Bhattacharya Debojit, Panda Anirban, Shil Suranjan, Goswami Tamal, Misra Anirban
Department of Chemistry, University of North Bengal, Darjeeling, PIN. 734013, West Bengal, India.
Phys Chem Chem Phys. 2012 May 21;14(19):6905-13. doi: 10.1039/c2cp00053a. Epub 2012 Apr 3.
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.
我们设计并从理论上研究了三对不同的绿色荧光蛋白发色团及其与亚氨基氮氧化物偶联的基于不同同系物的双自由基。首先，在气相、水介质和血浆介质中，所有这些双自由基的几何结构都已在高自旋（HS）状态下进行了优化。在气相情况下，计算过程简单且成熟。然而，对于在水中的计算，我们采用了自己的N层集成分子轨道和分子力学（ONIOM）方法。同样，对于血液相计算，采用了极化连续介质模型（PCM）方法。利用这些优化后的几何结构，在无限制密度泛函理论框架下，采用破缺对称性（BS）方法估计了这些双自由基在不同介质中的磁交换耦合常数（J）值。为了在ONIOM方法中获得BS解，我们进行了ONIOM-BS计算，其中对内部高层（双自由基体系）进行BS计算，而外部水层保持在低水平。类似地，在PCM方法的BS计算中也采用了PCM-BS技术。我们发现，这些双自由基在照射适当波长的光后，能够将其磁性从反铁磁性的反式转变为铁磁性的顺式。使用含时密度泛函理论（TDDFT）技术，确定了气相和水介质中每对双自由基使非荧光暗反式双自由基转变为其相应的亮荧光顺式异构体所需的光波长。这种颜色变化确实是相关双自由基磁态变化的一个标志。在此，我们还计算了零场分裂（ZFS）参数（D）、菱形ZFS参数（E）和ZFS大小（a2）。通过计算，我们满怀期望地认为，如果合成这些双自由基，那么它们可能会作为一种成功的、无害的磁共振成像造影剂（MRICA）来替代其他基于金属的造影剂。
