Abstract
Mapping and control of proteins and oligonucleotides on metallic and nonmetallic surfaces are
important in many respects. Electrochemical techniques based on single-crystal electrodes and scanning
probe microscopies directly in aqueous solution (in situ SPM) have recently opened perspectives for such
mapping at a resolution that approaches the single-molecule level. De novo design of model proteins has
evolved in parallel and holds promise for testing and controlling protein folding and for new tailored protein
structural motifs. In this report we combine these two strategies. We present a scheme for the synthesis
of a new 4-R-helix bundle carboprotein built on a galactopyranoside derivative with a thiol anchor aglycon
suitable for surface immobilization on gold. The carboprotein with thiol anchor in monomeric and dimeric
(disulfide) form, the thiol anchor alone, and a sulfur-free 4-R-helix bundle carboprotein without thiol anchor
have been prepared and investigated for comparison. Cyclic and differential pulse voltammetry (DPV) of
the proteins show desorption peaks around -750 mV (SCE), whereas the thiol anchor desorption peak is
at -685 mV. The peaks are by far the highest for thiol monomeric 4-R-helix bundle carboprotein and the
thiol anchor. This pattern is supported by capacitance data. The DPV and capacitance data for the thiolated
4-R-helix bundle carboproteins and the thiol anchor hold a strong Faradaic reductive desorption component
as supported by X-ray photoelectron spectroscopy. The desorption peak of the sulfur-free 4-R-helix bundle
carboprotein, however, also points to a capacitive component. In situ scanning tunneling microscopy (in
situ STM) of the thiol anchor discloses an adlayer with small domains and single molecules ordered in
pin-striped supramolecular structures. In situ STM of thiolated 4-R-helix bundle carboprotein monomer shows
a dense monolayer in a broad potential range on the positive side of the desorption potential. The coverage
decreases close to this potential and single-molecule structures become apparent. The in situ STM contrast
is also strengthened, indicative of a new redox-based tunneling mechanism. The data overall suggest that
single-molecule mapping of natural and synthetic proteins on well-characterized surfaces by electrochemistry
and in situ STM is within reach.
Original language | English |
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Journal | Journal of the American Chemical Society |
Volume | 125 |
Pages (from-to) | 94-104 |
ISSN | 0002-7863 |
Publication status | Published - 2003 |