4P092
4P092
Quantum chemical DMRG study of biolobiological
gical dinuclear metal
metal2+9complexes: ∆ desaturase and {{[[Cu(NH3)3]2O2} complex(Institute for Molecular Science1) Jakub Chalupský1, Yuki Kurashige1, Takeshi Yanai1

We present quantum chemical study of two biologically relevant dinuclear transition
metal complexes based on DMRG-CASSCF and DMRG-CASPT2 calculations. Thesemethods, allowing us to use sufficiently large active spaces for dinuclear complexes, canprovide useful information about electronic structure of low-lying states as well as theenergetics of studied processes. We address two problems in this poster, barrier hight of
one of the reactions in catalytic cycle of ∆9 desaturase and energetics of isomerization of{[Cu(NH3)3]2O2}2+ complex.
Stearoyl-ACP ∆9 desaturase is the non-heme iron-containing enzyme involved in fattyacid metabolism and its regulation in plants, which catalyzes the conversion of thesaturated stearic-acid chain into the unsaturated oleic-acid chain. It undergoes multistep catalytic cycle, whose mechanism is under investigation by the experiments as wellas theoretical DFT-based QM/MM calculations. Although several alternatives of its
mechanism have been already revealed by theory, reliable prediction of the barrierheight for one of the key steps – abstraction of the first hydrogen atom from thesaturated stearic-acid chain – seems to be very difficult to make using DFT or
conventional CASSCF/CASPT2 approaches. We thus provide a study of height of thisbarrier based on DMRG-CASSCF/CASPT2 methodology, which allows for qualitatively
correct description of the electronic structure of the active site. We have found that thistype of calculations is practically feasible even for such a large system (model of active
site involves almost 70 atoms), and it provides very important results. Our DMRGCASPT2 calculations have shown that from the two alternatives of the mechanism
studied so far, one (so called “protonated” version of the mechanism, geometry change

during the studied step is shown in the figure) is significantly more favored than theother (water-containing version). Although a lot of work still needs to be done for otheralternatives of the mechanism of catalytic cycle of ∆9 desaturase, already these firstobtained results seems to be very promising, and we expect this project to be a highly
important application of DMRG approach to biologically relevant chemical problems.
Another interesting problem is the energy of isomerization between bis(µ-oxo) andperoxo forms of {[Cu(NH3)3]2O2}2+ complex, which are mimicking structural motives invarious copper-containing enzymes, for example tyrosinase. Accurate predictions of therelative energies of these structures have been shown to be extremely difficult to makeby nowadays theoretical methods – they seem to require either RASPT2 with the size ofactive space practically at its limit, or CCSD(T) including corrections formulticonfigurational character of the reference wave function. We thus consider theproblem of energetics of isomerization of {[Cu(NH3)3]2O2}2+ complex (in the figure) to be
an important case for testing the applicability of DMRG approach on systems, which

require including many orbitals on one center in the active space (Cu 3d and 4d shell –10 orbitals on a single atom), because it is expected from the principles of DMRG thatthis type of systems might be problematic to describe correctly. This study thus should
provide important information about way of dealing with such systems and possible
limitations of DMRG-CASSCF/CASPT2 approach, which could be of high importance tothe theoretical chemists in this field.