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Abstract: The advent of recombinant DNA technology allowed to site‐specifically insert, delete, or mutate almost any amino acid in a given protein, significantly improving our knowledge of protein structure, stability, and function. Nevertheless, a quantitative description of the physical and chemical basis that makes a polypeptide chain to efficiently fold into a stable and functionally active conformation is still elusive. This mainly originates from the fact that nature combined, in a yet unknown manner, different properties (i.e., hydrophobicity, conformational propensity, polarizability, and hydrogen bonding capability) into the 20 standard natural amino acids, thus making difficult, if not impossible, to univocally relate the change in protein stability or function to the alteration of physicochemical properties caused by amino acid exchange(s). In this view, incorporation of noncoded amino acids with tailored side chains, allowing to finely tune the structure at a protein site, would facilitate to dissect the effects of a given mutation in terms of one or a few physicochemical properties, thus much expanding the scope of physical organic chemistry in the study of proteins. In this review, relevant applications from our laboratory will be presented on the use of noncoded amino acids in structure–activity relationships studies of hirudin binding to thrombin. [ABSTRACT FROM AUTHOR]

Proteins are large complex biomolecules that act as the effectors of essentially all cell functions. Due to the intrinsic complexity of protein architecture at the microscopic level and the inadequacy of theoretical methods to predict protein reactivity (ie, folding, stability, and function), protein engineering has emerged as a valuable tool to investigate structure–stability–activity relationships in proteins and nowadays recombinant DNA technologies are the "gold standard" for site‐specifically manipulating a given protein chain. The usefulness of current mutagenesis techniques, however, is limited by the relatively poor chemical diversity of the 20 DNA‐coded amino acids, such that it is difficult to precisely assign the observed change of protein stability or function to the variation of a single physicochemical property at a protein site (ie, hydrophobicity, conformational propensity, polarizability, hydrogen bonding, etc). In this article, we report relevant examples from our laboratory showing that chemical methods, that is, enzyme‐catalyzed semisynthesis and stepwise solid‐phase synthesis, allow to conveniently incorporate non‐natural amino acids with "tailored" side chains into small proteins and thus effectively transfer the structure–activity relationship methodology, typical of the medicinal chemistry approach on small molecules, to the study of folding, stability, and molecular recognition in macromolecular protein systems. [ABSTRACT FROM AUTHOR]