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The interaction of the self-assembled trinuclear ruthenium bowl 13+ , that displays three other accessible oxidation states, with oxo-anions is investigated. Using a combination of NMR and electrochemical experimental data, estimates of the binding affinities of 14+ , 15+ , and 16+ for both halide and oxo-anions were derived. This analysis revealed that, across the range of oxidation states of the host, both high anion binding affinities (>109 â M-1 for specific guests bound to 16+ ) and high selectivities (a range of >107 â M-1 ) were observed. As the crystal structure of binding of the hexafluorophosphate anion revealed that the host has two potential binding sites (named the α and ß pockets), the host-guest properties of both putative binding sites of the bowl, in all of its four oxidation states, were investigated through detailed quantum-based computational studies. These studies revealed that, due to the interplay of ion-ion interactions, charge-assisted hydrogen-bonding and anion-π interactions, binding to the α pocket is generally preferred, except for the case of the relatively large and lipophilic hexafluorophosphate anionic guest and the host in the highest oxidation states, where the ß pocket becomes relatively favourable. This analysis confirms that host-guest interactions involving structurally complex supramolecular architectures are driven by a combination of non-covalent interactions and, even in the case of charged binding pairs, simple ion-ion interactions alone cannot accurately define these recognition processes.
Assuntos
Ânions , Sítios de Ligação , Ligação de Hidrogênio , OxirreduçãoRESUMO
This work demonstrates photocatalytic CO2 reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4'-{Et2O3PCH2}2-2,2'-bipyridyl)(CO)3] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO2 to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl4 (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO2 reduction. The incorporation of -P(O)(OEt)2 groups, decoupled from the core of the catalyst by a -CH2- spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by 1H and 13C{1H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.
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Size and shape criteria for guest binding inside the cavity of an octanuclear cubic coordination cage in water have been established using a new fluorescence displacement assay to quantify guest binding. For aliphatic cyclic ketones of increasing size (from C5 to C11), there is a linear relationship between ΔG for guest binding and the guest's surface area: the change in ΔG for binding is 0.3 kJ mol(-1) Å(-2), corresponding to 5 kJ mol(-1) for each additional CH2 group in the guest, in good agreement with expectations based on hydrophobic desolvation. The highest association constant is K = 1.2 × 10(6) M(-1) for cycloundecanone, whose volume is approximately 50% of the cavity volume; for larger C12 and C13 cyclic ketones, the association constant progressively decreases as the guests become too large. For a series of C10 aliphatic ketones differing in shape but not size, ΔG for guest binding showed no correlation with surface area. These guests are close to the volume limit of the cavity (cf. Rebek's 55% rule), so the association constant is sensitive to shape complementarity, with small changes in guest structure resulting in large changes in binding affinity. The most flexible members of this series (linear aliphatic ketones) did not bind, whereas the more preorganized cyclic ketones all have association constants of 10(4)-10(5) M(-1). A crystal structure of the cage·cycloundecanone complex shows that the guest carbonyl oxygen is directed into a binding pocket defined by a convergent set of CH groups, which act as weak hydrogen-bond donors, and also shows close contacts between the exterior surface of the disc-shaped guest and the interior surface of the pseudospherical cage cavity despite the slight mismatch in shape.
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Effective molarity (EM) is a key parameter that determines the efficiency of a range of supramolecular phenomena from the folding of macromolecules to multivalent ligand binding. Coordination complexes formed between zinc porphyrins equipped H-bond donor sites and pyridine ligands equipped with H-bond acceptor sites have allowed systematic quantification of EM values for the formation of intramolecular H-bonds in 240 different systems. The results provide insights into the relationship of EM to supramolecular architecture, H-bond strength, and solvent. Previous studies on ligands equipped with phosphonate diester and ether H-bond acceptors were inconclusive, but the experiments described here on ligands equipped with phosphine oxide, amide, and ester H-bond acceptors resolve these ambiguities. Chemical double-mutant cycles were used to dissect the thermodynamic contributions of individual H-bond interactions to the overall stabilities of the complexes and hence determine the values of EM, which fall in the range 1-1000 mM. Solvent has little effect on EM, and the values measured in toluene and 1,1,2,2-tetrachloroethane are similar. For H-bond acceptors that have similar geometries but different H-bond strengths (amide and ester), the values of EM are very similar. For H-bond acceptors that have different geometries but similar H-bond strengths (amide and phosphonate diester), there is little correlation between the values of EM. These results imply that supramolecular EMs are independent of solvent and intrinsic H-bond strength but depend on supramolecular architecture and geometric complementarity.
