Macrodipole interaction of helical peptides in a self-assembled monolayer on gold substrate

Monolayers of helical peptides on gold substrates were prepared by a self-assembly approach aiming at fabricating a regular structure by the use of the specific interaction between neighboring helices. Two kinds of helical peptides were synthesized. One has a disulfide group at the N-terminal of the helix part, and the other, at the C-terminal of the same helix. Self-assembled monolayers (SAMs) were prepared by dipping a gold substrate into each peptide solution. The thickness and the molecular orientation of each monocomponent SAM indicated that the helical peptides were adsorbed on the surface with a preferred orientation parallel to the surface. However, those of an eqnimolar mixed SAM showed that the peptide took an appreciably vertical orientation on the surface. These observations indicated that an antiparallel helix packing is significantly more favorable than a parallel one. It is strongly suggested that the SAM structure is regulated by a dipolar interaction between helical peptides since the geometric fitting among the molecules in the parallel packing could be hardly different from that in the antiparallel packing.