Our work on targeting redox equilibria of malarial parasites propagating in red blood cells has led to the selection of six 1,4-naphthoquinones, which are active at nanomolar concentrations against the human pathogen Plasmodium falciparum in culture and against Plasmodium berghei in infected mice. With respect to safety, the compounds do not trigger hemolysis or other signs of toxicity in mice. Concerning the antimalarial mode of action, we propose that the lead benzyl naphthoquinones are initially oxidized at the benzylic chain to benzoyl naphthoquinones in a heme-catalyzed reaction within the digestive acidic vesicles of the parasite. The major putative benzoyl metabolites were then found to function as redox cyclers: (i) in their oxidized form, the benzoyl metabolites are reduced by NADPH in glutathione reductase-catalyzed reactions within the cytosols of infected red blood cells; (ii) in their reduced forms, these benzoyl metabolites can convert methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Studies on a fluorinated suicide-substrate indicate as well that the glutathione reductase-catalyzed bioactivation of naphthoquinones is essential for the observed antimalarial activity. In conclusion, the antimalarial naphthoquinones are suggested to perturb the major redox equilibria of the targeted infected red blood cells, which might be removed by macrophages. This results in development arrest and death of the malaria parasite at the trophozoite stage.

The ability of the diradical dicationic cyclobis­(paraquat-p-phenylene) (CBPQT2(•+)) ring to form inclusion complexes with 1,1′-dialkyl-4,4′-bipyridinium radical cationic (BIPY•+) guests has been investigated mechanistically and quantitatively. Two BIPY•+ radical cations, methyl viologen (MV•+) and a dibutynyl derivative (V•+), were investigated as guests for the CBPQT2(•+) ring. Both guests form trisradical complexes, namely, CBPQT2(•+)⊂MV•+ and CBPQT2(•+)⊂V•+, respectively. The structural details of the CBPQT2(•+)⊂MV•+ complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV•+ is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT2(•+) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT2(•+)⊂MV•+ and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT2(•+)⊂MV•+ complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 104 M–1 and (7.9 ± 5.5) × 104 M–1, respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k f and k b of (2.1 ± 0.3) × 106 M–1 s–1 and 250 ± 50 s–1, respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT(2+)(•+)⊂V•+ inclusion complex; in the case of the V•+ guest, the rate of disassociation (k b = 10 ± 2 s–1) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT(2+)(•+) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY•+ radical cation and π-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical–radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY•+ radical–radical interactions in solution as well as in the solid-state.

All reagents and starting materials were purchased from Acros and used without further purification. Thin layer chromatography (TLC) was performed on silica gel 60 F254 (E. Merck). Column chromatography was performed on silica gel (Silicycle, 230-400 mesh). The melting point was recorded on an Electrothermal 9100 instrument in open capillary tubes and is uncorrected. Deuterated solvents (Cambridge Isotope Laboratories and Acros) for NMR spectroscopic analyses were used as received. NMR spectra were recorded on a Varian Unity Plus 500 MHz spectrometer, with working frequency of 499.87 MHz for 1 H nuclei, and 125.7 MHz for 13 C nuclei, respectively. Chemical shifts are quoted in ppm relative to tetramethylsilane, using the residual solvent peak as a reference standard. High-resolution mass spectra were measured on a Micromass Q-Tof Ultima capable of running HR-ESI MS. UV-Vis spectra was recorded on a Shimadzu UV spectrophotometer (UV-1800). Infrared spectra were obtained using a Shimadzu IRAffinity-1 spectrometer equipped with a ZnSe Single reflection ATR. UV/Vis Titrations: Trifluoroacetic acid (CF 3 CO 2 H or TFA, Merck Uvasol, 99.5%, for spectroscopy) and triethylamine (NEt 3 , SDS, 99.3%, synthesis grade) were purchased from commercial sources and used without further purification. All analyses were carried out with spectroscopic grade acetonitrile CH 3 CN (Acros Organics, 99+% for spectroscopy), which was used as received. All solutions were protected from daylight to avoid any potential photochemical degradation. All stock solutions were prepared using an AG 245 Mettler Toledo analytical balance (precision 0.01 mg). The concentrations of the stock solutions of the targeted compounds were calculated by quantitative dissolution