Monolayer properties of eight diastereomeric two-chain surfactants at the air/water interface: a resultant of intramolecular and intermolecular forces

In situ UV Resonance Raman Micro-spectroscopic Localization of the Antimalarial Quinine in Cinchona Bark

Deep UV resonance Raman micro-spectroscopy (lambda(exc) = 244 nm) was applied for a highly sensitive, selective, and gentle localization of the antimalarial quinine in situ in cinchona bark. The high potential of the method was demonstrated by the detection of small amounts of the alkaloid in the plant material without any further sample preparation, where conventional (non-resonant) Raman microscopy was unsuccessful due to a strong fluorescence background. The resonance Raman spectrum of cinchona bark corresponds well with that of quinine; it can be distinguished from its diastereomer quinidine via the mode at 831 cm(-1), which is shifted to 843 cm(-1) in the case of quinidine. This vibration involves a bending motion within the side chain around the chiral center of quinine. Vibrations belonging to the quinoline ring (important for its antimalarial activity in forming pi-pi-interactions to hemozoin) and the vinyl group are resonantly enhanced in the UV Raman spectra. A convincing mode assignment is derived by means of a combination of NIR Raman spectroscopy and DFT calculations. The Raman spectra of quinine in cinchona bark are modeled by considering a hydrous environment that causes a shift of the band at 1362 compared with 1371 cm(-1) in anhydrous quinine. This intense vibration is therefore sensitive to the presence of an aqueous environment and is assigned mostly to a stretching motion within the quinoline ring. The presented results nicely show the sensitivity of Raman spectroscopy to monitor subtle differences within the molecular structure and the influence of a biological relevant hydrous environment and trace low concentrated pharmaceutical relevant active agents in plant material.

Ultrasensitivein situTracing of the Alkaloid Dioncophylline A in the Tropical LianaTriphyophyllum peltatumby Applying Deep-UV Resonance Raman Microscopy

UV resonance Raman microspectroscopy was applied for a localization of the antiplasmodial naphthylisoquinoline alkaloid dioncophylline A in very low concentrations in different parts of the samples (e.g., in the roots) of the tropical liana Triphyophyllum peltatum. The application of resonance Raman microspectroscopy was characterized by a very high sensitivity and selectivity. It was possible to assign the resonance Raman spectra of dioncophylline A, dioncophylline C, and dioncopeltine A by means of a combination of NIR Raman spectroscopy and DFT calculations. The UV resonance Raman spectra of T. peltatum are very well resembled by the spectra of dioncophylline A, while they can be clearly distinguished from the spectra of dioncophylline C and dioncopeltine A. This distinction between the various naphthylisoquinolines was possible by the two modes at 1356 and 1613 cm-1. These two modes were assigned to C=C stretching and CH bending vibrations. The presented results of a highly sensitive and selective in situ localization of the active agent dioncophylline A in different parts of the plant material of T. peltatum are of high importance for the acquisition of new antimalarials and for plant science in general.

Structural analysis of the anti-malaria active agent chloroquine under physiological conditions

UV resonance Raman spectroscopy was applied for a selective enhancement of molecular vibrations of the important antimalarial chloroquine under physiological conditions. The resonance Raman spectra of chloroquine at pH values resembling the pH value of blood and the pH value of the acid food vacuole of plasmodium can unambiguously be distinguished via Raman resonantly enhanced mode at 721 cm(-1). These vibrations are assigned to -(CH2)n- rocking mode of the chloroquine side chain and are expected to be influenced by protonation of chloroquine. Furthermore, vibrations belonging to the quinoline ring (important for pi-pi-interactions to hemozoin) are resonantly enhanced and can be studied selectively. A convincing mode assignment was performed by means of DFT calculations. These calculations proved that the different protonation states of chloroquine remarkably influence various vibrational modes, the molecular geometry, and molecular orbitals. The presented results are of significant relevance for a Raman spectroscopical localization of chloroquine inside the acid food vacuole of plasmodium, the study of pi-pi-interactions of chloroquine to the biological target molecules hematin and hemozoin and the protonation state of chloroquine during this docking process. The protonation of the weak base chloroquine is considered to be crucial for an accumulation inside the acid food vacuole of plasmodium and an object for resistances against this drug.

