How Shaped Light Discriminates Nearly Identical Biochromophores

We present a general mechanism for successful discrimination of spectroscopically indistinguishable biochromophores by shaped light. For this purpose we use nonadiabatic dynamics in excited electronic states in the frame of the field-induced surface hopping method driven by the experimentally shaped laser fields. Our findings show that optimal laser fields drive low-frequency vibrational modes localized in the side chains of two biochromophores, thus selecting the parts of their potential energy surfaces characterized by different transition dipole moments leading to different ionization probabilities. The presented mechanism leads to selective fluorescence depletion which serves as a discrimination signal. Our findings offer a promising perspective for using optimally shaped laser pulses in bioanalytical applications by increasing the selectivity beyond the current capability.

Figure 1

Schematic illustration of the optimal dynamic discrimination by shaped laser fields on the example of shaped pulse 1 maximizing the FMN-RBF fluorescence depletion ratio.

Figure 2

Upper panel: Temporal structure of pulse 1 for maximization (left-hand side) and of pulse 2 for minimization (right-hand side) of the depletion ratio D(FMN)/D(RBF). Middle panel: Ionized populations $P_{\mathrm{ion}}$ of RBF (black) and FMN [grey (red)] due to pulse 1 and pulse 2. Lower panel: Fluorescence depletion $D$ of RBF (black) and FMN [grey (red)] due to pulse 1 and pulse 2.

Figure 3

Fluorescence depletion ratio as a function of the IR pulse delay for unshaped UV pulse (black), pulse 1 [dark grey (red)], and pulse 2 [light grey (green)].

Figure 4

(a),(b) Average transition dipole moments for $S_1\to S_2–S_9$ transitions for the dynamics driven by pulse 1 (a) and 2 (b) for FMN [grey (red)] and RBF (black). The average is performed over the states $S_2–S_9$ and over the ensemble of trajectories. (c),(d) Average transition dipole moments $⟨{\mu }_{n9}⟩$ for $S_1–S_8\to S_9$ transitions weighted by the state populations as $⟨{\mu }_{n9}⟩=\sum _n|c_n{|}^2|{\mu }_{n9}|$, in the time window of the IR pulse (shaded) for pulse 1 (c) and 2 (d).

Figure 5

Selected averaged ground state normal mode displacements for RBF (upper panel) and FMN (lower panel) induced by pulse 1 (black) and pulse 2 [grey (red)].