Neuronal PAS protein 2 (NPAS2), a heme-binding transcriptional regulatory factor, is involved in circadian rhythms. Period homologue (Per) is another important transcriptional regulatory factor that binds to cryptochrome (Cry). The resultant Per/Cry heterodimer interacts with the NPAS2/BMAL1 heterodimer to inhibit the transcription of Per and Cry. Previous cell biology experiments indicate that mouse Per2 (mPer2) is also a heme-binding protein, and heme shuttling between mPer2 and NPAS2 may regulate transcription. In the present study, we show that the isolated PAS-A domain of mPer2 (PAS-A-mPer2) binds the Fe(III) protoporphyrin IX complex (hemin) with a heme:protein stoichiometry of 1:1. Optical absorption and EPR spectroscopic findings suggest that the Fe(III)-bound PAS-A-mPer2 is a six-coordinated low-spin complex with Cys and an unknown axial ligand. A Hg (2+) binding study supports the theory that Cys is one of the axial ligands for Fe(III)-bound PAS-A-mPer2. The dissociation rate constant of the Fe(III) complex from PAS-A-mPer2 (6.3 x 10 (-4) s (-1)) was comparable to that of the heme-regulated inhibitor (HRI), a heme-sensor enzyme (1.5 x 10 (-3) s (-1)), but markedly higher than that of metmyoglobin (8.4 x 10 (-7) s (-1)). As confirmed by a Soret absorption spectral shift, heme transferred from the holo basic helix-loop-helix PAS-A of NPAS2 to apoPAS-A-mPer2. The Soret CD spectrum of the C215A mutant PAS-A-mPer2 protein was markedly different from that of the wild-type protein. On the basis of the data, we propose that PAS-A-mPer2 is a heme-sensor protein in which Cys215 is the heme axial ligand.

Electron spin polarizations were studied for a system of excited triplet (T1) phthalocyanine (MTNPc; M = H2, Zn) and a nitroxide radical (TEMPO) in toluene solution by means of time-resolved EPR (TREPR) spectroscopy. TREPR spectra of the T1 state and the radical (R) were observed simultaneously. Spin polarizations of both signals were found to consist of two components with different decay rates. From analyses of the polarizations and the decay curves, it was found that the triplet polarizations are generated from anisotropic S1→T1 intersystem crossings and thermal populations. The initial and late polarizations on TEMPO were interpreted in terms of electron spin polarization transfer (ESPT) from T1 and a radical-triplet pair mechanism (RTPM) with a quartet precursor under J < 0 (J; exchange coupling parameter), respectively. The ESPT rate was determined and found to be dependent on temperature and the central atom(s) of MTNPc. In the H2TNPc system, the ESPT rate is much slower than the diffusion rate and axial ligation of TEMPO remarkably accelerates the ESPT rate in the ZnTNPc system.

The substituent effect on the g-tensor of polycrystalline 2,6-di-tert-butyl phenoxyl radical derivatives diluted in diamagnetic crystals was investigated using multifrequency ESR spectroscopy and DFT calculations. It was revealed that the g-tensors of the series of phenoxyl radical derivatives essentially have an orthorhombic symmetry. For some radicals, the hyperfine-splitting tensors from the para groups were resolved. The interpretations and the assignments of the spin-Hamiltonian parameters were confirmed with computer simulations in all bands. The DFT-calculated g-tensors were consistent with the experimental g-tensors. Furthermore, the shifts Delta(g) from the free electron ge were analyzed in details as the sum of three contributions. The spin-orbit interactions were found to be the dominant factor with regard to the Delta(g). With a focus on the s-o term, thus, the relationship of the g-values and the electronic excited states was explained by visualizing the molecular orbitals of the phenoxyl radical derivatives. This study thus showed the very significant potential of the combination of a multi-frequency ESR approach and a DFT calculation to advanced ESR analysis, particularly, g-tensor analysis, even for a powder-sample radical.

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A computational model to simulate electron spin polarization in the three-spin-1/2 system composed of the molecular excited triplet state of (tetraphenylporphinato)zinc(II) (ZnTPP) and the doublet ground state of the 3-(N-nitronyl-notroxide) pyridine (3-NOPy) stable radical is proposed. The model is based on numerical solutions of the stochastic Liouville equation for the diffusively rotating system where the magnetic dipolar, isotropie Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure, whereas the intersystem crossing processes between the singlet and triplet states of ZnTPP are considered in terms of kinetic equations for the relevant spin density matrices. Additional longitudinal and transversal paramagnetic relaxation caused by relative rotation motions of the ZnTPP and 3-NOPy moieties is taken into consideration in the form of the generalized relaxation operator.

We study the spin state of the π radical assemblies, which are created through converting nonmagnetic terbium (2,3,7,8,12,13,17,18-octaethylporphyrinate) (2,3,7,8,12,13,17,18-octaethylporphyrindiate) (Tb­(Hoep)­(oep)) selectively either into [Tb­(oep)2] or deethyl-[Tb­(oep)2] radical molecule by removing the H atom and further ethyl group, with the injection of tunneling electrons using a scanning tunneling microscope. The Kondo resonance, formed by screening the radical spin by the conduction electron, shows the dip and peak shapes for [Tb­(oep)2] or deethyl-[Tb­(oep)2], respectively, due to the difference in the coupling between the two oep ligands. When these two types of molecules are created next to each other, the Kondo resonance appears as a mixture of the peak and dip at the interface of the two. This is interpreted as the result of the hybridization of the two different π radical orbitals, suggesting a possible control of the Kondo state by designing the molecular orbitals.