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Reply to “Comment on ‘Chirality-Induced Electron Spin Polarization and Enantiospecific Response in Solid-State Cross-Polarization Nuclear Magnetic Resonance’” Jose I. Santos,† Iván Rivilla,*,‡ Fernando P. Cossío,‡ F. Javier García-García,§ Jon M. Matxain,⊥ Marek Grzeliczak,⊥ Shobeir K. S. Mazinani,∥ Jesus M. Ugalde,*,⊥ and Vladimiro Mujica*,∥ †
SGIker-UPV/EHU, Centro “Joxe Mari Korta”, Tolosa Hiribidea, 72, E-20018 Donostia-San Sebastian, Spain Department of Organic Chemistry I, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Centro de Innovación en Química Avanzada (ORFEO−CINQA), and Donostia International Physics Center (DIPC), Paseo de Manuel Lardizabal 3, 20018 Donostia-San Sebastián Spain § ICTS-Centro Nacional de Microscopía Electrónica, UCM, Av. Complutense S/N, 28040 Madrid, Spain ⊥ Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K.1072, 20080 Donostia, Euskadi, Spain ∥ School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
Downloaded by 5.189.202.149 at 12:25:56:095 on June 03, 2019 from https://pubs.acs.org/doi/10.1021/acsnano.9b00946.
‡
electrochemical reactions,5−8 in field-induced enantiomer separation,9 and in enantiospecific adsorption-induced changes in superconductivity.10 Electron spin polarization, Pz, along a molecular axis z, is induced by a combination of time-inversion symmetry breaking and enhanced spin−orbit interaction in chiral molecules. It is defined as the normalized difference in spin density along the molecular axis associated with any process of electron transfer, electron transport, or bond polarization along a chiral center. That is, the spin polarization is expressed as
n a Comment to our article “Chirality-Induced Electron Spin Polarization and Enantiospecific Response in SolidState Cross-Polarization Nuclear Magnetic Resonance” in ACS Nano.1 Venkatesh et al. rephrase one of our main conclusions: “The authors attributed such differences to transient changes in T1 relaxation times resulting from an interaction between the electron spins and the radio frequency contact pulses used in the CPMAS experiment and discussed this proposed phenomenon in terms of the chirality-induced spin selectivity (CISS) effect.” Then they add: “We disagree with the authors conclusion that the CISS effect plays a role in the different signal intensities observed in the CPMAS solid-state NMR spectra of crystalline enantiomers. Putting aside any fundamental arguments about how radiofrequency electromagnetic fields might interact with electron spins in diamagnetic solids at typical NMR magnetic field strengths, or how slight chirality induced differences in electron spin polarization could affect the intensity of CPMAS NMR signals, the most straightforward explanation for the reported differences in CPMAS signal intensities of enantiomers is that the enantiomers exhibit distinct proton longitudinal relaxation times (T1(1H)). It is a well-known phenomenon in solidstate NMR spectroscopy that the T1 of abundant, high-γ nuclei such as 1H and 19F is highly dependent upon sample purity, crystallinity, and particle size.” Going straight to the heart of the matter: what Venkatesh et al. call “slight chirality induced dif ferences in electron spin polarization” is a very robust and measurable effect that has been described in tens of publications. The importance of the CISS effect has been shown in electron photoemission experiments,2 in STM and AFM direct measurements of conductance,3,4 in variations of the electrode overpotential in
I
© XXXX American Chemical Society
Pz =
N+1/2, z − N −1/2, z N+1/2, z + N −1/2, z
where N+1/2,z and N−1/2,z are the spin populations for each spin component, ±1/2, along the z-direction. Pz can be as large as 60% in some cases, and there is no physical reason a priori to neglect its effect in the context of the NMR CP MAS experiment, particularly because this electron spin polarization occurs in the presence of an external magnetic field that enhances the effect. In our article, we explore the consequences of the CISS effect in the context of CP MAS NMR, and our results are very consistent with what has been found in other research areas. Arguing that chirality-induced differences in electron spin polarization are small without testing them, and without any analysis of the physics involved, is hardly an argument to neglect their importance. Comments on Alternative Explanations. Before going into the detail of the alternative explanations suggested by Venkatesh et al., it is important to realize that the authors of the Comment to our article do not dispute the fact that the observed differences in the CP MAS NMR experiments are Received: February 1, 2019
