Ultrafast charge migration in tyrosine and tryptophan: Branch energy bias and aromatic ring participation

We investigate ultrafast hole migration in the amino acids tyrosine and tryptophan after strong-field ionization. The initial junction-localized hole state is prepared using constrained density functional theory, and its subsequent dynamics are followed using time-dependent density functional theory. In the time domain, both molecules show delayed but finite involvement of the aromatic unit, with a much shorter timescale for aromatic engagement in tryptophan (Trp) than in tyrosine (Tyr). Frequency-domain analysis reveals distinct beating energies associated with long-distance hole redistribution between the branch and the ring. These features can be understood within a simplified two-level Rabi picture, which captures the dominant peaks in the hole dynamics. We further show that the constrained density functional theory (cDFT) site-energy landscape connects the energetics of initial hole localization to the subsequent quantum-beat signatures: in both molecules, the terminal oxygen on the functional branch is the energetically favored hole site, explaining the initial tendency of a junction-launched hole to move toward the branch. Our results may provide a practical framework for interpreting and potentially controlling ultrafast charge migration in complex molecules.