Despite its importance as a target in anti-cancer therapeutics and the numerous rational-based inhibitor design efforts aimed at it, there are only limited data available on structural-thermodynamic relationships of interactions of the N-terminal ATP-binding domain of Hsp90 (N-Hsp90). Here, we redress this by presenting an investigation of binding of nucleotides and ansamycin compounds to this domain. Interactions of nucleotides with N-Hsp90 are relatively weak (>10 microM) and are strongly enthalpy driven over the temperature range 10-25 degrees C. Geldanamycin (GA) and its analogues 17-AAG [17-(allylamino)-17-demethoxy-GA] and 17-DMAG (17-N,N-dimethylaminoethylamino-17-demethoxy-GA) bind more strongly and have a dominant favourable enthalpic contribution over the temperature range investigated. We investigated the temperature dependence of the enthalpic contribution to binding. We found that while the ansamycin compounds have the commonly observed negative value, the nucleotides show a negligible or even a positive DeltaC(p) of binding. These data represent the first observation of a single binding site for which interactions with different ligands result in both negative and positive DeltaC(p) values. By addressing the likely impact of the potential contributions from protein-ligand interactions, we are able to attribute the anomalous DeltaC(p) for the nucleotides largely to a change in the conformation of the domain structure and local motion in the lid region of N-Hsp90 with the concomitant exposure of hydrophobic amino acid side chains.