Leslie A. Hull

Thermal decomposition of dimethyldioxirane

Abstract The thermal decomposition of dimethyldioxirane (DMDO) was followed by uv/vis spectroscopy at 340 nm. At high concentration of DMDO (≈0.1M) the decomposition was found to to have an induction period followed by rapid decay. The rapid decay was shown to be caused by presence of reaction products. At low concentration of DMDO (≈0.02M) the decomposition was found to be first order with an Ea of 24.9 kcal/mol. Based on this result it is estimated that the dioxybiradical produced by cleavage of the OO bond shows an activation energy of about 10 kcal/m for reclosure back to the DMDO.

Stabilization of helical peptides by mixed spaced salt bridges

Whether or not surface salt bridges have a strong stabilizing effect on the native structure in proteins remains uncertain. Previous studies of model peptides have shown that salt bridges spaced at i,i +4 along the chain are more stabilizing than those spaced at i,i +3, with a preference for the order acid-base rather than base-acid from N to C terminus. An analysis of the effect of spacing the ion pairs in short helical peptides is presented, in which acidic and basic side chains spaced two or three residues apart alternate along the chain. The mixed spacing proves to be stabilizing relative to pure spacings. A control peptide in which salt bridges were spaced uniformly three residues apart proved to form a beta-sheet structure rather than alpha-helix. This is due to formation of a silk-like apolar face consisting of alanine side chains; the mesoscopic structure formed by these sheets can be imaged by scanning microscopy.