how does nmr determine protein structure?

Question

Hello! I need help because I need to analyse this article https://pubmed.ncbi.nlm.nih.gov/2001363/ but the thing is, I have never heard of NMR before and my professor won't reply to my emails and I've been desperately trying to understand this for a week now. I found several articles similar to this one but I really can't understand them so I'm going to explain what I think it is happening even though I'm probably very wrong (I would like to apologise for my ignorance by the way, but I'm really trying...)

So, I think they used NMR to get information about the protein and basically they determine the atoms of the protein corresponding to each spectral peak in different NMR spectra and so we get a lot of information, like NOEs and the coupling constant J. With this information, they were able to calculate/simulate the structure. Those figures that we can find in the article are the result of statistical analysis of that data obtained by NMR? I know I'm probably wrong but I'm really lost...

Also I think they found the beta strands by looking at the simulated structure but I think the RMS graphs might give be involved in how they concluded that we're dealing with mostly beta strands?

I would appreciate it if someone could explain what I'm clearly not understanding...

Answer 1

Here is a fairly simple version of this complex NMR principle-

NMR spectroscopy is the only method that allows the determination of three-dimensional (3D) structures of proteins molecules in the solution phase and for proteins in the size range between 5 and 25 kDa.

In nature certain atomic nuclei are intrinsically magnetic. However, only a limited number of isotopes display a physical property, called spin.

For example the hydrogen nucleus (¹H), which is a proton. The spinning of a proton generates a magnetic moment. This moment can take either of two orientations, or spin states (called α and β), only when an external magnetic field is applied. The energy difference between these states is proportional to the strength of the imposed magnetic field. The α state has a slightly lower energy and hence is slightly more populated. But the spinning proton in an α state can also be raised to an excited state (β state) by applying a pulse of electromagnetic radiation (a radio-frequency). In these circumstances, the spin will change from α to β (aka magnetic resonance). For a sample, first it is transiently magnetized which causes this spin effect. This magnetization is transferred from an excited nucleus to an unexcited one if they are less than about 5 Å apart. This effect provides a means of detecting the location of atoms relative to one another in the three-dimensional structure of the protein. The readout is graphical in nature and it is these graphs that help us lead to the type of structure of the protein. Here is a reference- https://www.ncbi.nlm.nih.gov/books/NBK22393/ (Fig 4.5-4.6)