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The association constants for a family of 96 closely related zinc porphyrin-pyridine ligand complexes have been measured in two different solvents, toluene and 1,1,2,2-tetrachloroethane (TCE). The zinc porphyrin receptors are equipped with phenol side arms, which can form intramolecular H-bonds with ester or amide side arms on the pyridine ligands. These association constants were used to construct 64 chemical double mutant cycles, which measure the free energy contributions of intramolecular H-bonding interactions to the overall stability of the complexes. Measurement of association constants for the corresponding intermolecular H-bonding interactions allowed determination of the effective molarities (EM) for the intramolecular interactions. Comparison of ligands that feature amide H-bond acceptors and ester H-bonds at identical sites on the ligand framework show that the values of EM are practically identical. Similarly, the values of EM are practically identical in toluene and in TCE. However, comparison of two ligand series that differ by one degree of torsional freedom shows that the values of EM for the flexible ligands are an order of magnitude lower than for the corresponding rigid ligands. This observation holds for a range of different supramolecular architectures with different degrees of receptor-ligand complementarity and suggests that in general the cost of freezing a rotor in supramolecular complexes is of the order of 5 kJ/mol.
Assuntos
Metaloporfirinas/química , Piridinas/química , Zinco/química , Ligação de Hidrogênio , Ligantes , Substâncias Macromoleculares/síntese química , Substâncias Macromoleculares/química , Metaloporfirinas/síntese química , Modelos Moleculares , Conformação MolecularRESUMO
The host-guest chemistry of the octanuclear cubic coordination cage [Co(8)L(12)](16+) (where L is a bridging ligand containing two chelating pyrazolyl-pyridine units connected to a central naphthalene-1,5-diyl spacer via methylene "hinges") has been investigated in detail by (1)H NMR spectroscopy. The cage encloses a cavity of volume of ca. 400 Å(3), which is accessible through 4 Å diameter portals in the centers of the cube faces. The paramagnetism of the cage eliminates overlap of NMR signals by dispersing them over a range of ca. 200 ppm, making changes of specific signals easy to observe, and also results in large complexation-induced shifts of bound guests. The cage, in CD(3)CN solution, acts as a remarkably size- and shape-selective host for small organic guests such as coumarin (K = 78 M(-1)) and other bicyclic molecules of comparable size and shape such as isoquinoline-N-oxide (K = 2100 M(-1)). Binding arises from two independent recognition elements, which have been separately quantified. These are (i) a polar component arising from interaction of the H-bond accepting O atom of the guest with a convergent group of CH protons inside the cavity that lie close to a fac tris-chelate metal center and are therefore in a region of high electrostatic potential; and (ii) an additional component arising from the second aromatic ring (aromatic/van der Waals interactions with the interior surface of the cage and/or solvophobic interactions). The strength of the first component varies linearly with the H-bond-accepting ability of the guest; the second component is fixed at approximately 10 kJ mol(-1). We have also used (1)H-(1)H exchange spectroscopy (EXSY) experiments to analyze semiquantitatively two distinct dynamic processes, viz. movement of the guest into and out of the cavity and tumbling of the guest inside the host cavity. Depending on the size of the guest and the position of substituents, the rates of these processes can vary substantially, and the rates of processes that afford observable cross-peaks in EXSY spectra (e.g., between free and bound guest in some cases; between different conformers of a specific host·guest complex in others) can be narrowed down to a specific time window. Overall, the paramagnetism of the host cage has allowed an exceptionally detailed analysis of the kinetics and thermodynamics of its host-guest behavior.