Mapping Antimalarial Pharmacophores as a Useful Tool for the Rapid Discovery of Drugs Effective in Vivo: Design, Construction, Characterization, and Pharmacology of Metaquine

Resistant strains of Plasmodium falciparum and the unavailability of useful antimalarial vaccines reinforce the need to develop new efficacious antimalarials. This study details a pharmacophore model that has been used to identify a potent, soluble, orally bioavailable antimalarial bisquinoline, metaquine (N,N'-bis(7-chloroquinolin-4-yl)benzene-1,3-diamine) (dihydrochloride), which is active against Plasmodium berghei in vivo (oral ID(50) of 25 micromol/kg) and multidrug-resistant Plasmodium falciparum K1 in vitro (0.17 microM). Metaquine shows strong affinity for the putative antimalarial receptor, heme at pH 7.4 in aqueous DMSO. Both crystallographic analyses and quantum mechanical calculations (HF/6-31+G) reveal important regions of protonation and bonding thought to persist at parasitic vacuolar pH concordant with our receptor model. Formation of drug-heme adduct in solution was confirmed using high-resolution positive ion electrospray mass spectrometry. Metaquine showed strong binding with the receptor in a 1:1 ratio (log K = 5.7 +/- 0.1) that was predicted by molecular mechanics calculations. This study illustrates a rational multidisciplinary approach for the development of new 4-aminoquinoline antimalarials, with efficacy superior to chloroquine, based on the use of a pharmacophore model.

Structural, topographical, and shear characteristics of milk protein and monoglyceride monolayers spread at the air-water interface

In this contribution we are concerned with the study of structure, topography, and surface rheological characteristics under shear conditions of monoglyceride (monopalmitin and monoolein) and milk protein (beta-casein, kappa-casein, caseinate, and WPI) spread monolayers at the air-water interface. Combined surface chemistry (surface film balance and surface shear rheometry) and microscopy (Brewster angle microscopy: BAM) techniques have been applied in this study to pure emulsifiers (proteins and monoglycerides) spread at the air-water interface. To study the shear characteristics of spread films, a homemade canal viscometer was used. The experiments have demonstrated the sensitivity of the surface shear viscosity (eta(s)) of protein and monoglyceride films at the air-water interface, as a function of surface pressure (or surface density). The surface shear viscosity was higher for proteins than for monoglycerides. In addition, eta(s) was higher for the globular WPI than for disordered beta-casein and caseinate due to the strong forces acting on spread globular proteins. This technique makes it possible to distinguish between beta-casein and caseinate spread films, with the higher eta(s) values for the later due to the presence of kappa-casein. The eta(s) value varies greatly with the surface pressure (or surface density). In general, the greater the surface pressure, the greater the values of eta(s). Finally, the eta(s) value is also sensitive to the monolayer structure, as was observed for monoglycerides with a rich structural polymorphism (i.e., monopalmitin).

Structure-activity relationships in 4-aminoquinoline antiplasmodials. The role of the group at the 7-position

Antiplasmodial activities versus the chloroquine sensitive D10 strain of Plasmodium falciparum of a series of N(1),N(1)-diethyl-N(2)-(4-quinolinyl)-1,2-ethanediamines with 11 different substituents at the 7-position on the quinoline ring have been investigated in vitro. Electron-withdrawing groups at the 7-position have been shown to lower the pK(a) of both the quinoline ring nitrogen atom and the tertiary amino nitrogen in the alkyl side chain. The quinoline nitrogen pK(a) ranges from 6.28 in the nitro derivative to 8.36 in the amino derivative, while the tertiary amino nitrogen has a pK(a) ranging between 7.65 in the trifluoromethyl derivative and 10.02 in the amino derivative. Calculation suggests that the resulting pH trapping of these compounds in the parasite food vacuole ranges between about 7% of that observed in chloroquine for the NO(2) derivative and 97% in the amino derivative. A direct proportionality between antiplasmodial activity normalized for pH trapping and beta-hematin inhibitory activity was observed. Activity could not be correlated with any other observed physical parameter. The beta-hematin inhibitory activity of these derivatives appears to correlate with both the hematin-quinoline association constant and the electron-withdrawing capacity of the group at the 7-position (Hammett constant). For the compounds under investigation, the hematin association constant is in turn influenced by the lipophilicity of the group at the 7-position.