A
DOI: 10.1021/acsnano.9b00946 ACS Nano XXXX, XXX, XXX−XXX
Letter to the Editor
Cite This: ACS Nano XXXX, XXX, XXX−XXX
ACS Nano
Letter to the Editor
due to changes in T1(1H). The central disagreement is then about the origin of the difference. An interesting point to notice is that Venkatesh et al. chose the most complex case of CP of 1H to 13C, as opposed to the case we considered in detail in our article, namely, CP of 1H to 15N. Taking into account that we have been exploring an entirely new field, and the fact that we do not have a detailed theory yet to analyze the case of multiple nuclei, it would have been more relevant within the current context to analyze the same case we are considering, instead of focusing on a different, and more complex, system. Regarding a comparison of the experimental conditions and how “sample purity, crystallinity and particle size” might influence the experimental results, we agree of course that there are many reports in the literature that indicate this type of dependence and that this is an important factor to be taken into account. Furthermore, we did take it into account in our experiments. Solvent and Recrystallization. As indicated in our article and the Supporting Information, we did not use water as a solvent precisely to prevent impurities. In fact, (see article) we recrystallized our pure L and D samples, such as phenylglycine, in a mixture of solvents (EtOH/H2O/toluene, 1:0.2:0.7, v/v) and heated them to 100 °C to promote the solubility and the subsequent recrystallization of the amino acids. Water is difficult to eliminate in organic samples and introduces impurities. Also, the crystals were filtered and then dried into a rotavap at 70 °C under vacuum. We further dried our sample in a vacuum pump for 48 h at a temperature close to the melting point of the amino acid and lyophilized to further dehydrate the sample. Due to the difficulties in eliminating water and the risk of introducing impurities as Venkatesh et al. point out themselves in the introduction of their comment, it is difficult to understand why they chose to use this solvent at all. Size Dependency. To exclude the effect of particles size on the intensities of L and D signals, we conducted scanning electron microscopy (SEM) analysis of the samples, before and after recrystallization. We focused on the most relevant case, the aspartic acid, for which the largest L/D signal area ratio was found.1 Notice that these samples were further used to carry out the statistical analysis (see below), and taking into account that obtaining a single signal in the 15N CP NMR experiment takes around 20 h, we decided to focus only on the case with the largest L/D signal area ratio. Four different aspartic acid samples were prepared for the NMR experiment. Namely, the recrystallized and unrecrystallized L- and D-aspartic acids. SEM images of these four different samples were obtained (see Figure 1). The prepared samples contain particles of diameters above 1 μm. Importantly, no differences in size is observed for L and D cases, where similar patterns are observed. These sizes are much larger than the nanoparticle radii where size-dependent NMR intensities have been reported in the literature.11 Hence, we conclude that the size dependency can be ruled out as a potential cause of the observed L/D signal differences. Statistical Analysis. Another alternative explanation to the observed intensity differences in the signals of L- and D-amino acids is related to the experimental error and statistical meaning of the obtained results. In ref 1, all of the given data were obtained by single measurements for each sample, and hence, no statistical analysis was carried out. Using the samples described above for L- and D-aspartic acid, we have now carried out five measurements per sample. In Figure 2, the obtained
Figure 1. SEM images obtained for the recrystallized and unrecrystallized L- and D-aspartic acid, showing the same distribution of particle size for the two enantiomers.
