Study on the Interaction of Functionalized Doped Graphene Quantum Dots by S-Allylcysteine and Serum Albumin Using Fluorescence Spectroscopy and Molecular Docking Techniques

This study investigates the interaction mechanisms between human serum albumin (HSA) and two structurally distinct ligands S-allyl-cysteine (SAC) and S, N-<i>co</i>-doped graphene quantum dots functionalized by S-allyl-cysteine (DGQD/SAC) using multispectroscopic and computational approaches. Steady-state and time-resolved fluorescence measurements revealed distinct quenching mechanisms: SAC exhibited static quenching through ground-state complex formation (K_SV = 2 × 10^-4 ppm¹ at 298 K) with preserved HSA conformation (Δα-helix < 10%), while DGQD/SAC showed dynamic-dominated quenching (<i>K</i>_SV = 0.2648 ppm^-1 at 298 K and <i>K</i>_q = 26.48 × 10⁶ ppm^-1s^-1) accompanied by partial protein unfolding (15% α-helix reduction). Förster resonance energy transfer (FRET) analysis confirmed donor-acceptor distances of 2.85 nm for HSA-DGQD/SAC, within optimal range for energy transfer (0.5R₀< <i>r</i> < 1.5R₀). Circular dichroism (CD) spectra demonstrated SAC's localized binding at Sudlow's site I, whereas N, S-GQD/SAC induced tertiary structure perturbations. Thermodynamic profiling revealed entropy-driven binding for both ligands (Δ<i>S</i> > 0), with SAC showing temperature-enhanced affinity (<i>K</i>_a increased from 1.2474 to 1.9902 ppm^-1, 298-318 K). These findings provide critical insights for designing HSA-based delivery systems, highlighting SAC's structural preservation advantages and DGQD/SAC's tunable interfacial interactions.