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Identity proton-transfer reactions between 21 acids, Y-X-H, and their conjugate bases, (-)X-Y, were studied according to the reaction scheme, Y-X-H + (-)X-Y --> (Y-X-H...(-)X-Y)(cx) --> [Y-X...H...X-Y](ts) --> (Y-X..H-X-Y)(cx) --> Y-X(-) + H-X-Y, where cx indicates an ion-molecule complex and ts indicates the proton-transfer transition state. All species were optimized at the MP2/6-311+G level, and these geometries were used for single-point calculations by other methods: coupled-cluster, DFT (gas phase), and a polarizable continuum aqueous solvent model (COSMO). All methods gave enthalpies of deprotonation which correlate well with experimental measurements of deltaH(ACID) (gas) or pK(a) (aq). Calculated gas-phase enthalpies of deprotonation (deltaH(ACID)) and enthalpies of activation (deltaH(#)) are poorly correlated except for small, carefully selected sets. This result stands in contrast to the many aqueous phase Brönsted correlations of kinetic and equilibrium acid strength. On the other hand, gas-phase enthalpies of complexation and deltaH(#) are well correlated, indicating that factors which stabilize the transition state are at work in the bimolecular ion-molecule complex although to a smaller degree. We infer that intermoiety electrostatic and other interactions, similar within the complex and the transition state, but absent in the separated reactants (products), cause the lack of correlation between deltaH(ACID) and the other two quantities. Such differences are strongly attenuated in water because reactants and products do interact with polar/polarizable matter (the solvent) if not with each other. Charge distributions (NPA) were computed, allowing calculation of Bernasconi's "transition state imbalance parameter". Such measures provide intuitively satisfactory trends, but only if the reaction termini, X, are kept the same. As X is made more electronegative, the magnitude of the apparent imbalance increases, a result of greater negative charge on X in the transition state. This result gives additional support for the importance of the ion-triplet structure, [YX(-)...H(+)...(-)XY], to the stability of the transition state. Additional qualitative support for this conclusion is provided by the inverse relationship between the activation barrier and the charge on the in-flight hydrogen in the transition state, and by the dominance of polar over resonance substituent effects on the stability of the transition state. Calculations also show that the "nitroalkane anomaly", well established in solution, does not exist in the gas phase. The COSMO model partly reproduces this anomaly and performs adequately except when strong, specific intermolecular forces such as hydrogen bonding between solvent and anions are important.

BRCA1 is a breast and ovarian cancer tumor suppressor protein that associates with BARD1 to form a RINGRING heterodimer. The BRCA1BARD1 RING complex functions as an ubiquitin (Ub) ligase with activity substantially greater than individual BRCA1 or BARD1 subunits. By using NMR spectroscopy and site-directed mutagenesis, we have mapped the binding site on the BRCA1BARD1 heterodimer for the Ub-conjugating enzyme UbcH5c. The results demonstrate that UbcH5c binds only to the BRCA1 RING domain and not the BARD1 RING. The binding interface is formed by the first and second Zn(2+)-loops and central alpha-helix of the BRCA1 RING domain, a region disrupted by cancer-predisposing mutations. Unexpectedly, a second Ub-conjugating enzyme, UbcH7, also interacts with the BRCA1BARD1 complex with similar affinity, although it is not active in Ub-ligase activity assays. Thus, binding alone is not sufficient for BRCA1-dependent Ub-ligase activity.

A circular permutein of sperm whale myoglobin in which the G helix is C-terminal, the H helix is N-terminal, and 16 amino acids link the H helix to the A helix has been expressed in Escherichia coli. The permutein sequence begins with Gly121 (using the numbering scheme for the wild-type protein) and terminates with Pro120. The ligand binding function of the permutein was assayed using stopped-flow methods and shown to be essentially identical to that of the wild-type protein. In addition, one- and two-dimensional NMR studies of the cyanomet isoform of the permutein show a nativelike structure with a heme binding pocket very similar to that of the wild-type myoglobin. Although the structure and function of the permutein resemble those of the wild-type myoglobin, the permutein is less stable to chemical denaturation by 5.2 kcal/mol.