spectra are collected for recrystallized and unrecrystallized L and D samples. A quick glance at these spectra clearly shows a difference between L and D signals for both the recrystallized and unrecrystallized samples. As shown, D values are higher than L ones for all measurements. We have integrated the areas for all measured signals, and the calculated values are collected in Table 1, along with the calculated mean values, standard deviations, and variances. Similar to the intensities observed in Figure 2, all signal integrated areas are clearly larger for the D enantiomer, regardless of whether the sample is recrystallized or not. Based on these data, the mean values, standard deviations, and variances have been calculated. Focusing on the data obtained for the recrystallized samples, the calculated average value is 2.917.687,4 ± 209.711,24 for the L enantiomer and 3.972.083,0 ± 261.724,43 for the D enantiomer. Clearly, the up-limit of the L enantiomer is much lower than the low limit of the D enantiomer. Given that there is no overlap between the intensities of the two enantiomers, no further t test analysis is required. This might be needed for other samples, where the differences in intensities could be lower. The calculated values indicate that in both recrystallized and unrecrystallized samples, L and D values are statistically different. Hence, the observed differences in peak intensities and peak areas of L and D enantiomers are not due to statistical issues. Impurities. It is well-documented in the literature that the presence of paramagnetic impurities would have, for all practical purposes, prevented acquiring data for the case CP of 1H to 15N we considered in our work.12 The fact that we were able to run the experiment repeatedly under standard conditions argues by itself against the presence of paramagnetic impurities. In this context, it is important to notice that the contact time used by Venkatesh et al. is 500 μs, compared to 2 ms in our work. This difference might be crucial in realizing that their samples might indeed have been contaminated with paramagnetic impurities because this condition would require the use of very short contact times as is their case. B
DOI: 10.1021/acsnano.9b00946 ACS Nano XXXX, XXX, XXX−XXX
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Letter to the Editor
Figure 2. Obtained spectra for the recrystallized (left) and unrecrystrallized (right) samples, for both L- (red) and D-aspartic acid (black).
Table 1. Measured Signal Areas for Recrystallized and Unrecrystallized L- and D-Aspartic Acid, with Average Values, Standard Deviations, and Variances recrystallized
unrecrystallized
L
measurement measurement measurement measurement measurement
1 2 3 4 5
mean standard deviation variance
D
L
D
3.066.698 3.080.338 2.958.477 2.563.303 2.919.621
3.901.283 4.058.600 4.329.459 3.964.679 3.606.394
2.820.774 2.413.788 2.570.687 2.557.244 2.590.623
4.843.075 4.686.203 4.868.607 3.962.292 4.590.044
2.917.687,4 209.711,24 4.39788 × 1010
3.972.083,0 261.724,43 6.84997 × 1010
2.590.623,2 4.590.044,2 2.14379 × 1010
4.146.416,96 369.100,65 1.36235 × 1011
Milling and Size Dispersity. As mentioned above, we have not detected any size-dependent effect as a function of milling conditions. We are aware that the issue of size dependence of the intensity in CP MAS experiments with wellcharacterized nanosystems is an important one that we are currently addressing. Final Comments and Conclusions. Although we acknowledge that there are still many valid issues in assessing the full potential of an NMR-based enantiospecific technique, we believe Venkatesh et al. are criticizing our work and disregarding the importance of the CISS effect, without a proper scientific evaluation of this essential point. The authors of this Comment seem to attribute all our consistent observations to what amounts to essentially random effects controlling the relaxation time of the abundant nuclei. In doing so, they leave out of their explanation a crucial point, namely, how to reconcile the random variation justification with our very systematic result that the intensity of the D isomer was larger than the intensity of the L isomer in all cases considered. We are aware that our findings challenge some widely accepted ideas about how to use NMR-based techniques to obtain enantiospecific response of pure materials, and we are currently conducting experiments to further validate our theoretical model. In particular, increasing the spinning speed of the chamber to reduce the importance of the dipole−dipole mechanism on the polarization transfer and increase the relative contribution of the indirect J-coupling; see eq 1 of ref 1, which is the one sensitive to the spin polarization effect. These efforts should contribute to a wider acceptance
and appreciation of the deep implications of the CISS effect for CP NMR.