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Extracellular vesicles (EVs) are lipid bilayer structures released by all cells that mediate cell-to-cell communication via the transfer of bioactive cargo. Because of the natural origin of EVs, their efficient uptake by recipient cells, capacity to stabilize and transport biomolecules and their potential for cell/tissue targeting and preferential uptake by cancer cells, they have enormous potential for bioengineering into improved and targeted drug delivery systems. In this work, we investigated the use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool to measure the loading of platinum-based chemotherapeutic agents. The EV loading of oxaliplatin via co-incubation was demonstrated, and LA-ICP-MS imaging showed greater efficiency of delivery to colorectal cancer cells compared to free oxaliplatin, leading to enhanced cytotoxic effect. Further, the impact of EV co-loading with a porphyrin (C5SHU, known as 'C5') photosensitizer on oxaliplatin delivery was assessed. Fluorescence analysis using nano-flow cytometry showed dose-dependent EV loading as well as a trend towards the loading of larger particles. Exposure of OXA-C5-EV-treated colorectal cancer cells to light indicated that delivery was enhanced by both light exposure and porphyrins, with a synergistic effect on cell viability observed between oxaliplatin, EVs and light exposure after the delivery of the co-loaded EVs. In summary, this work demonstrates the utility of LA-ICP-MS and mass spectrometry imaging in assessing the loading efficiency and cellular delivery of platinum-based therapeutics, which would also be suitable for agents containing other elements, confirms that EVs are more efficient at delivery compared to free drugs, and describes the use of light exposure in optimizing delivery and therapeutic effects of EV-mediated drug delivery both in combination and independently of porphyrin-based photosensitizers.
Assuntos
Neoplasias Colorretais , Vesículas Extracelulares , Terapia a Laser , Porfirinas , Humanos , Oxaliplatina/farmacologia , Espectrometria de Massas , Neoplasias Colorretais/tratamento farmacológicoRESUMO
Free energy contributions due to intramolecular phosphonate diester-phenol H-bonds have been measured for 20 different supramolecular architectures in cyclohexanone solution. High throughput UV/Vis titrations were used in combination with chemical double mutant cycles to dissect out the contributions of different functional group interactions to the stabilities of over 100 different zinc porphyrin-pyridine ligand complexes. These complexes have previously been characterised in toluene and in 1,1,2,2-tetrachloroethane (TCE) solution. Intramolecular ester-phenol H-bonds that were measured in these less polar solvents are too weak to be detected in cyclohexanone, which is a more competitive solvent. The stability of the intermolecular phosphonate diester-phenol H-bond in cyclohexanone is an order of magnitude lower than in TCE and two orders of magnitude lower than in toluene. As a consequence, only seven of the twenty intramolecular phosphonate diester-phenol interactions that were previously measured in toluene and TCE could be detected in cyclohexanone. The effective molarities (EM) for these intramolecular interactions are different in all three solvents. Determination of the EM accounts for solvent effects on the strengths of the individual H-bonding interactions and the zinc porphyrin-pyridine coordination bond, so the variation in EM with solvent implies that differences in the solvation shells make significant contributions to the overall stabilities of the complexes. The results suggest that steric effects lead to desolvation of bulky polar ligands. This increases the EM values measured in TCE, because ligands that fail to replace the strong interactions made with this solvent are unusually weakly bound compared with ligands that make intramolecular H-bonds.
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Clinical trials evaluating intrapleural photodynamic therapy (PDT) are ongoing for mesothelioma. Several issues still hinder the development of PDT, such as those related to the inherent properties of photosensitizers. Herein, we report the synthesis, photophysical, and photobiological properties of three porphyrin-based photosensitizers conjugated to truncated fatty acids (C5SHU to C7SHU). Our photosensitizers exhibited excellent water solubility and high PDT efficiency in mesothelioma. As expected, absorption spectroscopy confirmed an increased aggregation as a consequence of extending the fatty acid chain length. In vitro PDT activity was studied using human mesothelioma cell lines (biphasic MSTO-211H cells and epithelioid NCI-H28 cells) alongside a non-malignant mesothelial cell line (MET-5A). The PDT effect of these photosensitizers was initially assessed using the colorimetric WST-8 cell viability assay and the mode of cell death was determined via flow cytometry of Annexin V-FITC/PI-stained cells. Photosensitizers appeared to selectively localize within the non-nuclear compartments of cells before exhibiting high phototoxicity. Both apoptosis and necrosis were induced at 24 and 48 h. As our pentanoic acid-derivatized porphyrin (C5SHU) induced the largest anti-tumor effect in this study, we put this forward as an anti-tumor drug candidate in PDT and photo-imaging diagnosis in mesothelioma.
