An approach to long-range electron transfer mechanisms in metalloproteins: In situ scanning tunneling microscopy with submolecular resolution

Abstract

In situ scanning tunneling microscopy (STM) of redox molecules, in aqueous solution, shows interesting analogies and differences compared with interfacial electrochemical electron transfer ( Molecular long-range electron transfer (ET) in solid phases or liquid solution, in which the ET distance exceeds the structural extension of the donor and acceptor, has been in focus over the last decade and a half (1-5). Progress has rested on synthetic donor-acceptor molecules (1-3), metalloproteins (2, 4, 6, 7), and on new electrochemical systems in which electrons are brought to tunnel across well characterized, self-assembled films (8, 9). These achievements have prompted new theoretical efforts with notions such as directional tunneling along chemical bond networks (10), fluctuating tunnel barriers (11, 12), coherent and resonance ET (13), and self-consistent electronic-vibrational interaction (11).In a parallel development, scanning tunneling and atomic force microscopy (STM and AFM, respectively) have opened exciting new perspectives for mapping molecular adsorbate patterns (14, 15). The primary basis for adsorption patterns in vacuum or air is imaging of small and intermediate-size adsorbate molecules with molecular and submolecular resolution (14-17), combined with theoretical frames for tunneling through adsorbate molecules based on different methodologies (17-21). There are also reported experimental approaches to functional mapping of intermediate-size molecules, most prominently in the form of correlations between the tunnel current and the bias voltage. Target adsorbates for ex situ STM imaging have been, for example, benzene (22, 23), methylazulenes (17, 24), C 60 (25), alkyl and aryl thiolates (17,21,(26)(27)(28), and transition metal phthalocyanins (29) on highly oriented pyrolytic graphite or crystalline surfaces of electronically ''soft'' metals compatible with the adsorbates. STM imaging at the solid͞air interface to molecular and occasionally submolecular resolution also has been extended to biological macromolecules including DNA (30) and a number of redox and nonredox proteins (for an overview, see ref. 31). Functional properties addressed particularly have been the electrical potential distribution and conductivity patterns of the tunnel gap (21, 27-29), resonance tunneling via suitable highest occupied molecular orbitals, lowest unoccupied molecular orbitals, or (transition metal) redox levels (17-21), and the notion of orbital-specific mediation of the tunnel current (29).Combination of STM with concepts of long-range ET in chemical and, particularly, metalloprotein systems must, however, give explicit attention to the fact that the natural medium for most chemical and biological reactivity is aqueous solution. Extension of STM͞AFM to aqueous solution (in situ STM͞ AFM) is established (32, 33) but has raised issues related to adsorbate immobilization, ultrapure solutions, tip coating, independent tip and substrate potential control (32-34), and the fundamental STM and AFM phenomena. In situ...

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“…The surface density of the spots can be tuned by varying incubation time, and protein concentration. The vertical size of the protein extracted by STM data (Figure 6b) is smaller than the crystallographic dimension, as usual for protein adsorbed onto gold and imaged by STM [37,38]. In the case of the SiO 2 substrates, the technique for protein immobilization is based upon the self assembly of 3-mercaptopropyltrimethoxysilane (3-MPTS).…”