Abstract The present publication examines the transition from the already derived structural water window toward biological compatibility. It does not begin from biology as an independent external layer, but continues a previous chain of derivation: water as an ensemble of admissible self-consistent regimes with non-uniform stability; a minimal law for the stability of water regimes; preferred cluster regimes without external tuning; temperature redistribution without external tuning; the structural water window; a broad life-compatible regime; and a narrower enzyme-compatible regime. On this basis, the present work asks whether major classes of biological structures can be understood as naturally compatible not with arbitrary water, but with a narrow class of locally structured and dynamically reconfigurable water regimes. It is shown at the structural-compatibility level that DNA hydration, enzyme pockets, membrane interfaces, and the cytoskeletal hydration environment require one and the same general type of aqueous environment: neither domain-rigid nor chaotically diluted, but intermediate water in which local structural organization and rapid reconfiguration coexist. External sources, when present, are not used as the basis of the derivation, but only as points of comparison with measured or independently proposed pictures.
Balevsky et al. (2026) studied this question.