This special issue is dedicated to the 80th birthday of our esteemed colleague, Prof. Robert Kaptein, famous for his contributions to CIDNP, spin chemistry, and biomolecular NMR. In his PhD research Prof. Kaptein laid the theoretical framework for CIDNP and formulated the famous Kaptein rules, which describe the signs of the CIDNP signals. This development formed the basis for CIDNP as an efficient hyperpolarization method, providing unique information about short-lived radicals and radical pairs and leading to significant insight into the mechanisms of radical reactions. These developments laid the basis for an entire new field of science nowadays called spin chemistry. Later Prof. Kaptein developed laser photo-CIDNP as a selective and sensitive surface probe for studies of proteins and protein interactions in solution. His interest in spin hyperpolarization has not been limited to CIDNP: recently he also made valuable contributions in understanding the spin dynamics underlying PHIP/SABRE-derived polarization.

Prof. Kaptein has made prominent contributions as well to the computational and experimental methodology of biomolecular NMR and to the structure and dynamics of gene regulatory proteins and protein-DNA complexes. His laboratory developed among others non-selective homonuclear 3D NMR, restrained molecular dynamics, and methods for relaxation matrix calculations and protein structure validation. The structure of the Lac headpiece in 1985 was one of the first protein structures solved by NMR. This was followed by studies on the structure and dynamics of other gene regulatory proteins and protein-DNA complexes. Examples were the glucocorticoid receptor, the Arc repressor, and the POU domain of the Oct1 transcription factor. Central throughout his research have been the studies on the DNA complexes of the Lac repressor that not only deepened our understanding of protein-DNA recognition, but that also gave a molecular model for protein sliding along the DNA. These studies have been key in establishing NMR as a key method for studies on the structure and dynamics of proteins and protein complexes and an important stimulus in developing high-field NMR instrumentation and establishing national and international research infrastructures.

Prof. Robert Kaptein is an active member of the European NMR family. He established a world-class biomolecular NMR lab in Utrecht. Over many years, Prof. Kaptein has been and still is associate editor of the Journal of Biomolecular NMR. He also initiated the EMBO course for multidimensional NMR in structural biology and organized it for many years. For several years, he has been the Director of the Bijvoet Center for Biomolecular Research and Secretary General of the Royal Dutch Academy of Sciences (KNAW).

In this special issue, we find contributions from many colleagues who have worked with Prof. Robert Kaptein over these years. The contributions show current research in the topics to which Robert Kaptein has enormously contributed and present examples where these topics are still the focus of intense research.