMs to DNA denaturing, but without conclusive evidence. Furthermore, treatment of a plasmid DNA with compounds 2a–2e and KF, followed by the analysis by alkaline denaturing gel electrophoresis, did not provide evidence for the DNA cross-linking. MTT test performed on two human cancer cell lines (MCF7 breast adenocarcinoma and SUM159 pleomorphic breast carcinoma), with and without fluoride, indicated that 2a–2e or the corresponding QMs did not exhibit cytotoxic activity, in line with the lack of ability to cross-link DNA. The lack of reactivity with DNA and biological activity were explained by sequential formation of QMs where bifunctional cytotoxic reagent is probably never produced. Instead, the sequential generation of monofunctional QM followed by a faster hydrolysis leads to the destruction of biologically active reagent. The findings described here are particularly important for the rational design of new generation of QM precursor molecules that will attain desirable DNA reactivity and cytotoxicity. This work is licensed under a Creative Commons Attribution 4.0 International License.

A series of naphthalene-1,5- and 2,6-diamines and their N-methyl derivatives were synthesized and characterized. 1H NMR spectroscopy showed that an aqueous solution of naphthalene-2,6-di(NMe3I) (2) underwent considerable demethylation over a 24 h period after its initial preparation. Exchange of the iodide nucleophilic anion by the triflate non-nucleophilic counter-ion afforded a stable quaternary aminium salt. H/D α-position exchange in D2O solutions of naphthalene-2,6-di(NH2MeCl) (3·HCl) and naphthalene-2,6-di(NHMe2Cl) (4·HCl) with acidic +NH protons was observed, but not in those of naphthalene-2,6-di(NMe3I) or triflate (2 or 5). Exhaustive per-N-methylation of naphthalene-1,5-di(NMe2) (7) afforded only the mixed naphthalene-1-NMe3I-5-NHMe2I quaternary aminium-tertiary ammonium salt due to severe 1,8-type peri-strain.

We report the binding constants of CB[7] toward a series of naphthalene diammonium and 4, 4’-dipiperidinium derivatives and compare the results with those obtained previously for complex of CB[7] with 4, 9-di(NMe3I)diamantane by 1H NMR and X-ray crystallography. The nature of binding in the host·guest complexes was investigated using quantum mechanical tools.

Cancer stem cells (CSCs) are a subpopulation of cancer cells that share properties of embryonic stem cells like pluripotency and self-renewal and show increased resistance to chemo- and radiotherapy. Targeting CSC, rather than cancer cells in general, is a novel and promising strategy for cancer treatment. Novel therapeutic approaches, such as photodynamic therapy (PDT) have been investigated. A promising group of phototherapeutic agents are reactive intermediates - quinone methides (QMs). This study describes preparation of QM precursor, 2-hydroxy-3- hydroxymethylanthracene (2) and a detailed photochemical and photobiological investigation on similar anthracene derivatives 3 and 4. Upon photoexcitation with near visible light at λ > 400 nm 1 and 2 give QMs, that were detected by laser flash photolysis and their reactivity with nucleophiles has been demonstrated in the preparative irradiation experiments where the corresponding adducts were isolated and characterized. 3 and 4 cannot undergo photodehydration and deliver QM, but lead to the formation of phenoxyl radical and singlet oxygen, respectively. The activity of 1-4 was tested on a panel of human tumor cell lines, while special attention was devoted to demonstrate their potential selectivity towards the cells with CSC- like properties (HMLEshEcad). Upon the irradiation of cell lines treated with 1-4, an enhancement of antiproliferative activity was demonstrated, but the DNA was not the target molecule. Confocal microscopy revealed that these compounds entered the cell and, upon irradiation, reacted with cellular membranes. Our experiments demonstrated moderate selectivity of 2 and 4 towards CSC-like cells, while necrosis was shown to be a dominant cell death mechanism. Especially interesting was the selectivity of 4 that produced higher levels of ROS in CSC-like cells, which forms the basis for further research on cancer phototherapy, as well as for the elucidation of the underlying mechanism of the observed CSC selectivity based on oxidative stress activation.

