Raj Chaklashiya (Northwestern University and University of California, Santa Barbara, United States)
LinkedIn: @Raj Chaklashiya
Abstract: Dynamic Nuclear Polarization (DNP) continues to transform NMR spectroscopy by enhancing its signal by several orders of magnitude via polarization transfer from unpaired electron spins to nuclear spins, enabling studies of objects consisting of very few spins such as cells. However, maximizing its potential towards subcellular components consisting of even fewer spins would significantly benefit from optimization of signal enhancement towards its theoretical maximum, which is nontrivial to achieve due to the many different factors that go into signal enhancement. DNP Semianalytical Calculations via simple numerical model assumptions and first principles Quantum Mechanical Simulations via time-evolved Hamiltonian propagators are two distinct methods that can be used to predict DNP performance and assess potential sources of missing enhancement via comparison with experimental DNP frequency profiles. This talk is a tutorial that applies these methods to the analysis of DNP from a highly efficient DNP biradical, TEMTriPol-1, which consists of one part Trityl and another part TEMPO. Inclusion of the effects of microwave saturation and electron spin relaxation in semianalytical calculations and use of the QUEST Northwestern computing cluster in Spinevolution quantum mechanical simulations both provide key improvements to these methods that enable closer matching between experiment and simulation. The results point towards the critical need to understand the J-coupling distribution of DNP radicals to fully understand the underlying DNP mechanism and optimize DNP performance.
-
Dear Raj, very interesting study. Could you comment more on the role of the J coupling on the shape of the DNP profile? How would a reduction of J affect the profile? Have you studied other biradicals with different combinations of relaxation times/J couplings?
-
Dear Arianna, thank you, and thanks for the questions!
So regarding J coupling and the profile shape, we found that reducing the J coupling narrows the Trityl part of the DNP profile significantly, removing the “bump” in the DNP profile that shows up on the left side of the profile during experiment. It seems that reducing J has the net effect of making the overall DNP profile narrower, leading to clear discrepancies with the broader experimental DNP profile.
This is the first established biradical that I have done this kind of study on, but I have done simulation studies in the past of multiradicals and coupled monoradicals with varying dipolar, J, and/or relaxation times, and the results can get quite interesting. I’m happy to go more in-depth on this if you’d like! Here are a couple papers where we discuss those cases in detail:
Multi Electron Spin Cluster Enabled Dynamic Nuclear Polarization with Sulfonated BDPA –in our simulation section we see the impact of J coupling and relaxation times on coupled BDPA and find that there is a combination that matches experimental trends. J coupling matching the nuclear larmor frequency combined with a strong differential in t1e’s can result in an absorptive central feature in the DNP profile lineshape: https://pubs.acs.org/doi/full/10.1021/acs.jpclett.3c02428
Dynamic Nuclear Polarization Using Electron Spin Cluster – this paper has detailed simulations on Trityl-based multiradicals and the conditions in which dipolar couplings are strong enough to result in strong DNP enhancements, as well as the impact of relaxation time differentials on the DNP profile: https://pubs.acs.org/doi/full/10.1021/acs.jpclett.4c00182
-
-
Dear Raj, very nice talk! I would like to know that in second method, when you are observing for static case, the dipolar interaction is also present there (which is primarily averaged out in case of MAS), how the role of dipolar interaction and J-coupling can be separately understood?
Thank you.-
Hi Kuntal,
Thank you! Good question–so first to clarify, the dipolar coupling definitely plays a role in both the MAS and Static simulations–even though there is averaging of dipolar orientations that would make seeing their effects in the NMR spectra harder, its effects can still be seen in the DNP profile itself, because in both cases the dipolar coupling strength directly determines how coupled the two electron spins are–if they are too weakly coupled, the Cross Effect DNP cannot occur, while if they have strong enough coupling it can occur. So in this sense, it needs to be understood for both static and MAS.
As to your question–in these simulations I assume a dipolar coupling constant at 12.5 MHz because I don’t expect the distance between the Trityl electron and the TEMPO electron of TEMTriPol-1 to change. However, J-Coupling is a different story, as it is already known based on liquid state EPR experiments that there is a broad distribution of J couplings.
However, if we assume both the dipolar and J couplings can change, what happens, and how do the effects differentiate from one another? I think in this case, the key difference lies in how they impact the EPR spin populations:
– Dipolar coupling acts as a means to broaden electron spin populations. Stronger dipolar coupling results in broader EPR lines, while weaker dipolar coupling results in narrower EPR lines
– J coupling acts as a means of splitting electron spin populations. Stronger +J coupling results in a wider split in the EPR line, while weaker +J coupling results in a narrower split in the EPR line. And crucially, negative J coupling when strong vs weak can flip this dependence (which results in the better fit with -J as opposed to +J)From this, one can see how dipolar coupling alone would result in different a different input EPR line, and therefore a different DNP profile–while it can broaden the already existing electron spin populations, it cannot necessarily create new populations further away. This means it would be harder for a small “bump” to suddenly appear due to dipolar broadening as opposed to J couplings–that bump however could easily be created if a population were shifted due to J couplings.
The beauty of these simulations, however, is that it can be easy to test these hypotheses! We could input a range of dipolar coupling values and compare that with what happens if we input a range of J coupling values, and vice-versa, all while keeping the other coupling fixed.
Let me know if you have any more questions!
-
Ok, that clears the query, thank you!
-
No problem!
-
-
-
Leave a Reply