Adopt the observed right handed helical conformation with the two GDC-0032 active heads mutually antiparallel. Moreover, the 9a linker hydrophobicity warrants an overall compact structure of the free ligand in solution, but with significant solvent exposure of the two active heads, as observed in molecular dynamics simulations of free 9a in solution. All results reported here emphasize the importance of structural dynamics in IAPs interactions with inhibitors and provide new hints for the development of divalent lead compounds able to bind preferentially XIAP, cIAP1 and cIAP2, thereby introducing specificity, albeit partial, in their action on different apoptotic pathways. The program EOM describes a flexible molecule in solution, using an ensemble of typically 50 conformations extracted from a very large pool of conformations. The conformer pool is constructed by connecting domains treated as rigid bodies by self-avoiding linkers, where the dihedral angles of the linkers in the Ca�CCa space are selected randomly but biased to comply with the quasi-Ramachandran plot and the model generated is free from steric clashes. A genetic algorithm progressively refines the composition of the ensemble so that the average scattering pattern of the molecular conformations within the ensemble fits the experimental data within error bars. The process was repeated 200 times and the distribution of the radius of gyration and the SB 202190 structure maximum diameter were calculated and compared with those derived from the entire starting pool. This comparison yields some global features of the conformational space explored by the molecule as probed by SAXS. Models of both XIAP-BIR2BIR3 with and without the inhibitor were obtained using the program Bunch which moves domains as rigid bodies while describing the missing parts of the molecule as chains of dummy residues so as to fit the experimental scattering pattern. Models of the conformation in solution of XIAPBIR2BIR3 with 9a were obtained using the program Coral starting from the high-resolution model of BIR2-BIR3 complexed with 9a. Here, Coral was used exactly as the program Bunch but allowed us to impose a 5 A �� distance restraint between two atoms of the inhibitor on each side of a broken methylbenzene bond in the 9a central benzene. The core domain