Mechanism-based design of parasite-targeted artemisinin derivatives: synthesis and antimalarial activity of new diamine containing analogues

The potent antimalarial activity of chloroquine against chloroquine-sensitive strains can be attributed, in part, to its high accumulation in the acidic environment of the heme-rich parasite food vacuole. A key component of this intraparasitic chloroquine accumulation mechanism is a weak base "ion-trapping" effect whereupon the basic drug is concentrated in the acidic food vacuole in its membrane-impermeable diprotonated form. By the incorporation of amino functionality into target artemisinin analogues, we hoped to prepare a new series of analogues that, by virtue of increased accumulation into the ferrous-rich vacuole, would display enhanced antimalarial potency. The initial part of the project focused on the preparation of piperazine-linked analogues (series 1 (7-16)). Antimalarial evaluation of these derivatives demonstrated potent activity versus both chloroquine-sensitive and chloroquine-resistant parasites. On the basis of these observations, we then set about preparing a series of C-10 carba-linked amino derivatives. Optimization of the key synthetic step using a newly developed coupling protocol provided a key intermediate, allyldeoxoartemisinin (17) in 90% yield. Further elaboration, in three steps, provided nine target C-10 carba analogues (series 2 (21-29)) in good overall yields. Antimalarial assessment demonstrated that these compounds were 4-fold more potent than artemisinin and about twice as active as artemether in vitro versus chloroquine-resistant parasites. On the basis of the products obtained from biomimetic Fe(II) degradation of the C-10 carba analogue (23), we propose that these analogues may have a mode of action subtly different from that of the parent drug artemisinin (series 1 (7-16)) and other C-10 ether derivatives such as artemether. Preliminary in vivo testing by the WHO demonstrated that four of these compounds are active orally at doses of less than 10 mg/kg. Since these analogues are available as water-soluble salts and cannot form dihydroartemisinin by P450-catalyzed oxidation, they represent useful leads that might prove to be superior to the currently used derivatives, artemether and artesunate.

Structure-function relationships in aminoquinolines: effect of amino and chloro groups on quinoline-hematin complex formation, inhibition of beta-hematin formation, and antiplasmodial activity

Comparison of 19 aminoquinolines supports the hypothesis that chloroquine and related antimalarials act by complexing ferriprotoporphyrin IX (Fe(III)PPIX), inhibiting its conversion to beta-hematin (hemozoin) and hence its detoxification. The study suggests that a basic amino side chain is also essential for antiplasmodial activity. 2- And 4-aminoquinolines are unique in their strong affinity for Fe(III)PPIX, and attachment of side chains to the amino group has relatively little influence on the strength of complex formation. Association with Fe(III)PPIX is necessary, but not sufficient, for inhibiting beta-hematin formation. Presence of a 7-chloro group in the 4-aminoquinoline ring is a requirement for beta-hematin inhibitory activity, and this is also unaffected by side chains attached to the amino group. In turn, beta-hematin inhibitory activity is necessary, but not sufficient, for antiplasmodial activity as the presence of an aminoalkyl group attached to the 4-amino-7-chloroquinoline template is essential for strong activity. We thus propose that the 4-aminoquinoline nucleus of chloroquine and related antimalarials is responsible for complexing Fe(III)PPIX, the 7-chloro group is required for inhibition of beta-hematin formation, and the basic amino side chain is required for drug accumulation in the food vacuole of the parasite.

