No abstract

The triply hydrogen bonded complex between guanine and cytosine (1) is central to the structure of nucleic acids. The strength of the interaction is notable with a K, of ca. 104-105 M-l in chloroform.' Another complex (2) with a similar bonding pattern has also been found recently to have a K, of 1.7 X lo4 M-' in chloroform.2 The benefit of the third hydrogen bond is sug ' H Ar 1 2

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Changes in amino acid side chains have long been recognized to alter the range and distribution of phi, psi angles found in the main chain of polypeptides. Altering the range and distribution of phi, psi angles also alters the conformational entropy of the flexible denatured state and may thus stabilize or destabilize it relative to the comparatively conformationally rigid native state. A database of 12,320 residues from 61 nonhomologous, high resolution crystal structures was examined to determine the phi, psi conformational preferences of each of the 20 amino acids. These observed distributions in the native state of proteins are assumed to also reflect the distributions found in the denatured state. The distributions were used to approximate the energy surface for each residue, allowing the calculation of relative conformational entropies for each residue relative to glycine. In the most extreme case, replacement of glycine by proline, conformational entropy changes will stabilize the native state relative to the denatured state by -0.82 +/- 0.08 kcal/mol at 20 degrees C. Surprisingly, alanine is found to be the most ordered residue other than proline. This unexpected result is a result of the high percentage of alanines found in helical conformations. This either indicates that the observed distributions in the native state do not reflect the distributions in the denatured state, or that alanine is much more likely to adopt a helical conformation in the denatured state than residues with longer side chains.(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary R-and β-deuterium isotope effects for enolization reactions and equilibria have been determined by ab initio calculations, 1 H NMR spectroscopy, and triton exchange kinetics. Kinetic and equilibrium R-deuterium isotope effects for hydroxide ion-catalyzed enolization of acetaldehyde calculated by ab initio methods are normal and depend on the orientation of the secondary hydrogen with respect to the carbonyl group. The computed transition state structure indicates a small degree of bond rehybridization at the transition state. Experimentally measured secondary isotope effects on the deuteroxide ion-catalyzed proton exchange of acetophenone are k H /k D ) 1.08 ( 0.07 for R-CH 3 exchange and k H /k D ) 0.96 ( 0.08 for R-CH 2 D exchange. For R-CH 2 T exchange in water, the corresponding secondary isotope effect is k H /k D ) 1.06 ( 0.02, assuming the rule of the geometric mean is valid. These effects are smaller than the calculated equilibrium isotope effect for formation of the enolate ion-water complex: K H /K D ) 1.11-1.22 at the MP2 level. The normal kinetic isotope effects are smaller than might be expected due to a loss in hyperconjugation of the out-of-plane C-H bond and a lag in structural reorganization that contributes to the intrinsic barrier for proton transfer from carbon. Ionization of protonated acetone gives rise to an inverse secondary isotope effect of 0.97/D for the C-L bond adjacent to the carbonyl group and is consistent with a loss in hyperconjugation upon formation of the neutral ketone.