PAC spectroscopy was first applied in biochemistry more than 40 years ago in a study of serum albumin (Leipert et al., Nature 220:907-909, ). Over the following decades a number of such applications have appeared in the literature, demonstrating that PAC spectroscopy is a useful tool in the study of biomolecules. Selected prototypical examples with Cd, Ag, and Hg are presented in this work with a particular focus on the biological role of metal ions in catalysis, protein structure and function, protein-protein interaction in biochemical reaction paths, in vivo binding of mercury ions to barley, and the use of de novo designed proteins to systematically explore the interaction of proteins and metal ions. [ABSTRACT FROM AUTHOR]