“We wanted to examine the problem of the hybrid interface between biological and inorganic materials in an elemental fashion,” said Bob Willett of Lucent. “How do the building blocks of certain biological materials - amino acids - interact with a set of materials that are among the building blocks for a specific technology - microelectronics? By understanding the interactions of the components, one can build up in an empirical fashion more complex ensembles at these interfaces.”
Willett and colleagues tested solutions of fluorescently tagged peptides containing between eight and 10 residues of one of twenty amino acids. They looked at adhesion of the solutions to the surfaces of five metals: gold, palladium, platinum, titanium and aluminium; two semiconductors: gallium arsenide and aluminium gallium arsenide; and the insulators silicon nitride and silica.
By measuring the persistence of fluorescence after the surface was rinsed with water, the team was able to quantify the amount of peptide adhesion.
“Our results demonstrate a surprisingly large range of adhesion interactions,” said Willett. “The adhesion maps are an empirical tool for attempting to understand certain molecular interactions with inorganic surface states and, perhaps more importantly, provide an empirical guide for building nanostructures that are hybrids of peptide-based materials and inorganics.”
The researchers found that the amount of adhesion of the amino acids was related to the charge on the side-chain. In general, the peptides with charged side groups, either basic or acidic, showed greater adhesion than polar-but-uncharged and nonpolar peptides. The silica, silicon nitride and aluminium surfaces were, on the whole, more adherent than the gallium arsenide and palladium surfaces. And the non-oxidized metals - platinum, palladium and gold - mostly interacted only weakly with amino acids.
The team built on these results to design an inorganic surface that would specifically adhere to particular peptide chains. Using molecular beam epitaxy, they created a layered structure of gallium arsenide and aluminium gallium arsenide, which they etched to expose aluminium gallium arsenide veins. They matched the structure with a peptide sequence containing a centre section of Asp – an amino acid that adheres to aluminium gallium arsenide – surrounded on either side by series of the hydrophobic amino acid Leu, which doesn’t adhere to aluminium gallium arsenide.
As long as the sizes and spacing of the aluminium gallium arsenide veins matched with the length of the peptide sequences, the central aspartic acid sections of the peptides were able to adhere to the veins without interference from the leucine sections.
“This finding can be considered quite remarkable in that it shows that man-made structures may possibly be constructed at an important length-scale – that of enzymatic specificity,” said Willett. “A collaborator has loosely referred to these structures as ‘artificial enzymes’, a moniker that has yet to be fully substantiated.”
The team reckons that their results could have applications ranging from biomolecular detection to biomolecular manipulation.
“Once a specific adhesion site is established, the appropriate sensing devices can be built around the site,” said Willett. “Additionally, if certain molecular species can be adhered to designed surfaces, then interactions among different molecules can be promoted through the surface site design. In a broader sense, we think the information can be applied to basic biological molecule studies, such as X-ray analysis of proteins or intracellular peptide assays, or applied to new materials physics, such as examining the physics of complex molecular interactions on surfaces.”
The researchers reported their work in PNAS.