The article Self-assembly of dimeric tetraurea calix[4]pyrrole capsules (P. Natl. Acad. Sci. USA 2009, 106, 10455-10459), authored by Pablo Ballester and Guzmán Gil-Ramírez, has been highlighted on the Cover of Proceedings of the National Academy of Sciences of the United States of America on issue 26, a Molecular Recognition and Self-Assembly special feature edited by Julius Rebek, Jr.

Calix[4]pyrroles having extended aromatic cavities have been functionalized with 4 ureas in the para position of their meso phenyl substituents. This tetraurea calix[4]pyrrole dimerizes reversibly in dichloromethane solution and in the presence of 4,4′-bipyridine N-N′-dioxide, forming a cyclic array of 16 hydrogen bonds and encapsulating 1 molecule of bis-N-oxide.