REFERENCES (1) Santos, J. I.; Rivilla, I.; Cossío, F. P.; Matxain, J. M.; Grzelczak, M.; Mazinani, S. K. S.; Ugalde, J. M.; Mujica, V. Chirality-Induced Electron Spin Polarization and Enantiospecific Response in SolidState Cross-Polarization Nuclear Magnetic Resonance. ACS Nano 2018, 12, 11426−11433. (2) Göhler, B.; Hamelbeck, V.; Markus, T. Z.; Kettner, M.; Hanne, G. F.; Vager, Z.; Naaman, R.; Zacharias, H. Spin Selectivity in Electron Transmission Through Self-Assembled Monolayers of Double-Stranded DNA. Science 2011, 331, 894−897. (3) Aragonès, A. C.; Medina, E.; Ferrer-Huerta, M.; Gimeno, N.; Teixidó, M.; Palma, J. L.; Tao, N.; Ugalde, J. M.; Giralt, E.; DíezPérez, I.; Mujica, V. Measuring the Spin-Polarization Power of a Single Chiral Molecule. Small 2017, 13, 1602519−1602524. (4) Koplovitz, G.; Leitus, G.; Ghosh, S.; Bloom, B. P.; Yochelis, S.; Rotem, D.; Vischio, F.; Striccoli, M.; Fanizza, E.; Naaman, R.; Waldeck, D. H.; Porath, D.; Paltiel, Y. Single Domain 10 nm Ferromagnetism Imprinted on Superparamagnetic Nanoparticles Using Chiral Molecules. Small 2019, 15, 1804557−1804563. (5) Mishra, D.; Markus, T. Z.; Naaman, R.; Kettner, M.; Göhler, B.; Zacharias, H.; Friedman, N.; Sheves, M.; Fontanesi, C. SpinDependent Electron Transmission Through Bacteriorhodopsin Embedded in Purple Membrane. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 14872−14876. (6) Mondal, P. C.; Kantor-Uriel, N.; Mathew, S. P.; Tassinari, F.; Fontanesi, C.; Naaman, R. Chiral Conductive Polymers as Spin Filters. Adv. Mater. 2015, 27, 1924−1927. C
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(7) Mondal, P. K.; Fontanesi, C.; Waldeck, D. H.; Naaman, R. SpinDependent Transport through Chiral Molecules Studied by SpinDependent Electrochemistry. Acc. Chem. Res. 2016, 49, 2560−2568. (8) Kumar, A.; Capua, E.; Vankayala, K.; Fontanesi, C.; Naaman, R. Magnetless Device for Conducting Three-Dimensional Spin-Specific Electrochemistry. Angew. Chem., Int. Ed. 2017, 56, 14587−14590. (9) Banerjee-Ghosh, K.; Ben Dor, O.; Tassinari, F.; Capua, E.; Yochelis, S.; Capua, A.; Yang, S. H.; Parkin, S. S. P.; Sarkar, S.; Kronik, L.; Baczewski, L. T.; Naaman, R.; Paltiel, Y. Separation of Enantiomers by their Enantiospecific Interaction with Achiral Magnetic Substrates. Science 2018, 360, 1331−1334. (10) Alpern, H.; Katzir, E.; Yochelis, S.; Katz, N.; Paltiel, Y.; Millo, O. Unconventional Superconductivity Induced in Nb Films by Adsorbed Chiral Molecules. New J. Phys. 2016, 18, 113048−113055. (11) Dempah, K. E.; Lubach, J. W.; Munson, E. J. Characterization of the Particle Size and Polydispersity of Dicumarol Using Solid-State NMR Spectroscopy. Mol. Pharmaceutics 2017, 14, 856−865. (12) Bertmer, M. Paramagnetic Solid State Materials. Solid State Nucl. Magn. Reson. 2017, 81, 1−7.
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