Frequently, the interactions of a protein with its environment are not stable, but only transitory. The association between the interaction partners can, for example, dissociate as soon as the biological systems being examined are disintegrated for analysis. For this reason it is essential for the analysis of protein-protein interactions that the respective biomolecular interactions are covalently linked. In this context the time of fixation should be arbitrary and specific for the interaction. To meet this requirement, we make use of an expanded genetic code (Fig. 1). In this context, instead of a natural amino acid an artificial amino acid will be incorporated into the interaction domain of the transcription factor Gal4p in vivo at a defined position during biosynthesis (Figs. 3 and 5). For this purpose, azidophenylalanine and benzoylphenylalanine, which are derived from the natural amino acid phenylalanine, are used as artificial amino acids. The additional side groups of these amino acids can then be photoactivated by UV light to form covalent links to the respective interaction partners. These protein complexes, which are very tightly bound to one another, can then be isolated, subsequently enriched, and identified by means of mass spectrometry.