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One result can be summarized with the help of Fig. ( 1) provide an example that touches biology, chemistry, and physics. The central part, called motte, resembles the active center, the moat is the equivalent of the hydration shell, and the bailey is the field between motte and moat, with no obvious function. The entire structure is reminiscent of the Chateau Gisors, built in 1096 and sketched in Fig. 1 a entrance and exit of ligands must occur through gates. A major part of Mb is taken up by amino acids that do not appear to have an obvious function we denote this part as “bailey.” Surrounding the protein proper is the hydration shell, consisting of one to two layers of water molecules with properties that are different from bulk water. The amino acids lining the xenon cavities are much more conserved than other amino acids in mammalian Mb they are thus likely to be important for function. Surrounding the heme group are five cavities, the heme cavity and four cavities denoted by Xe1 to Xe4 ( 5). 1 b gives a schematic cross section through Mb that shows the active center: a heme group with a central iron atom. Mb consists of 153 amino acids that fold into a structure that is ≈3 nm in diameter, as depicted in Fig. It plays roles other than O 2 storage, serves as a prototype for complex systems, and yields insight into the chemistry and physics of soft matter and of chemical reactions. Since then, the situation has changed: Mb is no longer fully understood. Mb was essentially written off as a topic of serious research. Thirty years ago the textbook function of Mb, storage of dioxygen at the heme iron, was considered to be simple, fully understood, and consequently boring. Mb is a monomeric protein that gives muscle its red color.
![hydrogen atom hydrogen atom](https://i.pinimg.com/originals/d4/67/8c/d4678cb1bc5a37eb21b71441ed9a6d6c.jpg)
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The recent breakthrough shows how careful studies of proteins, in particular of Mb, impact many different fields. It demonstrates how far advances in x-ray sources and computers have moved the field of protein structure determination since the path-breaking work of Kendrew et al. This work follows the pioneering experiments of Moffat and collaborators ( 2). ( 1) have used 2.2-ns x-ray pulses to observe the motion of carbon monoxide (CO) through myoglobin (Mb) and the relaxation of the protein from 3.2 ns to 3 ms after photodissociation. In a tour de force in this issue of PNAS, Bourgeois et al.