UC Irvine

The electronic transitions of indole, the aromatic UV chromophore of the amino acid tryptophan, are characterized by using state-of-the-art multiconfigurational methods in gas-phase and aqueous solution, revealing the electronic spectrum up to 10 eV. Bidimensional near-ultraviolet (2D-NUV) electronic spectra of indole are simulated using the sum-over-states approach, based on ab initio calculations, accounting for different experimental set-ups, including rephasing (KI = - k1 + k2 + k3), quasi-absorptive (PP) and double quantum coherence (KIII = k1 + k2 - k3) signals, and both one-color (2D-NUV) and two-colors (2D-NUV/Vis) regimes. In order to obtain accurate energies of high lying excited states and reliable 2DES spectra, extravalence virtual orbitals have been included using the restricted active space technique. The 2D-NUV spectrum of indole shows off-diagonal signals due to the correlation of the GS → Lb and GS → La transitions, an indole "fingerprint" in the NUV region that differentiate it from other aromatic chromophores in proteins. Further indole-specific transitions are resolved in the whole Vis-NUV range. A background-free region bellow the ionization potential, which shows no absorption signals in the monomer, can be used to resolve charge transfer states in coupled chromophore aggregates with a two-color 2D-NUV/Vis experimental set-up. Fundamental information for design of the 2DES experiments of indole is provided, including possible experimental pulse configurations that improve spectral resolution, revealing anharmonicities and selecting transitions. The proposed 2DES experiments could provide unprecedented level of detail for tracking indole electronic transitions in proteins, laying the groundwork for the use of nonlinear ultrafast optical spectroscopy for the study of protein structure and dynamics in solution. © 2014 Elsevier B.V. All rights reserved.