Applied Magnetic Resonance

Cerebral metabolic rate of oxygen (CMRO2) is a robust marker of brain health. It represents the amount of oxygen consumed by the brain, and it has been proved to be more sensitive indicator than oxygenation level and cerebral blood flow alone. Quantitative assessment of CMRO2 provides a useful insight into the viability of the brain tissue, the progression of a brain disease or action of a treatment. Therefore, there is a growing interest in developing methods that can quantify CMRO2, despite its complexity. Over the past years, many magnetic resonance imaging (MRI)-based methods and near-infrared spectroscopy (NIRS)-based methods have been developed for CMRO2 quantification. Here, we review the available approaches based on these two techniques, their advantages, and disadvantages. Examples of application of these approaches in animal models, neonates and adults under normal and different physiological conditions are provided. Physiological correlates such as cerebral blood flow, venous oxygen saturation and oxygen extraction fraction in addition to CMRO2 values found in the literature, are presented as well. We also show how the benefits of these two techniques can be combined to create a multimodal NIRS-MRI technique that can provide novel data, allowing better understanding of CMRO2 and oxidative metabolism in the brain.

In this minireview, modern multifrequency electron paramagnetic resonance (EPR) spectroscopy, in particular, at high magnetic fields, is shown to provide detailed information about structure, motional dynamics and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin–quinone donor–acceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $$ {\text{P}}_{865}^{ \cdot + } $$ $$ {\text{Q}}_{\text{A}}^{ \cdot - } $$ in frozen solution reaction centers from the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P865QA. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the QA binding site does not change under photoreduction of the quinone—in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of “classical” spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.

Bảy công thức dạng vesicular CER(−), CER(+), CER(I), PC(−), PC(+), DAG(B) và DAG(I) đã được phát triển sử dụng ceramide IIIB (CER), phosphatidylcholine (PC), hoặc diacylglycerides (DAG) làm thành phần lipid chính. B và I chỉ định chất hoạt động mặt phân cực hydrophilic Brij 58 và Imwitor 375, (+) và (−) biểu thị tiềm năng zeta dương và âm. Ảnh hưởng của độ nhớt vi mạch của lớp màng lipid đến đặc điểm của các vesicle như hệ thống giao thuốc qua da đã được thảo luận rộng rãi, nhưng không tìm thấy kết quả tương đương và có thể tái lặp. Để đo độ nhớt vi mạch bằng cộng hưởng spin điện tử, chất dò spin lipophilic 14-doxylpalmitoylic acid methyl ester (DPME) đã được đưa vào lớp mạch đôi. Thời gian tương quan quay (τc) từ 0,3 đến 1,8 ns đã được tính toán. Độ nhớt động (η) của tám hỗn hợp triglycerides chuỗi trung bình khác nhau với dầu ricin trong khoảng từ 25 đến 948 mPa · s, cũng như các giá trị τc tương ứng của DPME trong các hỗn hợp này, đã được xác định để thiết lập một đường chuẩn cho việc ước lượng độ nhớt vi mạch. Độ nhớt vi của các màng vesicle tăng từ 40 đến 565 mPa · s theo thứ tự sau: DAG(B) = DAG(I) < PC(−) ≈ CER(I) < CER(−) ≤ CER(+) < PC(+). Độ nhớt vi là 49,8 ± 2,5 mPa · s trong các màng PC đậu nành chưa bão hòa tinh khiết giảm xuống còn 28,3–29,4 mPa · s khi thêm natri cholate, trong khi việc thêm dilauroylphosphatidylcholine trong các chu kỳ đóng băng–rã đông lặp lại hoặc 8,5% ethanol vào pha hydrophilic không có ảnh hưởng gì.

Intramolecular electron spin exchange has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two long-chain flexible nitroxide biradicals existing in fluid solutions in three spectroscopy-different spatial conformations as a function of temperature, solvent viscosity and polarity. Certain thermodynamic parameters of the conformational transitions were calculated from the EPR spectra. The process of spin-exchange in these biradicals dissolved in five different alcohols was compared with that in the non-polar solvents (toluene) and aprotic (acetonitrile), as well as with two other biradicals studied earlier, and with thermodynamic characteristics of the solvents. A distinct correlation was found between macroscopic (solvent viscosity) characteristics of solvents and thermodynamic parameters of the intramolecular conformational transitions.

Single quantum heteronuclear cross-polarization in solids is strongly sensitive to resonance offsets. In the presence of main field- or radio-frequency field gradients, the cross-polarization efficiency, therefore, shows a strong spatial dependence, which represents a new principle for localized NMR in solids. Since slices-selective excitation is achieved simultaneously to cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic with solid. The dependence of the localization efficiency on experimental and sample parameters is analyzed theoretically for a spin-1/2 system in the presence of a static or a radio-frequency magnetic field gradient. The resulting slice profiles and the calculated dependence of the slice thickness on the parameters of the basic cross-polarization procedures are discussed and confirmed experimentally on the example of1H-3C spin systems.

Recently, we introduced the pulsed triple electron resonance (TRIER) experiment, which correlates dipolar frequencies of molecules with three electron spins. These correlation patterns contain valuable information: in combination with double electron–electron resonance (DEER) they allow to interpret distance distributions of biological systems that exist in more than one conformation. Together with an improved sequence and new data processing, we were now for the first time able to obtain two-dimensional distance correlation maps of the previously investigated model compounds as well as of spin-labeled proteins. For this we applied two-dimensional approximate Pake transformation to TRIER data. This enabled us to get distance correlation plots from two triple-labeled protein samples that were in good agreement with DEER data and simulations. With such information it should then be possible to assign peaks in DEER distance distributions for macromolecules that can exist in more than one conformation.

Oligomers of poly(thiophene) and poly(ortho-phenyleneethynylene) having pendant S = 1/2 semiquinone radicals (as complexes of cobalt(III)) have been prepared and characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic measurements (superconducting quantum interferences device = SQUID magnetometry). Our results show that exchange coupling of semiquinone groups along a polythiophene backbone is greater than the corresponding coupling along a poly(ortho-phenyleneethynylene) backbone.