Controlling Excited‐State H‐Atom Transfer along Hydrogen‐Bonded Wires

Abstract

Hydrogen-bonded wires are involved in a large variety of chemical and biological processes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The main reaction associated with hydrogen bonds is proton or H-atom transfer, in which a charge accompanies the transferring proton. A topic of specific interest is proton transport through transmembrane ion channels ('proton wires'), because many transmembrane proteins create, control and use the proton gradient across biological membranes. Hydrogen-bonded wires of water molecules have been identified in numerous membrane-spanning proteins, such as bacteriorhodopsin [17][18][19][20][21][22], the bacterial photosynthetic reaction centre of Rhodobacter sphaeroides [23], the transmembrane channel formed by gramicidin [24][25][26], cytochrome oxidase [27] and other voltage-gated proton channels [28]. The enzymes carbonic anhydrase [29,30] and alcohol dehydrogenase [30] also contain proton relays along chains of water molecules embedded in the interior of the protein. Recently, hydrogen-bonded ammonia wires have been detected in ammonia/ammonium transporter (Amt) transmembrane proteins, which are essential for nitrogen metabolism in all species [31,32]. The translocation mechanisms along these 'proton wires' have become fields of intense theoretical study [5-8, 24-26, 29, 30, 33, 34]. Most of the time, the assumed mechanism is Grotthuss type, after the inventor (1806) of the process. The modern version of the Grotthuss mechanism consists of successive transfers of an excess proton along a hydrogen-bonded wire. These transfers require sequences of reorientation of the wire molecules, which constitutes the rate-limiting step of the mechanism [35]. The excess proton diffuses through a hydrogen bond network via the formation/breaking of covalent bonds, as shown schematically in