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Intermolecular complexes formed between metalloporphyrins and pyridine ligands equipped with multiple H-bond donors and acceptors have been used to measure the free energy contributions due to intramolecular ether-phenol H-bonding in the 24 different supramolecular architectures using chemical double mutant cycles in toluene. The ether-phenol interactions are relatively weak, and there are significant populations of partially bound states where between zero and four intramolecular H-bonds are made in addition to the porphyrin-ligand coordination interaction. The complexes were analyzed as ensembles of partially bound states to determine the effective molarities for the intramolecular interactions by comparison with the corresponding intermolecular ether-phenol H-bonds. The properties of the ether-phenol interactions were compared with phosphonate diester-phenol interactions in a closely related ligand system, which has more powerful H-bond acceptor oxygens positioned at the same location on the ligand framework. This provides a comparison of the properties of weak and strong H-bonds embedded in the same 24 supramolecular architectures. When the product of the intermolecular association constant and the effective molarity KEM > 1, there is a linear increase in the free energy contribution due to H-bonding with log EM, because the intramolecular interactions contribute fully to the stability of the complex. When KEM < 1, the H-bonded state is not significantly populated, and there is no impact on the overall stability of the complex. Intermolecular phosphonate diester-phenol H-bonds are 2 orders of magnitude stronger than ether-phenol H-bonds in toluene, so for the phosphonate diester ligand system, 23 of the 24 supramolecular architectures make intramolecular H-bonds. However, only 8 of these architectures lead to detectable H-bonding in the ether ligand system. The other 15 complexes have a suitable geometry for formation of H-bonds, but the ether-phenol interaction is not strong enough to overcome the reorganization costs associated with making intramolecular contacts, i.e., KEM < 1 for the ether ligands, and KEM > 1 for the phosphonate diester ligands. The values of EM measured for two different types of H-bond acceptor are linearly correlated, which suggests that EM is a property of the supramolecular acrchitecture. However, the absolute value of EM for an intramolecular phosphonate diester H-bond is about 4 times lower than the corresponding value for an intramolecular ether-phenol interaction embedded in the same supramolecular framework, which suggests that there may be some interplay of K and EM.
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The synthesis of a family of zinc porphyrins and pyridine ligands equipped with peripheral H-bonding functionality has provided access to a wide range of closely related supramolecular complexes featuring between zero and four intramolecular H-bonds. An automated UV/vis titration system was used to characterize 120 different complexes, and these data were used to construct a large of number of different chemical double mutant cycles to quantify the intramolecular H-bonding interactions. The results probe the quantitative structure-activity relationship that governs cooperativity in the assembly of complex molecular recognition interfaces. Specifically, variations in the chemical structures of the complexes have allowed us to change the supramolecular architecture, conformational flexibility, geometric complementarity, the number and nature of the H-bond interactions, and the overall stability of the complex. The free energy contributions from individual H-bonds are additive, and there is remarkably little variation with architecture in the effective molarity for the formation of intramolecular interactions. Intramolecular H-bonds are not observed in complexes where they are geometrically impossible, but there are no cases where excellent geometric complementarity leads to very high affinities. Similarly, changes in conformational flexibility seem to have limited impact on the values of effective molarity (EM). The major variation that was found for all of the 48 intramolecular interactions that were examined using double mutant cycles is that the values of EM for intramolecular carboxylate ester-phenol H-bonds (200 mM) are an order of magnitude larger than those found for phosphonate diester-phenol H-bonds (30 mM). The corresponding intermolecular phosphonate diester-phenol H-bonds are 2 orders of magnitude more stable than carboxylate ester-phenol H-bonds, and the large differences in EM may be due to some kind of compensation effect, where the stronger H-bond is harder to make, because it imposes tighter constraints on the geometry of the complex.
