Beyond Composition: Structure–Activity Relationships in Bioactive Deep Eutectic Systems

Deep eutectic systems (DESs) have evolved from sustainable solvent alternatives to promising bioactive platforms with reported antimicrobial, anti-inflammatory, regenerative, cryoprotective, and cytoprotective properties. However, despite the growing number of biological studies, the mechanistic basis of these effects remains poorly understood because biological activity is still interpreted predominantly from the chemical identity of the hydrogen-bond donor and acceptor, rather than from the supramolecular organization of the eutectic system itself. This review is intended to provide anyone interested in the biomedical and pharmaceutical applications of DESs with a conceptual framework for understanding how supramolecular organization may influence the biological performance of DES-based systems, without requiring extensive expertise in physical chemistry. It critically analyzes the current evidence linking DES structure with biological function. The literature reveals that many reported biological responses cannot be fully explained by the properties of the individual constituents alone, supporting the existence of emergent physicochemical behavior associated with eutectic formation. Current evidence further demonstrates that DESs are dynamic supramolecular systems characterized by hydrogen-bond networks, nanoscale heterogeneity, hydration-dependent structural rearrangement, and persistent local organization under biologically relevant conditions. These structural features generate localized physicochemical microenvironments capable of modulating membrane organization, protein hydration, osmotic balance, and biomolecular interactions, providing a plausible mechanistic basis for the diverse biological effects reported to date. Our analysis also highlights a fundamental disconnect between the extensive physicochemical characterization of DESs and the predominantly composition-based interpretation of their biological activity. While conventional Quantitative Structure–Activity Relationship (QSAR) approaches rely on molecular descriptors of individual components, they fail to capture the higher levels of organization that characterize these dynamic multicomponent systems. Based on concepts established in supramolecular chemistry, self-assembled biomaterials, colloidal science, and soft matter, we propose a Hierarchical Structure–Activity Relationship (H-SAR) framework in which biological activity emerges from successive levels of organization extending from molecular composition and hydrogen-bond networks to nanostructural organization, hydration-dependent restructuring, localized physicochemical microenvironments, and biological interfaces. This framework provides a mechanistic basis for interpreting DES bioactivity and could offer a conceptual roadmap for the rational design, predictive modeling, and biomedical translation of next-generation bioactive deep eutectic systems.