Electron paramagnetic resonance studies of free radicals: The research legacy of Janko N. Herak.

Abstract

This review presents a chronological analysis of the scientific contributions of Professor Janko N. Herak (1937-2025), a Croatian biophysicist whose research played a major role in the development of electron para-magnetic resonance (EPR) spectroscopy in Croatia and significantly advanced the study of free radicals in biological systems. Beginning with his early work at Duke University under the mentorship of Prof. Walter Gordy, Herak investigated radiation-induced radicals in nucleic acid bases, nucleosides, and related biomolecules using EPR spectroscopy. During subsequent decades at the Ruđer Bošković Institute and later at the University of Zagreb Faculty of Pharmacy and Biochemistry, he conducted extensive studies of free radicals in irradiated single crystals of nucleic acid constituents, elucidating mechanisms of radical formation, hydrogen transfer, and structural rearrangements. His research later expanded to the investigation of oxidative processes in biological macromolecules, particularly the slow oxidation of plasma lipoproteins, where EPR spectroscopy and kinetic modelling provided insight into radical-mediated lipid peroxidation and the physicochemical basis of lipoprotein modification. In parallel, Herak explored long-range charge migration in nucleobase crystals and investigated metal-nucleobase interactions, integrating experimental EPR and ENDOR spectroscopy with theoretical modelling. These studies contributed to a deeper understanding of radical stabilisation, electronic structure, and energy transfer in ordered molecular systems. By situating Herak's work within the broader development of radiation chemistry and magnetic resonance spectroscopy, this review highlights the methodological rigour, conceptual contributions, and lasting scientific impact of his research on free radicals and biologically relevant molecular systems.