Reinventing MASER: Parahydrogen-Induced Radio Amplification by Stimulated Emission of Radiation

Significance 

Nuclear Magnetic Resonance is a fascinating quantum‐mechanical effect that is used routinely in medical diagnostics and chemical analysis alike. The success of NMR can be attributed to numerous advancements over the years; including developing hyperpolarized contrast agents that enable real‐time imaging of metabolism in vivo. Recently, an intriguing publication elaborated on the first experimental observation of radio amplification by stimulated emission of radiation (RASER) of protons. RASER was recently discovered in a low-field NMR spectrometer incorporating a highly specialized radiofrequency resonator, where a high degree of proton-spin polarization was achieved by reversible parahydrogen exchange. Unlike lasers and masers, which employ self-organizing systems emitting coherent optical light and microwaves, RASER is induced by continuous coherent oscillation of radio waves at much lower frequencies through the coupling between nuclear-spin magnetization and an LC resonance circuit. RASER-based NMR spectroscopy is difficult to achieve using thermal nuclear-spin polarization due to the required magnetization. Hyperpolarization techniques allow for enhancement of nuclear-spin polarization by several orders of magnitude up to unity. The technique of signal amplification by reversible exchange (SABRE) can be used to provide a highly magnetized sample in the pioneering RASER demonstration.

In general, RASER activity, which results from the coherent coupling between the nuclear spins and the inductive detector, can overcome the limits of frequency resolution in NMR. To this end, scientists from the Wayne State University: Dr. Baptiste Joalland, Dr. Nuwandi Ariyasingha and Professor Eduard Y. Chekmenev in collaboration with Dr. Soeren Lehmkuhl and Professor Thomas Theis at the North Carolina State University and Professor Stephan Appelt at the RWTH Aachen University in Germany provided evidence showing that a commercial high-field NMR system with standard inductive detection could readily detect RASER when combined with the parahydrogen-induced polarization (PHIP) technique. Their work is currently published in the research journal, Angewandte Chemie International Edition.

In their approach, a commercial bench-top 1.4T NMR spectrometer in conjunction with pairwise parahydrogen addition producing proton hyperpolarized molecules in the Earth’s magnetic field (ALTADENA condition) or in a high magnetic field (PASADENA condition) was used to induce RASER without any radio-frequency excitation pulses. In other words, two main experimental protocols were followed: the ALTADENA condition and the PASADENA condition.

The authors reported that the RASER activity was observed with and without applications of RF-excitation pulses and under both ALTADENA and PASADENA conditions. Additionally, they established that J-coupling constants as well as chemical-shift differences could be measured with increased precision.

In summary, the new study reported on the RASER activity of two PHIP-hyperpolarized compounds using standard NMR hardware at concentrations as low as 40 mM and at estimated proton-polarization values of over 10% at the time of the detection. Their results demonstrated that RASER activity can be observed on virtually any NMR spectrometer and measure most of the important NMR parameters with high precision. In a statement to Advances in Engineering, Professor Eduard Y. Chekmenev, the lead author mentioned that the parahydrogen-induced RASER phenomenon presented could enable new applications in magnetic resonance (including medical diagnostics), quantum computing, data encryption-among other applications.

About the author

Prof. Eduard Y. Chekmenev, PhD in Physical Chemistry (supervisor Prof. Richard J. Wittebort) 2003, University of Louisville, KY, USA. Postdoctoral Fellow at NHMFL in Tallahassee, FL (Prof. Timothy Cross), Caltech (Prof. Daniel P. Weitekamp) and HMRI (Dr. Brian D. Ross). In 2009, Dr. Chekmenev started his hyperpolarization program at Vanderbilt University, and he was tenured in 2015. In 2018, he moved to Wayne State University (Detroit, MI) to continue his research on MR hyperpolarization. Research interests include development of methods of hyperpolarization and their Biomedical and industrial use.

References

Baptiste Joalland, Nuwandi M. Ariyasingha, Sçren Lehmkuhl, Thomas Theis, Stephan Appelt, Eduard Y. Chekmenev. Parahydrogen-Induced Radio Amplification by Stimulated Emission of Radiation. Angewandte Chemie International Edition 2020, volume 59; page 8654–8660.

Go To Angewandte Chemie International Edition

Nuwandi M. Ariyasingha Baptiste Joalland Hassan R. Younes Oleg G. Salnikov Nikita V. Chukanov Kirill V. Kovtunov Larisa M. Kovtunova Valerii I. Bukhtiyarov Igor V. Koptyug Juri G. Gelovani Eduard Y Chekmenev. Parahydrogen‐Induced Polarization of Diethyl Ether Anesthetic. Chemistry a European Journal. First published: 15 July 2020.

Go To Chemistry a European Journal

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