17 Lipoic acid has been also successfully employed to endow oligo- and polypeptides with a disulfide functionality at the N terminus. Stable peptide SAMs on gold surfaces have been obtained by functionalizing the peptide chain with a thiol, 15 a disulfide group 16 or the thiol side chain of cysteine. It has been shown that the associated electrostatic field promotes directional electron transfer (ET) over long distances, giving rise almost exclusively to the charge-separated state stabilized by the electric macrodipole. (iv) The peptide bond is polar (3.6 D per residue), so that, when the peptide chain folds in a helical structure, for example, α- or 3 10-helical conformation, the peptide dipole moments sum up to generate an electric macrodipole stabilized by head-to-tail interactions. O=C hydrogen bonds, cysteine–cysteine disulfide bridges, interchain dipole interactions, stacking interactions between aromatic moieties).12 Peptides expand the catalog of molecular interactions that stabilize self-assembled structures (interchain N–H 11 In the most popular alkanethiol SAMs, the ordering of the monolayer is driven by the strong Au/sulfur affinity (positional order) and the lateral van der Waals interaction between the tethered alkyl chains (orientational order).
10 (iii) Peptides show unique self-assembly properties, giving rise to the formation of nano- and mesoscopic supramolecular architectures.
The careful selection of amino-acid residues allows one to design ordered secondary structures, such as 3 10- and α-helical segments, β-strands and β-hairpin motifs. 9 (ii) The secondary structure adopted by the peptide backbone can be strictly controlled. Chemists are currently able to synthesize amino acids endowed with specific functions and showing well-defined three-dimensional structural properties. (i) Peptides can be easily functionalized. There are several reasons for investigating peptide-based SAMs. 4, 5, 6Īmong the supramolecular architectures obtained by self-assembly ( bottom-up approach), self-assembled monolayers (SAMs) have been demonstrated as the most suitable tool for modifying the surface properties of inorganic compounds (metals, semiconductors, polymers), paving the way for the realization of hybrid devices ( soft meets hard). 1, 2, 3 This strategy has been leading to important achievements in several fields, providing new concept, smart materials for tissue engineering, controlled drug release, nucleic acid and protein sensing, organic photovoltaics and molecular electronics. Self-assembly, that is, the spontaneous organization of molecular building blocks to generate a supramolecular architecture, is the most promising approach for the construction of controlled nanometric structures.
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