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- co -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 ppm 1 at 298 K) with preserved HSA conformation (Δα-helix < 10%), while DGQD/SAC showed dynamic-dominated quenching ( K SV = 0.2648 ppm –1 at 298 K and K q = 26.48 × 10 6 ppm –1 s –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 (0.5R 0 < r < 1.5R 0 ). 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 S > 0), with SAC showing temperature-enhanced affinity ( K 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.