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Electron Transfer and Redox Metalloenzyme Catalysis at the Single‐Molecule Level
Author(s) -
Hansen Allan G.,
Zhang Jingdong,
Christensen Hans E. M.,
Welinder Anne C.,
Wackerbarth Hainer,
Ulstrup Jens
Publication year - 2004
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1560/wy97-q5pq-f6vj-nln8
Subject(s) - chemistry , monolayer , electron transfer , azurin , scanning tunneling microscope , crystallography , redox , molecule , cyclic voltammetry , electrochemistry , analytical chemistry (journal) , electrode , inorganic chemistry , photochemistry , nanotechnology , organic chemistry , biochemistry , materials science
Voltammetry based on single‐crystal, atomically‐planar metal electrodes is novel in bioelectrochemistry. Together with in situ scanning tunneling microscopy (STM) directly in aqueous buffer, single‐crystal voltammetry has disclosed new detail in molecular adsorption and interfacial electron transfer (ET). Image interpretation requires, however, theoretical support, as STM represents both electronic and topographic features. Molecules with accessible redox levels offer other insight into electron tunneling mechanisms , addressed in detail for ET metalloproteins. We present here in situ STM of the blue redox metalloenzyme copper nitrite reductase ( Achromobacter xylosoxidans, Ax CuNiR) on Au(111) electrode surfaces modified by a self‐assembled cysteamine monolayer. Ax CuNiR displays strong nitrite reduction waves in this environment. Ax CuNiR/cysteamine/Au(111) surfaces were imaged at KNO 2 concentrations where most of the enzyme is in the enzyme‐substrate bound state. Molecular resolution for both cysteamine/Au(111) and Ax CuNiR/cysteamine/Au(111) electrode surfaces was achieved. The enzyme coverage is about 1.5 × 10 −13 mol cm −2 , which is low compared with an ideal close‐packed monolayer. The adlayer behaves as an assembly of individual molecules, reflected in distributions of molecular appearance, although a number of molecules do show the triangular shape of the trimeric Ax CuNiR structure. The apparent average molecular height is about 11 Å. This suggests that details of electronic structures and larger assemblies are needed to disentangle enzyme mechanisms at the single‐molecule level.