Using nanografting to achieve directed assembly of de novo designed metalloproteins on gold

Parallel three-helix bundle metalloproteins incorporating C-terminal thiol groups have been designed to orient vertically on a gold(111) surface. A small area of pre-assembled octadecanethiol was exchanged with the proteins under the action of an AFM tip. The resulting "nanografted" metalloprotein was imaged at low applied force. The measured height of the octadecanethiol monolayer was 2.2(1) nm and the mean height above this of the grafted metalloprotein layer was 3.1(4) nm. Thus the total height of the grafted metalloprotein is 5.3(4) nm. The predicted height of the vertically oriented metalloprotein is 5.2 nm. We take this as good evidence for spatial control of monolayer assembly of three-helix bundles with predictable orientation.