Quantifying Ultra-Low Pigment Solubilities by UV-Vis Spectroscopy: Implications for Toxicological Assessment

Accurate toxicological risk assessment of organic pigments is often hampered by their extremely low and poorly characterized solubilities, which obscure their true molecular bioavailability in biological systems. We present a comprehensive optical spectroscopic study of 18 pigments from 7 different structural classes, determining ultralow solubilities in DMSO and NMP through a novel iterative dilution procedure that deconvolutes particulate and molecular spectral signatures based on excitonic coupling theory. Rigorous exclusion of particulate contributions yielded true molecular extinction coefficients and optical band shapes. Results were consolidated by computational simulations of the molecular and crystalline pigment forms, respectively, at the TDDFT (PBE0/def2-SVP) level of theory. To translate these findings to biology, experimentally derived solubilities were extended to water and cytosol by integrating Abraham-type solvation models with composition-based pp-LFER frameworks. This combined approach revealed subnanomolar to subpicomolar solubilities in aqueous media and upper-bound cytosolic concentrations typically below 1 μM, providing a physicochemical basis to critically assess the relevance of toxicological end points, such as sensitization and genotoxicity, for tattooing and patch testing. Our results suggest that many negative toxicological findings may reflect insufficient bioavailability rather than a lack of intrinsic hazard.