Peroxynitrite-induced structural remodeling and aggregation of hemoglobin under nitroxidative stress

Peroxynitrite (PN), generated by the reaction of nitric oxide with superoxide, is a potent reactive nitrogen species (RNS) implicated in nitroxidative protein damage in cardiovascular and neurodegenerative diseases. Hemoglobin (Hb), the principal oxygen-transport protein of erythrocytes, is a key intravascular scavenger and target of PN; however, how controlled PN exposure translates into structural remodeling and aggregation of Hb remains poorly understood. Purified human Hb was exposed to PN across sub-stoichiometric to moderate oxidant excess and characterized using intrinsic fluorescence, UV-visible (UV-vis) spectroscopy, 2,4-dinitrophenylhydrazine (DNPH) carbonyl assay, Thioflavin T (ThT), 8-anilino-1-naphthalenesulfonic acid (ANS), Congo Red (CR), Fourier-transform infrared (FTIR) spectroscopy with Amide I deconvolution, dynamic light scattering (DLS) with zeta potential, x-ray powder diffraction (XRPD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and fluorescence microscopy. At low oxidant loads, PN caused concentration-dependent fluorescence quenching, carbonyl accumulation, and graded UV-visible absorbance increases, consistent with heme-centered oxidation and limited aromatic residue modification. At higher concentrations, Amide I deconvolution revealed collapse of α-helical content from 66.41% to 8.81% and β-sheet enrichment to 71.18%, accompanied by surface charge neutralization, increased hydrodynamic diameter, and enhanced nanoscale roughness. Despite β-sheet accumulation and ThT/CR-positive aggregate formation, XRPD, AFM, SEM, and fluorescence microscopy showed no long-range crystalline order or fibrillar morphology, demonstrating that PN drives Hb toward amorphous and oligomeric assemblies rather than canonical amyloid fibrils. Time-dependent assays at 37°C confirmed that PN modification lowers the nucleation barrier for thermally driven aggregation. These findings define physicochemical thresholds separating protective scavenging from structural destabilization and offer a framework for distinguishing nitroxidative protein aggregation from classical amyloid fibrillation.