Photo‐CIDNP study of interactions of serine proteinases with their protein inhibitors

Photochemically induced dynamic nuclear polarization was used to study the accessibility of surface tyrosine and tryptophan residues in proteinases, in their protein inhibitors and in the proteinase–inhibitor complexes. The accessibility probe is the triplet of 10‐(carboxyethyl) flavin formed by optical excitation. On complex formation we observe accessibility loss in the surface tyrosines and tryptophans in the proximity of the proteinase–inhibitor contact site, and in the case of bovine pancreatic trypsin inhibitor, in more distant tyrosines as well.