A theoretical study on photomagnetic fluorescent protein chromophore coupled diradicals and their possible applications

Abstract

We have designed and theoretically studied three different pairs of green fluorescent protein chromophores and their different homologue-based diradicals coupled with imino nitroxides. To begin with, the geometries of all these diradicals have been optimized at high spin (HS) state in the gas phase, in a water medium and in a blood plasma medium. The process of calculations is straightforward and well-established in the case of the gas phase. However, for calculations in water, we have adopted our own N-layer integrated molecular orbital and molecular mechanics (ONIOM) method. Similarly for the blood phase calculations, the polarized continuum model (PCM) method has been adopted. With these optimized geometries the magnetic exchange coupling constant (J) values are estimated for these diradicals in different media using the broken symmetry (BS) approach in an unrestricted DFT framework. In order to obtain the BS solutions in the ONIOM method, we have carried out ONIOM-BS, where the BS calculations are done for the inner high-level layer (diradical system) keeping the outer water layer at low level. In a similar fashion, a PCM-BS technique has also been adopted for the BS calculations in the PCM method. We have found that these diradicals have an ability to change their magnetic nature from antiferromagnetic in the trans form to ferromagnetic in the cis form upon irradiation of light with the appropriate wavelength. Using a time-dependent DFT (TDDFT) technique, the required wavelengths of light by which non-fluorescent dark trans diradicals turn into their corresponding bright fluorescent cis isomers are determined for each pair of diradicals for the gas and water media. This color change is indeed a signature of the change in magnetic state of the diradicals concerned. Here, we have also calculated the zero field splitting (ZFS) parameter (D), rhombic ZFS parameter (E) and ZFS magnitude (a2). From our calculations we ambitiously expect that if these diradicals are synthesized then they might be used as a successful, non-hazardous magnetic resonance imaging contrast agent (MRICA) in place of other metal-based contrast agents.

Photomagnetic and nonlinear optical properties in cis‐trans green fluoroprotein chromophore coupled Bis‐imino nitroxide diradicals

“…It is of interest to use the M06 functional to evaluate the transition energies responsible for trans to cis photo‐conversion. We have evaluated the excitation energies of these diradicals using the M06 functional and compare these values with the earlier results obtained using B3LYP functional and also with the experimental results (d)] which are furnished for the bare couplers. Table depicts π → π * transition energies for these diradicals with various functionals (UM06, UB3LYP, UPBE0, and UBP86) along with experimental values for the bare couplers from which these diradicals are designed.…”

“…Furthermore, for the reasons elaborated in the Introduction, we choose only the M06 functional for the higher quality basis‐set calculation, that is, with UM06/6–311++G(d,p) level of theory (Table ). In addition to the UM06/6–311++G(d,p) data, in Table we also list the UB3LYP/6–311++G(d,p) data taken from the previous study on these systems . For each of the DFT methods, Table lists the magnetic exchange coupling coefficients J , the change of these coefficients Δ J as the geometries change from trans to cis ‐conformers and the values for triplet and broken‐symmetry (BS) singlet spin‐multiplicities.…”

“…An interested reader may find a detailed description of the computational strategy adopted here. [59,63] However, a concise draft of the most relevant part of this section is given below. The Heisenberg Hamiltonian…”

“…There are different equations discussed elsewhere (see, e.g., Refs. [59,63] and references therein) which are used to evaluate the exchange coupling constants. However, in this work, we have used the well-accepted Yamaguchi formula…”

“…In addition to the UM06/ 6-31111G(d,p) data, in Table 2 we also list the UB3LYP/6-31111G(d,p) data taken from the previous study on these systems. [59] For each of the DFT methods, Table 2 lists the magnetic exchange coupling coefficients J, the change of these coefficients DJ as the geometries change from trans to cis-conformers and the hS 2 i values for triplet and broken-symmetry (BS) singlet spin-multiplicities. It is well-known [45] that a large amount of spin-contamination indicates that description of the associated electronic state is unreliable and that the calculated J values cannot be trusted.…”