A systematic study of the valence electronic structure of cyclo(Gly–Phe), cyclo(Trp–Tyr) and cyclo(Trp–Trp) dipeptides in the gas phase

Abstract

The electronic energy levels of cyclo(glycine–phenylalanine), cyclo(tryptophan–tyrosine) and cyclo(tryptophan–tryptophan) dipeptides are investigated with a joint experimental and theoretical approach.

Ab initio Circular Dichroism with the Yambo code: applications to dipeptides

“…In particular, the first and second transitions contribute to the absorption peak at ≈ 8.7 eV, while the 3rd, 4th, 5th and 7th transitions contribute to the absorption peak at ≈ 9 eV. The fact that most low-energy transitions give non-negligible contributions to the IP absorption spectrum is in agreement with the observation that most of the highest occupied and lowest unoccupied electronic states in this system, as discussed by some of us in a previous work [4], are not localized in a specific part of the molecule (in that case low intensity contributions would be expected to occur for transitions between pairs of states localized on separated and relatively "far" parts of the molecule). Also in c-GlyPhe conformation 2 (grey curve in panel (a) of Fig.…”

“…We report here on our implementation of CD calculations within the Density Functional Theory (DFT) framework in the Yambo code [1], and on its application to computing (absorption and) CD spectra of three cyclo-dipeptides, cyclo(Glycine-Phenylalanine), cyclo(Tryptophan-Tyrosine) and cyclo(Tryptophan-Tryptophan), of which some of us had previously characterized the electronic occupied states [4].…”

Ab initio Circular Dichroism with the Yambo code: beyond the Independent Particle approximation

“…The energy distributions of occupied electronic states obtained with the two chosen computational schemes (PW vs. GT bases) are in agreement with each other, apart from possible small rigid shifts. As for empty states, PW codes (in our case QE, red levels) can capture continuum delocalized electronic states [4]. This results in an overall higher density of states in the positive energy range with respect to GT basis codes.…”

“…In previous works we studied the electronic [4] and optical properties [5], both absorption and CD, of three CDPs containing aromatic groups, i.e. c-GlyPhe, c-TrpTyr and c-TrpTrp.…”

“…2, left panel. At the B3LYP level, photons with energy below 8.57 eV (vertical ionization potential of c-GlyPhe from total energy differences [4]) can only access neutral excitations. However, we notice that a continuum of transitions, from the HOMO to vacuum levels, enter the TD-B3LYP hamiltonian starting from 6.9 eV (continuous line).…”