Arsenic

About Arsenic

Arsenic is the element that explains why pyruvate dehydrogenase has a thiol-rich active site you'd never want to expose to it — As(III) binds vicinal cysteines tightly enough to shut down the entire complex, which is most of the textbook mechanism behind chronic arsenic toxicity. The redox couple between arsenite and arsenate is what makes the element interesting in groundwater geochemistry: under reducing conditions, As(III) desorbs from iron oxyhydroxides and goes mobile, which is the chemistry behind the Bangladesh and Bengal aquifer crisis where roughly 100 million people drink water carrying arsenic far above any drinking-water guideline. The material side is mostly III-V semiconductor work — gallium arsenide for high-frequency MMIC chips and red lasers, indium gallium arsenide for short-wave IR cameras and telecom photodiodes around 1550 nm. Medicinally arsenic trioxide had a fifteenth-century run as Fowler's solution and then a stunning twentieth-century rehabilitation when Shanghai hematologists showed it drives complete remission in acute promyelocytic leukemia by degrading the PML-RARA fusion protein. The Marsh test from 1836 still works: convert any As to arsine, decompose on a heated tube, weigh the silvery mirror.