  We theoretically analyze possible multiple conformations of protein molecules
immobilized by 1-pyrenebutanoic-acid-succinimidyl-ester (PASE) linkers on
graphene. The activation barrier between two bi-stable conformations exhibited
by PASE is confirmed to be based on the steric hindrance effect between a
hydrogen on the pyrene group and a hydrogen on the alkyl group of this
molecule. Even after the protein is supplemented, this steric hindrance effect
remains if the local structure of the linker consisting of an alkyl group and a
pyrene group is maintained. Therefore, it is likely that the kinetic behavior
of a protein immobilized with a single PASE linker exhibits an activation
barrier-type energy surface between the bi-stable conformations on graphene. We
discuss the expected protein sensors when this type of energy surface appears
and provide a guideline for improving the sensitivity, especially as an
oscillator-type biosensor.
