Hagen Wilfred R
Department of Biotechnology, Delft University of Technology, Building 58, Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
Molecules. 2024 May 11;29(10):2254. doi: 10.3390/molecules29102254.
The electron-electron, or zero-field interaction (ZFI) in the electron paramagnetic resonance (EPR) of high-spin transition ions in metalloproteins and coordination complexes, is commonly described by a simple spin Hamiltonian that is second-order in the spin : H=D[Sz2-SS+1/3+E(Sx2-Sy2). Symmetry considerations, however, allow for fourth-order terms when ≥ 2. In metalloprotein EPR studies, these terms have rarely been explored. Metal ions can cluster via non-metal bridges, as, for example, in iron-sulfur clusters, in which exchange interaction can result in higher system spin, and this would allow for sixth- and higher-order ZFI terms. For metalloproteins, these have thus far been completely ignored. Single-molecule magnets (SMMs) are multi-metal ion high spin complexes, in which the ZFI usually has a negative sign, thus affording a ground state level pair with maximal spin quantum number = ±, giving rise to unusual magnetic properties at low temperatures. The description of EPR from SMMs is commonly cast in terms of the 'giant-spin model', which assumes a magnetically isolated system spin, and in which fourth-order, and recently, even sixth-order ZFI terms have been found to be required. A special version of the giant-spin model, adopted for scaling-up to system spins of order ≈ 10-10, has been applied to the ubiquitous iron-storage protein ferritin, which has an internal core containing Fe ions whose individual high spins couple in a way to create a superparamagnet at ambient temperature with very high system spin reminiscent to that of ferromagnetic nanoparticles. This scaled giant-spin model is critically evaluated; limitations and future possibilities are explicitly formulated.
金属蛋白和配位络合物中高自旋过渡离子的电子顺磁共振（EPR）中的电子-电子相互作用或零场相互作用（ZFI），通常由一个简单的自旋哈密顿量来描述，该哈密顿量在自旋上是二阶的：H = D[Sz² - S(S + 1)/3] + E(Sx² - Sy²)。然而，当对称性考虑≥2时，允许存在四阶项。在金属蛋白EPR研究中，这些项很少被探讨。金属离子可以通过非金属桥簇集，例如在铁硫簇中，其中交换相互作用可导致更高的体系自旋，这将允许存在六阶及更高阶的ZFI项。对于金属蛋白，这些项迄今为止完全被忽略了。单分子磁体（SMM）是多金属离子高自旋络合物，其中ZFI通常具有负号，从而产生具有最大自旋量子数S = ±S的基态能级对，在低温下产生不寻常的磁性。SMM的EPR描述通常根据“巨自旋模型”进行，该模型假设一个磁隔离的体系自旋，并且已经发现需要四阶甚至最近的六阶ZFI项。巨自旋模型的一个特殊版本，用于放大到约为10 - 10的体系自旋，已应用于普遍存在的铁储存蛋白铁蛋白，其内部核心含有Fe离子，其单个高自旋以某种方式耦合，在室温下形成具有非常高体系自旋的超顺磁体，类似于铁磁纳米颗粒。对这个缩放后的巨自旋模型进行了严格评估；明确阐述了其局限性和未来可能性。
