Abstract
Abstract. Sensitivity being one of the main hurdles of nuclear magnetic resonance (NMR) can be gained by polarization techniques including chemically induced dynamic nuclear polarization (CIDNP). Kaptein demonstrated that the basic mechanism of the CIDNP arises from spin sorting based on coherent electron–electron nuclear spin dynamics during the formation and the recombination of a radical pair in a magnetic field. In photo-CIDNP of interest here the radical pair is between a dye and the molecule to be polarized. Here, we explore continuous-wave (CW) photo-CIDNP (denoted CW-photo-CIDNP) with a set of 10 tryptophan and tyrosine analogues, many of them newly identified to be photo-CIDNP active, and we observe not only signal enhancement of 2 orders of magnitude for 1H at 600 MHz (corresponding to 10 000 times in measurement time) but also reveal that polarization enhancement correlates with the hydrophobicity of the molecules. Furthermore, the small chemical library established indicates the existence of many photo-CIDNP-active molecules.
Highlights
Despite decades of development and impressive technological improvements, sensitivity remains the main hurdle of nuclear magnetic resonance (NMR) spectroscopy and imaging (Ardenkjaer-Larsen et al, 2015)
The tryptophan presented a higher signalto-noise enhancement (SNE) when polarized upon fluorescein irradiation when compared with the dye Atto Thio 12 (AT12) as shown in Fig. 1 and listed in Table 1, whereas tyrosine was better polarized in the presence of AT12 (Fig. 1) (Sobol et al, 2019)
In the case of the photoCIDNP reaction that is performed for all the experiments of this work, μ is positive since the radical pair is formed in a triplet state
Summary
Despite decades of development and impressive technological improvements, sensitivity remains the main hurdle of nuclear magnetic resonance (NMR) spectroscopy and imaging (Ardenkjaer-Larsen et al, 2015). The radical pair mechanism was proposed by Kaptein and Oosterhoff (1969) and by Closs (1969) 2 years after and remains the cornerstone of the CIDNP theory ever since. The interplay of nuclear-spin-dependent electron intersystem crossing into a singlet state, allowing the electron back-transfer, yields different radical pair recombination kinetics depending on the nuclear spin state. CIDNP can be used to study transient radicals that are too short lived for EPR (Closs and Trifunac, 1969; Morozova et al, 2008, 2007, 2005) to study protein structure (Kaptein et al, 1978) and folding (Hore et al, 1997; Mok et al, 2003; Mok and Hore, 2004), or to study the electron-transfer mechanism (Morozova et al, 2018, 2008, 2005, 2003). Robert Kaptein’s key role in the development of the theory underlying the CIDNP mechanism is crystallized in the Kaptein rules which capture the theory of CIDNP into
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