Anthrols 2-7 were synthesized and their photochemical reactivity investigated by irradiations in aq. CH3OH. Upon excitation with visible light (λ > 400 nm) in methanolic solutions they undergo photodehydration or photodeamination and deliver methyl ethers, most probably via quinone methides (QMs), with methanolysis quantum efficiencies ΦR = 0.02-0.3. Photophysical properties of 2-7 were determined by steady-state fluorescence and time-correlated single photon counting. Generally, anthrols 2-7 are highly fluorescent in aprotic solvents (ΦF = 0.5-0.9), whereas in aqueous solutions the fluorescence is quenched due to excited state proton transfer (ESPT) to solvent. The exception is amine 4 that undergoes excited state intramolecular proton transfer (ESIPT) in neat CH3CN where photodeamination is probably coupled to ESIPT. Photodehydration may take place via ESIPT (or ESPT) that is coupled to dehydration, or via a hitherto undisclosed pathway that involves photoionization and deprotonation of radical-cation, followed by homolytic cleavage of the alcohol OH group from the phenoxyl radical. QMs were detected by laser flash photolysis (LFP) and their reactivity with nucleophiles investigated. Biological investigation of 2-5 on human cancer cell lines showed enhancement of antiproliferative effect upon exposure of cells to irradiation by visible light, probably due to formation of electrophilic species such as QMs.

Kinon-metidi (QM, naziv po IUPAC-u: kinometani) su reaktivni međuprodukti koji su povezani s biološkom aktivnosti brojnih prirodnih i sintetski dobivenih spojeva. Njihova aktivnost temelji se na inhibiciji enzima ili, češće, na alkilaciji/križnom povezivanju DNA. QM se mogu generirati termičkim metodama, ali veliku prednost imaju fotokemijske metode, jer pružaju mogućnost prostorne i vremenske kontrole nad njihovim stvaranjem. Za konačni cilj – primjenu u fotokemoterapiji od presudne je važnosti razvoj prekursora za generiranje QM-a. Ovim pregledom bit će dan uvid u fotokemijske metode generiranja QM-a i razvoj njihove primjene u fotokemoterapiji. Ovo djelo je dano na korištenje pod licencom Creative Commons Imenovanje 4.0 međunarodna., Quinone-methides (QMs, IUPAC name: quinomethanes) are reactive intermediates that are associated with the biological activity of numerous natural and synthetic compounds. Their activity is based on the inhibition of enzymes or, more often, on the alkylation/cross-linking of DNA. QMs can be generated by thermal methods, but photochemical methods have great advantages because they provide the possibility of spatial and temporal control over their generation. For the final goal – application in photochemotherapy, the development of precursors for the generation of QMs is crucial. This review will provide insight into the photochemical methods of generating QMs and development of their use in photochemotherapy. This work is licensed under a Creative Commons Attribution 4.0 International License.

Six new naphthylphenols 7-12, bearing bulky hydroxymethyl substituents on the naphthalene, were synthesized and their photoreactivity investigated by preparative irradiations, fluorescence measurements, and laser flash photolysis. All derivatives (in S1) undergo deprotonation of the phenolic OH in the aqueous solution. As well, fluorescence quenching with HClO4 in the pH range 2-4 indicates that 7-12 can be protonated in S1. Formation of QMs most probably takes place sequentially, triggered by the phenol deprotonation. However, with the present data, a mechanism that involves simultaneous deprotonation and the loss of OH- cannot be ruled out. Photodehydration takes place only for 7, 9, 11 and 12, delivering the corresponding QMs which react with nucleophilic solvents giving the corresponding photosolvolysis products. The other less likely option for the formation of the observed solvolysis products from 7, 9, 11 and 12 may involve some radical species. Photodehydration of 8 and 10 was not observed which may be due to the anticipated high energy of the corresponding sterically-congested QM8 and QM10. The most efficient photosolvolyses were observed for the 2, 6-substituted naphthalenes.

