Conformational Identification of Individual Molecules

Molecular structure and conformation determine a molecule's behavior on many levels of complexity. Both chemical and physical properties such as reactivity, molecular packing, and protein folding are governed by conformational factors. In small molecules reactivity can be determined by the orientation of small subunits. For example in Taxotere/Taxol the conformation of one substituent determines the activity as inhibitor of cell division. In large molecules subtle conformational changes involving many ligands and bonds regulate the biological activity of enzymes. A classical example is the allosteric regulation of the oxygen affinity of heme-porphyrins. As a consequence of their biological importance, porphyrins, which are related to the heme group, have been used as model systems to mimic charge transfer steps, as components for the in vivo activation of drug precursors by light, and for optoelectronic devices. Here we present the first real-space conformational analysis of a medium-complexity molecule (a porphyrin with 173 atoms) using high-resolution scanning tunneling microscopy. This molecule's conformation is found to be dominated by four principal ligand bond rotations observed to readjust in response to a variety of substrate-adsorbate interactions. This phenomenon of conformational adaptation is new for epitaxy and layering in ultrathin organic layers and interfaces and has an impact of potential technological importance.