The helix-peptide dipole effect on single-molecule conductance was analysed experimentally and theoretically with a single 8mer helical peptide. The helical peptide was immobilized on a gold surface in two opposite directions of the helix dipole. Single-molecule conductance of the helical peptide was determined to be 2.4 × 10(-5) G(0) by scanning tunneling microscopy (STM) break-junction measurements under the condition of applied bias voltage parallel to the dipole, which was about 1.2-fold larger than that in the anti-parallel direction. Theoretical calculation also supports that the helix dipole influences the electron transport reaction depending on parallel or anti-parallel orientation of the dipole against the applied electric field.
The synthesis, properties, and conformation of shape-persistent macrocycles with extraannular oligoalkyl groups and an intraannular bis(hydroxymethyl)biphenylene diester are investigated. The key step in the preparation is the template-supported Glaser coupling of rigid bisacetylenes which allows obtaining the cyclic product in 81% yield. Contrary to the initial expectations, the compounds do not exhibit liquid crystalline phases. This could be explained by a nonplanar conformation of the molecules because the biphenylene bridge is longer than the diameter of the rings. On the other hand, scanning tunneling microscopy investigations indicate that the intraannular biphenylene bridge can be located in the same plane as the aromatic backbone of the macrocycles on graphite. Modeling involving molecular mechanics and molecular dynamics simulations unraveled this apparent inconsistency by showing that the molecules adopt a nonplanar conformation in the absence of the solid support whereas planarization takes place when they are adsorbed on the graphite surface.
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