The crystal structure of ligand-free tryptophanyl-tRNA synthetase~TrpRS! was solved at 2.9 Å using a combination of molecular replacement and maximum-entropy map0phase improvement. The dimeric structure~R ϭ 23.7, R free ϭ 26.2! is asymmetric, unlike that of the TrpRS tryptophanyl-59AMP complex~TAM; Doublié S, Bricogne G, Gilmore CJ, Carter CW Jr, 1995, Structure 3:17-31!. In agreement with small-angle solution X-ray scattering experiments, unliganded TrpRS has a conformation in which both monomers open, leaving only the tryptophan-binding regions of their active sites intact. The amino terminal aA-helix, TIGN, and KMSKS signature sequences, and the distal helical domain rotate as a single rigid body away from the dinucleotide-binding fold domain, opening the AMP binding site, seen in the TAM complex, into two halves. Comparison of side-chain packing in ligand-free TrpRS and the TAM complex, using identification of nonpolar nuclei~Ilyin VA, 1994, Protein Eng 7:1189-1195 shows that significant repacking occurs between three relatively stable core regions, one of which acts as a bearing between the other two. These domain rearrangements provide a new structural paradigm that is consistent in detail with the "induced-fit" mechanism proposed for TyrRS by Fersht et al.~Fersht AR, Knill-Jones JW, Beduelle H, Winter G, 1988, Biochemistry 27:1581-1587 Coupling of ATP binding determinants associated with the two catalytic signature sequences to the helical domain containing the presumptive anticodon-binding site provides a mechanism to coordinate active-site chemistry with relocation of the major tRNA binding determinants.

Bis(pyrrol-2-ylmetyhleneamine) ligands linked by alkyl spacers between the imine nitrogens are shown to give dinuclear dimers or mononuclear monomers, depending on the length of the alkyl linker, upon coordination with Cu II . Ligands containing ethylene or hexylene linkers −(CH 2 ) n − (n = 2, 6) give dinuclear dimers 5 and 8, while propylene and butylene chains (n = 3, 4) give mononuclear monomers 6 and 7. X-ray crystal structural analysis reveals that 5 exists as two very similar isomers 5a and 5b, in which the two ligands are bound to two Cu II centers to form a distorted helical conformation. In the unit cells of 8, two entirely different isomers

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