Fluconazole cocrystals were prepared with the intention to modify physicochemical properties of the antifungal agent. The well-known COOH···Narom heterosynthon was considered the key element in the cocrystals design strategy. Cocrystals of fluconazole with maleic, fumaric, and glutaric acid were identified in solution evaporation experiments with pharmaceutically acceptable dicarboxylic acids and their crystal structures are presented. Solid-state NMR as an alternative technique for providing structural information of cocrystals offered additional insights into hydrogen bonding and other inter- as well as intramolecular interactions.

Arsenate reductase (ArsC) from Staphylococcus aureus plasmid pI258 catalyzes the reduction of arsenate to arsenite and plays a role in bacterial heavy metal resistance. The high resolution x-ray structure of ArsC reveals the atomic details of the K ؉ binding site situated next to the catalytic P-loop structural motif of this redox enzyme. A full thermodynamic study of the binding characteristics of a series of monovalent cations (Li ؉ , Na ؉ , K ؉ , Rb ؉ , and Cs ؉ ) and their influence on the thermal stability of ArsC was performed with isothermal titration calorimetry, circular dichroism spectroscopy, and differential scanning calorimetry. Potassium has the largest affinity with a K a of 3.8 ؋ 10 3 M ؊1 , and the effectiveness of stabilization of ArsC by monovalent cations follows the binding affinity order:A mutagenesis study on the K ؉ binding side chains showed that Asn-13 and Asp-65 are essential for potassium binding, but the impact on the stability of ArsC was the most extreme when mutating Ser-36. Additionally, the thermal stabilization by K ؉ is significantly reduced in the case of the ArsC E21A mutant, showing the importance of a Glu-21-coordinated water molecule in its contact with K ؉ . Although potassium is not essential for catalysis, in its presence the k cat /K M increases with a factor of 5. Altogether, the interaction of K ؉ with specific residues in ArsC is an enthalpydriven process that stabilizes ArsC and increases the specific activity of this redox enzyme.It has been known for a long time that cations serve a variety of functions in proteins. They play a role in protein stabilization and/or are involved in catalytic processes of enzymes. Some of the questions to be answered in studies of ion-protein interactions are still those posed by Scatchard (1) in 1949: how many, how tightly, where, and why? Although the combination of kinetic, thermodynamic, and structural studies can provide these answers, complete studies are relatively rare, and often only partial answers are available. The analysis of metal binding sites in proteins is also becoming of considerable interest because of efforts to create novel metal binding sites, either by de novo design or by redesign of existing scaffolds (2, 3). Transition metal-containing enzymes and proteins that bind diva-

published as an Advance Article on the web 30th March 2001 L mol~1). The thermodynamic (K M B 104 analysis shows that for all studied systems monomer formation is an enthalpy-driven process kJ (*H M ¡ B [14 mol~1). The observed exothermic values result most likely from the attractive coordinative interactions *H M ¡ between Cu2`ion and 2-aminopyridine.Bis(2-aminopyridine)Bis(hexanoato-O)copper(II). The procedure for obtaining the monomeric forms of the complexes was similar to that described for the dimers except that higher