Why Neutrons Drip Off Nuclei

Home , Valley of stability

News & views begins with a protein sample that has been example, into the binding of a molecule called most macromolecules, the inherent structural applied to a special sample grid. Plunging it histamine in the core of the protein. flexibility and structural heterogeneity will into liquid ethane flash-freezes and traps the The developments in cryo-EM hardware instead probably be the resolution-limiting protein particles in a thin film of amorphous described by Yip, Nakane and their respec factor, regardless of the capabilities of the ice. Two-dimensional images of the individ tive colleagues have driven a major advance instrumentation available. For such less-stable ual particles in the sample grid, obtained by in the resolution of single-particle cryo-EM. specimens, the application of new sam applying a beam of electrons, are averaged Each team used hardware that tackled distinct ple-preparation technologies, together with computationally to yield a 3D structure. The aspects of cryo-EM imaging that had previ improvements in data-collection throughput 2D images are incredibly ‘noisy’ because a ously limited the resolution attainable. With and algorithm advances, will offer fresh ways low dose of electrons must be used to avoid these technologies, the increased signal-to- to probe the conformational landscapes of damaging the radiation-sensitive biolog noise ratio of cryo-EM images will expand the these complexes. Thus, although cryo-EM’s ical sample. As such, these images have technique’s applicability. For example, this resolution revolution might be nearing its historically been unsuitable for determining might include using the technique to deter end, more revolutions await in the years to structures at an atomic level of detail. How mine high-resolution structures of hetero come that will make this technique even more ever, the advances reported since 2013 have geneous samples such as those formed of powerful and applicable to the investigation allowed single-particle cryo-EM data to be col membrane proteins, or macromolecular of diverse biological questions. lected that rival those obtained using X-ray complexes that vary in conformation or crystallography. composition. Perhaps the melding of these Mark A. Herzik Jr is in the Department of The resolution revolution of cryo-EM has technologies will enable the determination Chemistry and Biochemistry, University continued to advance6. Yip et al. and Nakane of cryo-EM structures at a resolution beyond of California, San Diego, La Jolla, et al. harnessed technological improve even 1 Å. This once might have seemed a near California 92093, USA. ments to determine the structures of a stable impossible quest to embark upon. e-mail: [email protected] iron-storing protein called ferritin (termed However, these technologies represent the apoferritin in the absence of metals) to a elite echelon of cryo-EM instrumentation and resolution of approximately 1.2 ångströms. are currently out of reach for most institutes These structures are the highest-resolution because of the cost of purchase and operation. single-particle cryo-EM reconstructions so far Moving forward, these types of advance will determined, and the data are of sufficiently help us learn more about what is limiting the 1. Yip, K. M., Fischer, N., Paknia, E., Chari, A. & Stark, H. high quality to resolve the individual atoms attainable resolution and might therefore Nature 587, 157–161 (2020). in apoferritin (Fig. 1). This unprecedented feat enable the design of better instrumentation. 2. Nakane, T. et al. Nature 587, 152–156 (2020). would not have been thought feasible merely Although such high-resolution structures are 3. Zhang, K., Pintilie, G. D., Li, S., Schmid, M. F. & Chiu, W. Preprint at bioRxiv https://doi. a decade ago. not necessary to answer every biological ques org/10.1101/2020.08.19.256909 (2020). Yip and colleagues’ success relied on hard tion, the extra detail such hardware can pro 4. Kato, T. et al. Microsc. Microanal. 25, 998–999 (2019). ware advances, including components such as vide would limit inaccuracies in 3D structures 5. Cheng, Y. Science 361, 876–880 (2018). 6. Kühlbrandt, K. Science 343, 1443–1444 (2014). a spherical-aberration corrector plus a mono and provide a better platform for understand chromator device that applies a series of filters ing biological functions. Nevertheless, for This article was published online on 21 October 2020. to ensure that only electrons with a narrow spread of energies interact with the specimen, Nuclear physics thereby enhancing the resolution of the final image. Nakane and co-workers applied a dif ferent technology, a cold field-emission gun that also generates electrons with a narrow Why neutrons energy spread, together with a technology that reduces noise in each image by filtering out drip off nuclei those electrons that interact non-productively with the specimen. Moreover, Nakane et al. Calvin W. Johnson captured data with a next-generation, highly sensitive electron-detecting camera. The neutron drip line refers to the maximum number of In addition to analysing apoferritin, neutrons that can be packed into the atomic nuclei of each Nakane and colleagues obtained a structure chemical element. A mechanism has been proposed that could at 1.7 Å resolution of a form of the receptor for explain the long-debated origin of this drip line. See p.66 γ-aminobutyric acid type-A (GABAA) that was engineered to be more stable than the com mon form found in humans. This receptor is a protein complex that resides in the cell mem Whereas some people play extreme sports, that the mechanism responsible for the drip brane of neurons and is a target for numerous many nuclear physicists seek the thrill of line is more subtle than previously understood therapeutics. Obtaining such a high resolution extreme isotopes, by finding, for each chem and is related to deformation, a hallmark of by single-particle cryo-EM had been deemed ical element, the largest possible number of much of ordinary nuclear physics. near impossible for a biological specimen neutrons that can be held by an atom. This The strong nuclear force that binds protons such as this, one that exhibits a high level of boundary of nuclear existence, called the and neutrons together favours equal num flexibility in terms of its structural mobility neutron drip line, has not been fully mapped bers of each particle. By contrast, weaker but compared with structurally rigid molecules — although the construction of rare-isotope longer-range electrostatic repulsion discour such as apoferritin. The structure reveals facilities1 will bring the goal closer. Moreover, ages the accumulation of protons in atoms. details of the GABAA receptor that have never even the theoretical location of the drip line is Competition between these two forces pro been seen before, providing insights, for uncertain2,3. On page 66, Tsunoda et al.4 argue duces the valley of stability — the V-shaped

