He has held guest professorships at the Universities of Stony Brook (US), Strasbourg (France), La Plata (Argentina), Copenhagen/Lyngby (Denmark), and Nagoya (Japan).
Krebs has published more than 750 scientific peer-reviewed papers in international journals; besides a number of review articles, he was editor and co-editor of three books, and he is co-author of 13 patents.
He served for several years in leading positions of German and European science organizations, e.g., as a chief referee (IC) for the Deutsche Forschungsgemeinschaft (DFG).
Starting with his thesis on trithiocarbonic acid and its chemistry, and supported by his profound experience gained during his postdoctoral work at Brookhaven National Laboratory, he was one of the pioneers of chemical crystallography in Germany in the sixties.
Further investigations include a large number of mono- and polynuclear sulphur complexes of transition metals such as iron, nickel, cobalt, zinc or molybdenum.
His pioneering crystal structures of purple acid phosphatase from kidney bean and of catechol oxidase from sweet potatoes have become classical results of metalloenzyme research papers.