title: Coupled-cluster studies of extensive green fluorescent protein models using the reduced virtual space approach.
abstract: Accurate predictions of photoexcitation properties are a major challenge for modern methods of theoretical chemistry. We show here how approximate coupled-cluster singles and doubles (CC2) calculations in combination with the reduced virtual space (RVS) approach can be employed in studies of excited states of large biomolecular systems. The RVS-CC2 approach is used for accurately predicting optical properties of the p-hydroxybenzylidene-dihydroimidazolinone (p-HBDI) chromophore embedded in green fluorescent protein (GFP) models using quantum mechanical calculations in combination with large basis sets. We study the lowest excited states for the isolated and protein-embedded chromophore in two different protonation states, and show how omitting high-lying virtual orbitals in the RVS calculation of excitation energies renders large-scale CC2 studies computationally feasible. We also discuss how the error introduced by the RVS approach can be systematically estimated and controlled. The obtained CC2 excitation energies of 3.13-3.27 and 2.69-2.77 eV for the two protonation states of different protein models are in excellent agreement with the maxima of the experimental absorption spectra of 3.12-3.14 and 2.61-2.64 eV, respectively. Thus, the calculated energy splitting between the excited states of the two protonation states is 0.44-0.52 eV, which agrees very well with the experimental value of 0.48-0.51 eV. The calculations at the RVS-CC2 level on the protein models show the importance of using large QM regions in studies of biochromophores embedded in proteins.
authors.affiliation: BASF SE , Quantum Chemistry Group, GVM/M - B009, D-67056 Ludwigshafen, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Send
authors.forename: Carl-Mikael
authors.initials: CM
authors.lastname: Suomivuori
authors.forename: Ville R I
authors.initials: VR
authors.lastname: Kaila
authors.forename: Dage
authors.initials: D
authors.lastname: Sundholm
mesh_terms: D000818:Animals; D005240:Feasibility Studies; D049452:Green Fluorescent Proteins / Q000737:chemistry*; D006860:Hydrogen Bonding; D037402:Hydrozoa; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory
chemical_list: D049452:Green Fluorescent Proteins