Assuntos
Porfirinas/química , Ligação de Hidrogênio , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Espectrofotometria Ultravioleta , TermodinâmicaRESUMO
A family of four biscarbamates (AA) and four bisphenols (DD) were synthesized, and H-bonding interactions between all AAâ¢DD combinations were characterized using (1)H NMR titrations in carbon tetrachloride. A chemical double mutant cycle analysis shows that there are no secondary electrostatic interactions or allosteric cooperativity in these systems, and the system therefore provides an ideal platform for investigating the relationship between chemical structure and chelate cooperativity. Effective molarities (EMs) were measured for 12 different systems, where the number of rotors in the chains connecting the two H-bond sites was varied from 5 to 20. The association constants vary by less than an order of magnitude for all 12 complexes, and the variation in EM is remarkably small (0.1-0.9 M). The results provide a relationship between EM and the number of rotors in the connecting chains (r): EM ≈ 10r(-3/2). The value of 10 M is the upper limit for the value of EM for a noncovalent intramolecular interaction. Introduction of rotors reduces the value of EM from this maximum in accord with a random walk analysis of the encounter probability of the chain ends (r(-3/2)). Noncovalent EMs never reach the very high values observed for covalent processes, which places limitations on the magnitudes of the effects that one is likely to achieve through the use of chelate cooperativity in supramolecular assembly and catalysis. On the other hand, the decrease in EM due to the introduction of conformational flexibility is less dramatic than one might expect based on the behavior of covalent systems, which limits the losses in binding affinity caused by poor preorganization of the interaction sites.
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Chemical double mutant cycles have been used in conjunction with new H-bonding motifs for the quantification of chelate cooperativity in multiply H-bonded complexes. The double mutant cycle approach specifically deals with the effects of substituents, secondary interactions, and allosteric cooperativity on the free energy contributions from individual H-bond sites and allows dissection of the free energy contribution due to chelate cooperativity associated with the formation of intramolecular noncovalent interactions. Two different doubly H-bonded motifs were investigated in carbon tetrachloride, chloroform, 1,1,2,2-tetrachloroethane, and cyclohexane, and the results were similar in all cases, with effective molarities of 3-33 M for formation of intramolecular H-bonds. This corresponds to a free energy penalty of 3-9 kJ mol(-1) for formation of a bimolecular complex in solution, which is consistent with previous estimates of 6 kJ mol(-1). This result can be used in conjunction with the H-bond parameters, alpha and beta, to make a reasonable estimate of the stability constant for formation of a multiply H-bonded complex between two perfectly complementary partners, or to place an upper limit on the stability constant expected for a less complementary system.
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The free energy of complexation of supramolecular complexes containing phenol-carbamate H-bonds is an additive function of the number of H-bonds, with a constant increment of 6 kJ mol(-1) per interaction in carbon tetrachloride.
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A zinc porphyrin equipped with four amide H-bonding sites provides a rigid molecular receptor for the study of cooperative multipoint binding interactions. The interaction of this receptor with a variety of pyridine ligands bearing zero, one, and two H-bonding sites has been studied using UV/vis absorption, (1)H and (31)P NMR spectroscopy, and isothermal titration calorimetry in five different solvents. The results are analyzed in terms of a bound state that populates an ensemble of different complexes in which zero, one, or two of the potential H-bond interactions are formed. The key parameter that determines the behavior of the system is the product of the association constant for the H-bond interaction, K(H), and the effective molarity for the intramolecular interaction, EM. In the system reported here, EM is 0.1-1 M for all of the intramolecular interactions. For strong H-bonds (large K(H) in nonpolar solvents), all of the interactions are formed in the complex and the fully bound state dominates. In this case, additional binding interactions produce incremental increases in complex stability. However, for weaker H-bonds (small K(H) in polar solvents), the formation of additional interactions does not lead to an increase in the overall stability of the complex, due to the population of partially bound states.
