Peptide-terminated monolayers were formed through a Huisgen cycloaddition reaction between an α-helical peptide containing two propargylglycine unnatural functional groups 20 Å apart and an alkanethiol self-assembled monolayer (SAM) on a gold surface containing 25% surface density of reactive azide terminal groups. The azide- and peptide-terminated surfaces were imaged by scanning tunneling microscopy (STM) using a low tunneling current of 10 pA. On the peptide-terminated surface, oblong features ~30 Å long and ~20 Å wide were observed and attributed to individual surface-bound α-helical peptides oriented parallel to the gold surface. These features covered an area of the surface corresponding to a density of 0.11 ± 0.01 peptides nm(-2), compared with a theoretical density of ~0.14 peptides nm(-2) for a fully reacted surface. Finally, no evidence of peptide aggregation was observed on either short (<10 nm) or long (~100 nm) length scales.

The introduction of scanning probe microscopy (SPM) techniques revolutionized the field of condensed matter science by allowing researchers to probe the structure and composition of materials on an atomic scale. Although these methods have been used to make molecular- and atomic-scale measurements on biological systems with some success, the biophysical sciences remain on the cusp of a breakthrough with SPM technologies similar in magnitude to that experienced by fields related to solid-state surfaces and interfaces. Numerous challenges arise when attempting to connect biological molecules that are often delicate, dynamic, and complex with the experimental requirements of SPM techniques. However, there are a growing number of studies in which SPM has been successfully used to achieve subnanometer resolution measurements in biological systems where carefully designed and prepared samples have been paired with appropriate SPM techniques. We review significant recent innovations in applying SPM techniques to biological molecules, and highlight challenges that face researchers attempting to gain atomic- and molecular-level information of complex biomolecular structures.

Scanning tunneling microscopy is used to study monolayers of 1-adamantanethiolate as they are exposed to gas-phase atomic hydrogen. H-atom reaction results in complete removal of the organic monolayer. The relaxation of the reconstruction present at the gold–sulfur interface results in the formation of gold-atom islands, as well as the addition of gold atoms to extant surface defects such as steps and pits. Characterization of these changes shows that for 1-adamantanethiolate monolayers, 0.18 ± 0.033 monolayers of gold adatoms participate in bonding with thiolate sulfur atoms. This results in a 1:1 Au:S ratio, in contrast to the 1:2 Au:S ratio reported for n-alkanethiolate monolayers. The difference in adatom density implies a qualitative difference in binding between n-alkanethiols and 1-adamantanethiols.

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