Role of land-surface changes in arctic summer warming.

A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO2). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.

Free radical reactions with proteins and enzymes: the inactivation of pepsin.

The reactions of free radicals produced by ionizing radiation with pepsin have been studied by steady-state inactivation measurements and by pulse radiolysis. In de-aerated solutions thehydroxyl radical has been found to be the most efficient of the primary free radicals generated from water in causing inactivation. The reactions of the more selective oxidizing inorganic radical anions Br2-. and (SCN)2-., with pepsin have also beenexamined. In the case of the thiocyanate radical anion (SCN)2-., the inactivation efficiency is found to depend on SCN- concentration, an effect shown to arise from a reversible redox reaction involving the tryptophan and (SCN)2-. radicals. The results demonstrate that tryptophan residue plays an essential role in the enzyme activity of pepsin.