We have demonstrated that quinone methide (QM) precursors can be introduced in the peptide structure and used as photoswitchable units for peptide modifications. QM precursor 1 was prepared from protected tyrosine in the Mannich reaction, and further used as a building block in peptide synthesis. Moreover, peptides containing tyrosine can be transformed into a photoactivable QM precursor by the Mannich reaction which can afford monosubstituted derivatives 2 or bis-substituted derivatives 3. Photochemical reactivity of modified tyrosine 1 and dipeptides 2 and 3 was studied by preparative irradiation in CH3OH where photodeamination and photomethanolysis occur. QM precursors incorporated in peptides undergo photomethanolysis with quantum efficiency phi(R) = 0.1-0.2, wherein the peptide backbone does not affect their photochemical reactivity. QMs formed from dipeptides were detected by laser flash photolysis (lambda(max) approximate to 400 nm, iota = 100 mu s- 20 ms) and their reactivity with nucleophiles was studied. Consequently, QM precursors derived from tyrosine can be a part of the peptide backbone which can be transformed into QMs upon electronic excitation, leading to the reactions of peptides with different reagents. This proof of principle showing the ability to photochemically trigger peptide modifications and interactions with other molecules can have numerous applications in organic synthesis, materials science, biology and medicine.

Metal complexes with ML or ML2 stoichiometry have been isolated in the reaction of Zn(NO3)2, ZnBr2 or M(NO3)2/NaBF4, M = Zn(II), Co(II) or Ni(II), with either amino acid or amine substituted tridentate nitrogen ligands based on bis(2-picolyl)amine (bpa) or bis(2-quinaldyl)amine (bqa). The stoichiometry (M : L = 1 : 1 or 1 : 2) and stereochemistry (mer, trans-fac or cis-fac) of the products have been studied by NMR and IR spectroscopy, X-ray single crystal analysis and quantum-chemical calculations with an implicit SMD solvation model.

Derivatives of p-cresol 1-4 were synthesized, and their photochemical reactivity, acid-base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9-11) or diamine is in the monocationic form (pH 7-9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.

Excitation of 2-hydroxy-3-(diphenylhydroxymethyl)anthracene (7) to S1 initiates photodehydration giving the corresponding quinone methide (QM) that was detected by laser flash photolysis (LFP) in 2, 2, 2-trifluoroethanol TFE (λ = 580 nm, τ = 690 ± 10 ns). The QM decays by protonation, giving cation (λ = 520 nm, τ = 84 ± 3 μs) which subsequently reacts with nucleophiles. The rate constants in the reactions with nucleophiles were determined by LFP, whereas the adducts were isolated via preparative photolyses. The photogeneration of QMs in the anthrol series is important for potential use in biological systems since the chromophore absorbs at wavelengths > 400 nm. Antiproliferative investigations conducted with 2-anthrol derivative 7 on three human cancer cell lines showed higher activity for irradiated cells.

Ribosomal protein biosynthesis is a key cellular process because proteins are the main cellular building blocks on all scales of life. Ensuring substrates for protein biosynthesis, evolutionarily conserved aminoacyl-tRNA synthetases (aaRSs) covalently couple amino acids to cognate tRNAs. This makes aaRSs important targets for natural and man-made antibiotics. A good example are isoleucyl-tRNA-synthetases (IleRS) which are susceptible to inhibition with mupirocin, a naturally produced antibiotic from the bacterium Pseudomonas fluorescens. Bacterial IleRSs cluster into two clades differing in susceptibility to mupirocin inhibition. IleRS1, present in most pathogenic bacteria, are up to 9 orders of magnitude more susceptible to mupirocin inhibition than bacterial IleRS2, which share similarities with eucaryotic IleRS, thus making IleRS1 favorable pharmaceutical targets. To investigate the origin of mupirocin insensitivity in IleRS2, crystal structures of IleRS2, and IleRS1 in a complex with mupirocin and a nonhydrolyzable analog of the reaction intermediate, Ile-AMS, were solved. Bacillus megaterium was chosen because it has genomic copies of both IleRS1 and IleRS2. By analyzing the solved structures we have shown that mupirocin binding to the two IleRS types, although similar, differs in the number of established mupirocin interactions and steric rearrangements which coincide with mupirocin binding. In IleRS2, mupirocin lacks two active site interactions compared to IleRS1 and the binding coincides with steric rearrangements of key active site parts. In contrast, the IleRS1 active site seems preformed for high-affinity mupirocin binding, with only a little steric rearrangement taking place upon the mupirocin binding. Thus, we propose a mechanism wherein the differences between mupirocin resistance of IleRS1 and IleRS2 are not only correlated to the differences in the interactions mupirocin establishes in the active site, but also to the steric rearrangements (or lack thereof) of the IleRS active site that accompany mupirocin binding.