Introduction

A foldamer [1] can be defined as a (macro)molecular strand that is capable of adopting a well-defined, thermodynamically favored conformation in solution [2]. In order to provide a broader description that can be extended to interfaces as well, we view the foldamer's conformational preference as being the result of the action of internal and external boundary conditions, which involve various types of interactions (Fig. 13.1).

In solution in the isolated single foldamer molecule, only internal boundary conditions in the form of intramolecular interactions are involved in stabilizing a specific folded chain conformation. Both covalent internal constraints, such as linkage geometry, rotational preferences etc. (see Chapter 1), and noncovalent intramolecular interactions, such as H-bonding, metal-ligand coordination, p,p-stacking, coulombic, dipolar, and van der Waals interactions, etc. (see Chapter 2), govern the overall conformational preference. In addition, the foldamer's interaction with its surrounding has to be considered giving rise to external boundary conditions. Obviously, solvation that involves solvophilic and solvophobic interactions, such as the hydrophobic effect, as well as intramolecular nanophase separation (see Chapter 3) play an additional important role. Furthermore, endo- or exo-complexation of guest molecules (see Chapter 7) as well as dimerization and further aggregation (see Chapter 4) can give rise to folding. No further detailed discussion of the conformational preference in solution is needed here and the reader is referred to Part 1 of this book.

At interfaces, however, external boundary conditions in the form of interfacial interactions become crucial and often dictate molecular conformation. Several enthalpic and entropic parameters lead to certain conformational preferences that are often mingled with self-assembly processes, i.e. the external boundary conditions favor certain adsorption conformations and concurrently direct self-assembly (Fig. 13.1). This interplay leads to formation of hierarchically organized

Fig. 13.1 Overview of internal and external boundary conditions influencing the conformational preferences of foldamers and their assemblies.

materials that not only mimic structural evolution in Nature, for example biomi-neralization, but also hold exciting prospects for various new applications, involving for instance supramolecular electronics.

Here, we want to focus on foldamers at interfaces due to the high scientific interest and technological relevance of this field of research. While the question of how additional interactions at the interface interfere with the folding process and formation of the final secondary structure or higher order assemblies is of fundamental interest, the implication for technology including interfacial processes in recognition, sensors, patterning, catalysis among others is most significant. In addition, a profound knowledge of the underlying principles is key to understand-

ing biological phenomena, such as protein denaturation at interfaces and bio-mineralization. In the first part of this review we will propose a general description of folding at interfaces while in the second part we will highlight and discuss important examples from the literature that have been arranged with respect to the involved secondary structure motifs with particular focus on solid-liquid interfaces.

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