Macrocycle-Encircled Proton

Gazing down the contents list of the Journal of the American Chemical Society, the eye is drawn to the concise, provocative title, "Encircled Proton." The paper, it turns out, describes the "strikingly unusual occurrence" of a proton that is encircled by a dicationic macrocycle and also hydrogen-bonded to two inwardly oriented carbonyl oxygen atoms of amide groups (J. Am. Chem. Soc., DOI: 10.1021/ja0724745). Because the distance between those two oxygen atoms is only 2.45 ??, the hydrogen bonds in the O···H···O bridge are unusually short and very strong. Scientists have postulated that "such bonds exist within protein interiors, and although still subject to debate, these symmetrical amide O···H···O bonds may be an important component of transition-state stabilization in enzymes," according to Victor W. Day, Kristin Bowman-James, and coworkers at the University of Kansas, Lawrence. Their discovery was serendipitous: When they treated the macrocycle with a few drops of concentrated sulfuric acid, they produced and crystallized a complex containing three HSO₄^- counterions. An X-ray crystal structure and NMR spectra led them to the surprising conclusion that a lone proton had taken up residence in the macrocycle's cavity.