The present volume contains the proceedings of the 23rd Mos bach Colloquium on "Protein-Protein Interactions". It includes the paper by Prof. S. LIFSON who unfortunately was unable to attend. Discussions were included whenever possible in order to make the complete proceedings accessible to the future reader. The colloquium was generously supported by the firms who are corporate members of the Gesellschaft für Biologische Chemie. Dr. W. B. GRATZER and Dr. L. JAENICKE, as weIl as some of the invited speakers, were a tremendous help in preparing the program and we wish to thank them for their helpfuI suggestions and advice. Our appreciation extends to Prof. E. AUHAGEN and Prof. H. GIBIAN, and their secretaries, who shared the burden of organizing the colloquium. In addition, we wish to express our gratitude to Springer-Ver Iag, and especially to Dr. H. MAYER-KAUPP for editoriaI help and for the rapid publication of this volume. We hope that the reader, Iike the audience in the Mosbach market hall, will gain an impression of the present state of protein research and of the way in which intermolecular interactions hold specific implications for the structure and function of proteins. Regensburg and Würzburg, autumn 1972 R. JAENICKE E. HELMREICH Contents Introduction. R. JAENICKE (Regensburg) 1 Molecular Forees. S. LIFSON (Rehovot). 3 Structure, Function and Dynamics of a ReguIatory Enzyme: Aspartate Transcarbamylase. H. K. SCHACHMAN (Berkeley) 17 Discussion. 54.