Conformational dynamics and interfacial organization of bovine serum albumin-gold nanourchin conjugates probed by...

The dynamic molecular orientation and conformational changes of bovine serum albumin‑gold nanourchin (BSA-GNU) conjugate were investigated using various optical techniques. Interfacial changes showed a nonlinear tyrosine (Tyr) response at 278 nm, dynamic corona formation observed in time-resolved UV-Vis spectroscopy, and CMOS imaging showed nonlinear intensity profiles. Fluorescence emissions at ∼295 nm (Tyr) and ∼ 385 nm (tryptophan, Trp) initially increased due to reduced Tyr quenching and BSA structural rearrangement, then decreased as enhanced quenching accompanied growth in the GNUs' hydrodynamic diameter. FT-NIR spectra showed decreased absorption intensities consistent with protein adsorption. Time-resolved SERS revealed temporal variations in relative intensities and shifts in vibrational band positions, particularly in amide I and III regions, indicating protein unfolding, denaturation, and secondary-structure rearrangements. Fourier cross-correlation (FCC) showed maximal correlation in the first 30 s due to rapid BSA rearrangement, followed by frequency shifts and phase decorrelation from partial unfolding before reaching a new steady state. Fast Fourier transform (FFT) analysis of SERS revealed complex plasmon-protein coupling, with oscillation amplitudes reflecting coupling strength and accelerated decay beyond 60 s, indicating increased conformational heterogeneity. The scattered wavelengths (570-640 nm) varied with polarization, showing minima at 100° and 280°. Polarization-dependent intensity changes, asymmetric nonlinear patterns, and reduced optical power for BSA-GNU compared with BSA alone indicate conformational and orientational dynamics of BSA at the plasmonic interface. MATLAB modelling showed ≈ 95% GNU coverage by BSA within 30 μ s at 0.05 M.