Following the discovery of cisplatin, much effort has been devoted to the exploration of transition metal complexes as cytotoxic agents. We have recently introduced the highly efficient C(2)-symmetrical salan-Ti(IV) family of complexes, demonstrating high cytotoxicity toward colon and ovarian cells and enhanced hydrolytic stability in mixed organic/water solutions. The effect of stereochemistry is hereby reported, by comparing the cytotoxic activity and hydrolysis of pure enantiomers and their racemic mixture for four complexes of this family with different aromatic substitutions: para-Me, para-Cl, ortho-Cl, and ortho-OMe. These complexes include the trans-diaminocyclohexyl bridge, which enables ligand-to-metal chiral induction to give solely the Δ isomer when starting from the R,R ligand and vice versa. Different activity is obtained for the different stereochemical forms (Δ, Λ, and rac) in two of the four complexes, where for the other two either all forms are inactive or all are highly active. Additionally, where not all are of similar activity, the racemic mixture is the least active of the three. We therefore conclude that the salan ligand is essential for the fruitful biological interaction, which probably involves a chiral cellular target. The activity of the racemate differing from that expected from a simple mixture of enantiomers operating separately may be explained by the involvement of a polynuclear active species, where different metal centers might be of different configurations. This is particularly supported by the different polynuclear products of hydrolysis obtained from an optically pure complex and from the racemic one, as analyzed crystallographically. The former is an all-R,R chiral C(1)-symmetrical homodimer, while the latter is an achiral R,R-S,SC(i)-symmetrical heterodimer obtained through chiral recognition.

Working together to uncover the truth: A molecule-sized diagnostic system combining several recognition elements and four fluorescence-emission channels enabled the identification of a wide range of pharmaceuticals on the basis of distinct photophysical processes. The molecular sensor (see simplified representation; ID = identification) was also used to analyze drug concentrations and combinations in urine samples in a high-throughput manner.

The effect of stereochemistry on the cytotoxicity of highly active and hydrolytically stable N-methylated Ti(IV) salan complexes is reported. Four bis(isopropoxo) complexes incorporating N-methylated salan ligands with different aromatic substitution patterns have been prepared in racemic and optically active forms for the first time by ligand-to-metal chiral induction from trans-diaminocyclohexyl-based chiral ligands. The configuration of the metal center that derives from that of the ligand has an enormous influence on cytotoxicity, with the racemic mixture mostly being more active than the single enantiomers that are of either similar or different activity. This implies that the active species is a salan-bound heterochiral polynuclear compound, interacting with a chiral target. Four additional complexes of achiral salan and chiral labile sec-butoxo ligands, analyzed as racemic and as homochiral, revealed no influence of stereochemistry, supporting early dissociation of the labile ligands to give the polynuclear products.

The ability to query enzyme molecules individually is transforming our view of catalytic mechanisms. Quiescin sulfhydryl oxidase (QSOX) is a multidomain catalyst of disulfide-bond formation that relays electrons from substrate cysteines through two redox-active sites to molecular oxygen. The chemical steps in electron transfer have been delineated, but the conformational changes accompanying these steps are poorly characterized. Here we use single-molecule Förster resonance energy transfer (smFRET) to probe QSOX conformation in resting and cycling enzyme populations. We report the discovery of unanticipated roles for conformational changes in QSOX beyond mediating electron transfer between redox-active sites. In particular, a state of the enzyme not previously postulated or experimentally detected is shown to gate, via a conformational transition, the entrance into a sub-cycle within an expanded QSOX kinetic scheme. By tightly constraining mechanistic models, smFRET data can reveal the coupling between conformational and chemical transitions in complex enzymatic cycles.

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