title: Carbon-binding designer proteins that discriminate between sp2- and sp3-hybridized carbon surfaces.
abstract: Robust and simple strategies to directly functionalize graphene- and diamond-based nanostructures with proteins are of considerable interest for biologically-driven manufacturing, biosensing, and bioimaging. Here, we identify a new set of carbon-binding peptides that vary in overall hydrophobicity and charge and engineer two of these sequences (Car9 and Car15) within the framework of E. coli thioredoxin 1 (TrxA). We develop purification schemes to recover the resulting TrxA derivatives in a soluble form and conduct a detailed analysis of the mechanisms that underpin the interaction of the fusion proteins with carbonaceous surfaces. Although equilibrium quartz crystal microbalance measurements show that TrxA::Car9 and TrxA::Car15 have similar affinities for sp(2)-hybridized graphitic carbon (Kd = 50 and 90 nM, respectively), only the latter protein is capable of dispersing carbon nanotubes. Further investigation by surface plasmon resonance and atomic force microscopy reveals that TrxA::Car15 interacts with sp(2)-bonded carbon through a combination of hydrophobic and π-π interactions but that TrxA::Car9 exhibits a cooperative mode of binding that relies on a combination of electrostatics and weaker π stacking. Consequently, we find that TrxA::Car9 binds equally well to sp(2)- and sp(3)-bonded (diamondlike) carbon particles whereas TrxA::Car15 is capable of discriminating between the two carbon allotropes. Our results emphasize the importance of understanding both bulk and molecular recognition events when exploiting the adhesive properties of solid-binding peptides and proteins in technological applications.
authors.affiliation: Department of Chemical Engineering, University of Washington, Seattle, Washington 98195-1750, United States.
authors.forename: Brandon L
authors.initials: BL
authors.lastname: Coyle
authors.forename: Marco
authors.initials: M
authors.lastname: Rolandi
authors.forename: François
authors.initials: F
authors.lastname: Baneyx
mesh_terms: D002244:Carbon / Q000737:chemistry*; D004926:Escherichia coli / Q000737:chemistry; D010316:Particle Size; D010455:Peptides / Q000737:chemistry*; D058525:Quartz Crystal Microbalance Techniques; D013499:Surface Properties; D013879:Thioredoxins / Q000737:chemistry*
chemical_list: D010455:Peptides; D013879:Thioredoxins; D002244:Carbon