Assuntos
Amidas/química , Ligação de Hidrogênio , Metaloporfirinas/química , Piridinas/química , Zinco/química , Sítios de Ligação , Calorimetria/métodos , Fenômenos Químicos , Cinética , Ligantes , Espectrofotometria Ultravioleta/métodosRESUMO
Nitrones undergo dynamic exchange in chloroform at room temperature through two mechanisms-hydrolysis and recombination or hydroxylamine addition/elimination; this dynamic exchange is harnessed to select a nitrone-based bis(amidopyridine) receptor for diacids from a group of four nitrones through its binding to a glutaric acid-based target.
Assuntos
Óxidos de Nitrogênio/química , Aminopirina/química , Clorofórmio/química , Glutaratos/química , Hidrólise , Hidroxilamina/química , Espectroscopia de Ressonância Magnética , TemperaturaRESUMO
Dipeptidyl peptidase IV (DPP-IV also referred to as CD-26) is a serine protease enzyme with remarkable diagnostic and prognostic value in a variety of health and disease conditions. Herein, we describe a simple and real-time colorimetric assay for DPP-IV/CD-26 activity based on the aggregation of gold nanoparticles (AuNPs) functionalized with the peptide substrates: Gly-Pro-Asp-Cys (GPDC) or Val-Pro-ethylene diamine-Asp-Cys (VP-ED-DC). Cleavage of the substrates by DPP-IV resulted in aggregation of the AuNPs with accompanying color change in the solution from red to blue that was monitored using either a UV-visible spectrophotometer or by the naked eye. Factors, such as time course of the reaction, stability of the functionalized AuNPs and the structure of the substrate that influence the cleavage reaction in solution were investigated. The effects of potential interference from serum proteins (lysozyme, thrombin and trypsin) on the analytical response were negligible. The detection limits when GPDC or VP-EN-DC functionalized AuNPs were used for DPP-IV assay were 1.2U/L and 1.5U/L, respectively. The VP-EN-DC method was preferred for the quantitative determination of DPP-IV activity in serum because of its wide linear range 0-30U/L compared to 0-12U/L for the GPDC assay. Recoveries from serum samples spiked with DPP-IV activity, between 5 and 25U/L, and using the VP-EN-DC modified AuNPs method ranged between 83.6% and 114.9%. The two colorimetric biosensors described here are superior to other conventional methods because of their simplicity, stability, selectivity and reliability.
Assuntos
Técnicas Biossensoriais/métodos , Colorimetria/métodos , Colorimetria/normas , Dipeptidil Peptidase 4/sangue , Dipeptidil Peptidase 4/metabolismo , Ouro/química , Nanopartículas Metálicas/química , Bioensaio , Humanos , Limite de DetecçãoRESUMO
[Structure: see text] A simple receptor and substrate are used to probe the relationship between transition-state charge and the level of rate acceleration that can be created by stabilizing the transition state through hydrogen bonding. Pericyclic reactions are accelerated less than 2-fold by the receptor, whereas a conjugate addition reaction is accelerated more than 30-fold. Therefore, substrate polarization by hydrogen bonding would only appear to be effective for reactions that generate significant charge at the transition state.
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Maleimidas/química , Ligação de Hidrogênio , Cinética , Modelos Moleculares , Estrutura MolecularRESUMO
The selective recognition-mediated reaction between a nitrone bearing a urea recognition site and a maleimide bearing a proton switchable recognition site can be turned 'on' and 'off' by the addition of base and acid respectively.
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Maleimidas/química , Óxidos de Nitrogênio/química , Ureia/química , Concentração de Íons de Hidrogênio , Estrutura Molecular , Fatores de TempoRESUMO
The protein/ligand docking software GOLD, which was originally developed for drug discovery, has been used in a virtual screen to identify small molecules that bind with extremely high affinities (K ≈ 107 M-1) in the cavity of a cubic coordination cage in water. A scoring function was developed using known guests as a training set and modified by introducing an additional term to take account of loss of guest flexibility on binding. This scoring function was then used in GOLD to successfully identify 15 new guests and accurately predict the binding constants. This approach provides a powerful predictive tool for virtual screening of large compound libraries to identify new guests for synthetic hosts, thereby greatly simplifying and accelerating the process of identifying guests by removing the reliance on experimental trial-and-error.