magic’. But, moving away from stability, as the c Drip line balance between protons and neutrons shifts, previous magic numbers can be replaced d with new ones5, for example at 16 neutrons. b Both theory4,5 and experiment6 show that, as neon and magnesium isotopes collect more than 16 neutrons, the energies of the low a Neutron Unbound est-energy states that have 2 and 4 units of

Binding energy neutron angular momentum become markedly lower (see Figure 3 of the paper4), which is a typical

Nucleus sign of increased deformation. Tsunoda and colleagues’ proposed mech Neutron number anism draws on concepts familiar to nuclear physicists — in particular, the competition Figure 1 | A mechanism for the neutron drip line. The maximum number of neutrons that can be added between deformation and mean shell struc to an atomic nucleus corresponds to a boundary called the neutron drip line. Tsunoda et al.4 suggest that ture. But some questions remain. For exam the mechanism responsible for the drip line is linked to nuclear deformation. a, They consider a spherical ple, although the authors’ calculations were nucleus that has a particular binding energy — the difference between the total energy of the nucleus and the highly detailed, requiring supercomputers, energy of its components. b, If neutrons are added, the nucleus deforms to the shape of an ellipsoid, and the they largely ignored the unbound (continuum) binding energy rises. c, If more neutrons are packed in, the nucleus becomes even more deformed, and the single-particle states that have an essential role binding energy increases further. d, As the drip line is approached, the nucleus becomes less deformed. If in defining the drip line for lighter nuclei than even more neutrons are added, the binding energy falls, and the neutrons do not bind to the nucleus. those considered here. Moreover, although the authors drew on ab initio interactions surface that corresponds to stable nuclei small. With ‘magic’ numbers of protons or between protons and neutrons, they made when the energy per nucleus is plotted as a neutrons, akin to the filled electron shells that empirical tweaks to the single-particle ener function of the number of protons and neu drive the chemical inertness of noble gases, gies, which are part of the potential energies trons. The bottom of this valley is associated this energy gap is large, and deformation is of the shells. Such ‘by-hand’ adjustments leave with the most stable isotopes, which have just suppressed. the robustness of the proposed mechanism the right mix of protons and neutrons. Add Aware of this picture, Tsunoda et al. calcu unclear. neutrons to these isotopes, and you move up lated the various contributions to the binding But the biggest question concerns the drip- the valley walls. energy, such as the mean (monopole) energy line mechanism for even heavier elements than However, neutrons cannot be added generated from adding a neutron to a shell, those considered here. This mechanism could forever. It takes energy to pull atomic nuclei the energy from deformation and the energy be driven either by the mean shell structure apart, because their total energy is less than from protons and neutrons pairing up. They and leaking of neutrons into the continuum, that of their components. This deficit is or by the competition between deformation called the binding energy (shown as negative “The