SummaryThe indole based zwitterion 2 forms stable dimers held together by H-bond assisted ion pairs. Dimerisation was confirmed in the solid state and studied in solution using dilution NMR experiments. Even though zwitterion 2 forms very stable dimers even in DMSO, their stability is lower than of an analogous pyrrole based zwitterion 1. As revealed by the X-ray crystal structure the two binding sites in 2 cannot be planar due to steric interactions between the guanidinium group and a neighbouring aromatic CH. Hence the guanidinium moiety is twisted out of planarity from the rest of the molecule forcing the two monomers in dimer 2·2 to interact in a non-ideal orientation. Furthermore, the acidity of the NHs is lower than in 1 (as determined by UV-pH-titration) also leading to less efficient binding interactions.

The imidopyrrolecarboxylate 3(-) unexpectedly forms stable dimers (K(ass) = 130 M(-1) in CHCl(3)/DMSO, 1 : 1, v/v) despite the fact that two anions have to interact. The dimer is more stable than an analogous neutral amidopyrrolecarboxylic acid dimer (K(ass) < 10 M(-1)) underlining the importance of charged H-bonds compared to neutral ones.

Keywords: Molecular recognition / Ion-molecule reactions / Anions / Cations / Microcalorimetry / GuanidiniumThe synthesis of three new indole-based guanidinium cations 3, 4, and 16, that feature two different types of anion binding sites, either an acylguanidium cation (3) or an aromatic guanidinium cation (4) or both (16), is presented. NMR binding studies with N-acetylalaninecarboxylate as substrate in dimethylsulfoxide (DMSO) show that the acylated guanidinium cation is a significantly better anion binding site than the aromatic cation by at least one order of magnitude. Therefore, in dication 16, which possesses both binding sites, stepwise formation of the 1:1 and the 1:2 complex is observed with similar affinities for each binding site to those deter-

Enzymes are among the most effective catalysts known so far both in terms of their efficiency as well as selectivity. They can achieve rate accelerations of up to a factor of 10 11 and more, and sometimes react exclusively with only one single stereoisomer of a substrate if required. Enzymes make use of various different mechanisms of catalysis such as transition‐state binding, acid–base catalysis, nucleophilic catalysis, entropic control of reactions, and so on, with roughly half the known enzymes using metal ions while the other half solely relies on organic functional groups. Owing to these fascinating features of enzymes, chemists have tried to mimic enzymes early on developing artificial enzyme models. Over the years, a multitude of functional artificial enzyme models for very different types and classes of enzymes were developed, some closely mimicking the mode of action of their natural counterparts whereas others just adopt some general features found in enzymes. We briefly discuss some selected examples of artificial enzyme mimics in this review. Of course, the following examples are only a few picked out of a great variety. We do not attempt to provide a complete overview in respect to enzymes that are mimicked or models that were developed, but rather focus on the different approaches and concepts used to imitate an enzyme. The choice of examples is purely reflecting our personal preferences and is not implied to indicate that any work not mentioned is less interesting. We have arranged the examples of enzyme mimics and presented roughly according to their increasing chemical and structural complexity, starting with simple metal complexes (Section 1), then discussing cyclodextrin and related enzyme mimics (Section 2) before focusing on polymer‐ and dendrimer‐derived systems (Section 3). Miniproteins and oligopeptides with defined structures are then discussed (Section 4) before we end with a brief description of catalytic antibodies (Section 5).