CC2 Benchmark for Models of Phenylalanine Protein Chains: 0–0 Transition Energies and IR Signatures of the ππ* Excited State

Abstract

Extensive benchmarking calculations are presented to assess the accuracy of the standard approximate coupled cluster singles and doubles method (CC2) in studying * excited states properties of model protein chains containing a phenylalanine residue, namely capped peptides, whose ground state conformers adopt the prototypical secondary structural features of proteins. First, the dependence with the basis-set of the CC2 excitation energies, CC2 geometry optimizations and amide A region frequencies of the lowest * excited state in a reference system, the N-acetyl-phenylalaninylamide, are investigated and the results are compared with experimental data. Second, at the best level of theory determined, the CC2/aug(N,O,)-cc-pVDZ//CC2/cc-

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

“…In this case the trans NH is involved in an H-bond with the backbone and the cis NH in a   H-bond to either an aromatic residue of the sequence (cf. Supplementary information of ref 369 ) or to a -NH-Bn benzylamide cap. 78,79 This type of doubly bonded amide structure was also observed in a conformer of capped phenylalanine monohydrate.…”

“…However, CC2 methods has recently proven to provide valuable benchmarks on relatively large species. 369 Beyond transition energies, the same approach can also simulate Franck-Condon activity of electronic transitions and thus provide additional assignment information. Although popularized by recent simulations of vibronic spectra of protonated species, 370 its use in assigning UV spectra of neutrals remains widely unexploited.…”

Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects

“…This accuracy is far better than that reached by CC2 calculations to predict adiabatic electronic transitions, the relative errors being typically of the order of ~100 cm -1 for the phenyl chromophore. 45,46 -Remarkably, the identification of EI-ESE in the ion pairs considered in this work, together with the identification of Franck-Condon patterns and an analysis of the fragmentation pattern, enabled a complete conformational analysis based on UV spectroscopy only. While conformerselective IR spectroscopy is legitimately considered as a powerful source of structural information, the cost of these experiments and their related frequency calculations appear useless in this study, i.e.…”

“…This accuracy is far better than that reached by CC2 calculations to predict adiabatic electronic transitions, the relative errors being typically of the order of ∼100 cm −1 for the phenyl chromophore. 45,46 …”

Excited States Computation of Models of Phenylalanine Protein Chains: TD-DFT and Composite CC2/TD-DFT Protocols

“…Among the single reference methods, the time-dependent density functional theory (TD-DFT) [ 6 ], which describes excited states within response theory, exhibits a favorable cost–performance ratio, but even to only perform a qualitative exploration of the PES, the functional has to be judiciously chosen according to the nature of the excited states considered [ 7 , 8 , 9 , 10 , 11 ]. In this context, we developed an original, innovative computational strategy in order to document the conformer selective dynamics of capped peptide building blocks (including the phenylalanine (Phe) residue), serving as models of proteins [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Gas-phase isolation was investigated first, enabling cross-checking between the experimental data and relevant quantum chemistry methods in order to validate the theoretical approach.…”