Quantum Dynamics of Vibrationally-Assisted Electron Transfer beyond Condon approximation in the Ligand-Receptor Complex

Abstract

We investigate the quantum dynamics of ligand--receptor electron transfer and conformational response in a prototypical viral binding complex, using the SARS-CoV-2 Spike protein bound to the human ACE2 receptor as a model system. Treating the ACE2--Spike interface as an open quantum system embedded in a biological environment, we simulate how vibrational interactions and environmental memory reshape the coupled receptor--ligand dynamics and modulate vibrationally assisted electron transfer (VA-ET). Using a Non-Markovian Stochastic Schr"odinger Equation (NMSSE) approach, we simulate electron transfer between donor and acceptor states in ACE2 modulated by a specific vibrational mode of the Spike protein. The influence of environmental memory (non-Markovian dynamics) and non-Condon effects (vibrational modulation of electronic coupling) are analyzed in detail. In the Markovian limit with an Ohmic bath, population dynamics reduce to exponential kinetics, and extracted transfer rates agree with semiclassical Marcus--Jortner predictions in the appropriate regime. Beyond the Markovian, high-temperature limit, we observe clear deviations: non-exponential decay, coherent oscillatory features, and enhanced sensitivity to the vibrational frequency. Incorporating off-diagonal system--bath coupling alongside diagonal coupling shows that nuclear motion can dynamically gate electron tunneling, sharpening the frequency selectivity of the VA-ET mechanism. Finally, a structured (sub-Ohmic) environmental spectral density with long-lived correlations (``memory'') preserves electronic--vibrational coherence over longer times, amplifying vibrational selectivity under non-Condon coupling. Our results support the proposition that ACE2--Spike binding may exploit vibrational assistance and quantum coherence as a molecular recognition mechanism.