Ultrafast Autofluorescence of Bilirubin as a Sensor of Albumin Conformation in Blood Serum

Pathological processes are often accompanied by alterations of protein conformations in blood serum, making investigation of these structural rearrangements highly relevant for clinical diagnostics. Conformation of albumin, the predominant protein in blood serum, is known to be a sensor of pathologies in the human organism; however, label-free methods for its assessment directly in blood serum samples are lacking. In this work, we present a novel analytical methodology for evaluating albumin conformation using the fluorescence parameters of intrinsic blood serum fluorophores excited in the visible range. We first estimate the contribution of various endogenous fluorophores excited in the vicinity of 400 nm to both steady-state fluorescence and fluorescence decay across picosecond and nanosecond time scales, showing that one of the dominant fluorophores is bilirubin, an albumin ligand. In model experiments, we then demonstrate that the structural and photophysical features of bilirubin make its fluorescence decay at picosecond time scale sensitive to conformation of the protein-bilirubin complex. As a final step, it is demonstrated that changes in the ultrafast fluorescence decay parameters of the bilirubin are sensitive enough to detect biologically relevant differences in albumin conformation in serum across different patients. Specifically, we observed statistically significant differences in blood serum albumin's conformation for patients of different age groups (≤34 years and ≥65 years), suggesting that bilirubin may serve as a promising intrinsic sensor for assessing albumin conformational modifications in blood serum.