mechanism responsible and the mean potential-energy profiles that energies in Figure 4 of the paper4). If adding define magic numbers — or some combination a neutron increases the binding energy, the for the drip line is more of the two. For these heavier elements, the drip neutron sticks. Otherwise, the energetically subtle than previously line is associated with rapid neutron-capture disfavoured neutron ‘drips off’. Note that there understood.” nucleosynthesis7 — a process that forges many is also a proton drip line, driven by increasing heavy elements, such as iodine, gold and the electrostatic repulsion. rare-earth metals. To fully understand this Nuclear physicists have long assumed that discovered that, as neutrons are added to process, we have to go to nuclear extremes. the neutron drip line is governed by the pref nuclei of elements ranging from fluorine to erence of the strong nuclear force for proton– magnesium, the monopole contribution rises Calvin W. Johnson is in the Department of neutron symmetry. In the nuclear-shell model, steadily. At the same time, the nuclei initially Physics, San Diego State University, San Diego, protons and neutrons occupy quantum shells, become increasingly deformed, magnifying California 92182, USA. much like electrons in an atom, and each shell the rise in the binding energy. But, as even e-mail: [email protected] has a particular potential energy. The think more neutrons are added, the nuclei become ing was that if the proton–neutron symmetry difficult to deform, and the deformation decreases, the potential energy also falls, to contribution falls more quickly than does the point at which adding a neutron lowers the the increase in the monopole contribution. binding energy. When that happens, the binding energy falls, But protons and neutrons do not stay in giving rise to the drip line (Fig. 1). Intrigu a single shell. Instead, driven by the strong ingly, although the pairing contribution is nuclear force, they jump from shell to shell, non-negligible, it is approximately constant forming different configurations (see Figure 1 and so does not drive the drip line. 4 1. Geesaman, D. F., Gelbke, C. K., Janssens, R. V. F. & of the paper ). Like a flock of birds wheeling in The rise and fall of deformation for these Sherrill, B. M. Annu. Rev. Nucl. Part. Sci. 56, 53–92 (2006). the sky, protons and neutrons move collec neutron-rich nuclei touches on another 2. Afanasjev, A. V., Agbemava, S. E., Ray, D. & Ring, P. tively. For example, they can pair up with dance topic: magic numbers far from nuclear sta Phys. Rev. C 91, 014324 (2015). 3. Neufcourt, N., Cao, Y., Nazarewicz, W., Olsen, E. & partners, such as electrons in a superconduc bility. At the bottom of the valley of stability, Viens, F. Phys. Rev. Lett. 122, 062502 (2019). tor, and they can produce deformed (ellipsoid) 20 is a magic number. For example, calcium 4. Tsunoda, N. et al. Nature 587, 66–71 (2020). nuclei that rotate, throwing off γ-rays. It turns (with 20 protons) has many stable isotopes 5. Sorlin, O. & Porquet, M.-G. Prog. Part. Nucl. Phys. 61, 602–673 (2008). out that nuclei are easily deformed — espe that are difficult to deform, and calcium-40 6. Church, J. A. et al. Phys. Rev. C 72, 054320 (2005). cially when the energy gap between shells is (with 20 protons and 20 neutrons) is ‘doubly 7. Kajino, T. et al. Prog. Part. Nucl. Phys. 107, 109–166 (2019).