Structural specificity of chloroquine-hematin binding related to inhibition of hematin polymerization and parasite growth

Considerable data now support the hypothesis that chloroquine (CQ)-hematin binding in the parasite food vacuole leads to inhibition of hematin polymerization and parasite death by hematin poisoning. To better understand the structural specificity of CQ-hematin binding, 13 CQ analogues were chosen and their hematin binding affinity, inhibition of hematin polymerization, and inhibition of parasite growth were measured. As determined by isothermal titration calorimetry (ITC), the stoichiometry data and exothermic binding enthalpies indicated that, like CQ, these analogues bind to two or more hematin mu-oxo dimers in a cofacial pi-pi sandwich-type complex. Association constants (K(a)'s) ranged from 0.46 to 2.9 x 10(5) M(-1) compared to 4.0 x 10(5) M(-1) for CQ. Remarkably, we were not able to measure any significant interaction between hematin mu-oxo dimer and 11, the 6-chloro analogue of CQ. This result indicates that the 7-chloro substituent in CQ is a critical structural determinant in its binding affinity to hematin mu-oxo dimer. Molecular modeling experiments reinforce the view that the enthalpically favorable pi-pi interaction observed in the CQ-hematin mu-oxo dimer complex derives from a favorable alignment of the out-of-plane pi-electron density in CQ and hematin mu-oxo dimer at the points of intermolecular contact. For 4-aminoquinolines related to CQ, our data suggest that electron-withdrawing functional groups at the 7-position of the quinoline ring are required for activity against both hematin polymerization and parasite growth and that chlorine substitution at position 7 is optimal. Our results also confirm that the CQ diaminoalkyl side chain, especially the aliphatic tertiary nitrogen atom, is an important structural determinant in CQ drug resistance. For CQ analogues 1-13, the lack of correlation between K(a) and hematin polymerization IC(50) values suggests that other properties of the CQ-hematin mu-oxo dimer complex, rather than its association constant alone, play a role in the inhibition of hematin polymerization. However, there was a modest correlation between inhibition of hematin polymerization and inhibition of parasite growth when hematin polymerization IC(50) values were normalized for hematin mu-oxo dimer binding affinities, adding further evidence that antimalarial 4-aminoquinolines act by this mechanism.

Synthesis, antimalarial activity, and molecular modeling of tebuquine analogues

Tebuquine (5) is a 4-aminoquinoline that is significantly more active than amodiaquine (2) and chloroquine (1) both in vitro and in vivo. We have developed a novel more efficient synthetic route to tebuquine analogues which involves the use of a palladium-catalyzed Suzuki reaction to introduce the 4-chlorophenyl moiety into the 4-hydroxyaniline side chain. Using similar methodology, novel synthetic routes to fluorinated (7a, b) and a dehydroxylated (7c) analogue of tebuquine have also been developed. The novel analogues were subjected to testing against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. Tebuquine was the most active compound tested against both strains of Plasmodia. Replacement of the 4-hydroxy function with either fluorine or hydrogen led to a decrease in antimalarial activity. Molecular modeling of the tebuquine analogues alongside amodiaquine and chloroquine reveals that the inter-nitrogen separation in this class of drugs ranges between 9.36 and 9.86 A in their isolated diprotonated form and between 7.52 and 10.21 A in the heme-drug complex. Further modeling studies on the interaction of 4-aminoquinolines with the proposed cellular receptor heme revealed favorable interaction energies for chloroquine, amodiaquine, and tebuquine analogues. Tebuquine, the most potent antimalarial in the series, had the most favorable interaction energy calculated in both the in vacuo and solvent-based simulation studies. Although fluorotebuquine (7a) had a similar interaction energy to tebuquine, this compound had significantly reduced potency when compared with (5). This disparity is possibly the result of the reduced cellular accumulation (CAR) of fluorotebuquine when compared with tebuquine within the parasite. Measurement of the cellular accumulation of the tebuquine analogues and seven related 4-aminoquinolines shows a significant relationship (r = 0.98) between the CAR of 4-aminoquinoline drugs and the reciprocal of drugs IC50.