Nuclear magnetic resonance scanners are now extremely sensitive. A quantum sensor developed by a team headed by Professor Jörg Wrachtrup at the University of Stuttgart and researchers at the Max Planck Institute for Solid State Research in Stuttgart, now makes it possible to use nuclear magnetic resonance scanning to even investigate the structure of individual proteins atom by atom. In the future, the method could help to diagnose diseases at an early stage by detecting the first defective proteins.
Green laser light transmitted via an optical fibre excites nitrogen atoms in a diamond, causing it to fluoresce with a red light. The brightness of a nitrogen atom at the edge of the diamond lattice allows conclusions to be drawn about the magnetic signals from a sample on the surface of the sensor.
Many diseases have their origins in defective proteins. As proteins are important biochemical motors, defects can lead to disturbances in metabolism. Defective prions, which cause brain damage in BSE and Creutzfeldt- Jakob disease, are one example. Pathologically changed prions have defects in their complex molecular structure.
The problem: individual defective proteins can likewise induce defects in neighbouring intact proteins via a sort of domino effect and thus trigger a disease. It would therefore be very useful if doctors could detect the first, still individual prions with the wrong structure. It has, however, not been possible to date to elucidate the structure of one individual biomolecule.
In an article published in Science, a team of researchers from Stuttgart has now presented a method that can be used in the future for the reliable investigation of individual biomolecules. This is important not only for fighting diseases, but also for chemical and biochemical basic research.
The method involves the miniaturisation as it were of the nuclear magnetic resonance tomography (NMR) known from medical engineering, which is usually called MRI scanning in the medical field. NMR makes use of a special property of the atoms - their spin.
In simple terms, spin can be thought of as the rotation of atomic nuclei and electrons about their own axis, turning the particles into tiny, spinning bar magnets. How these magnets behave is characteristic for each type of atom and each chemical element. Each particle thus oscillates with a specific frequency.
In medical applications, it is normal for only one type of atom to be detected in the body – hydrogen, for example. The hydrogen content in the different tissues allows the interior of the body to be distinguished with the aid of various contrasts.
The spin frequency of the magnetic moment of an atom which has just been measured is transferred to the magnetic moment in the NV centre, which can be seen with a special optical microscope as a change in colour.
The quantum sensor achieves such high sensitivity, as it can store frequency signals of an atom. One single measurement of the frequency of an atom would be too weak for the quantum sensor and possibly too noisy.
The memory allows the sensor to store many frequency signals over a longer period of time, however, and thus tune itself very precisely to the oscillation frequency of an atom – in the same way as a high-quality short-wave receiver can clearly resolve radio channels which are very close to each other.
This technology has other advantages apart from its high resolution: it operates at room temperature and, unlike other high-sensitivity NMR methods used in biochemical research, it does not require a vacuum. Moreover, these other methods generally operate close to absolute zero - minus 273.16ºC - necessitating complex cooling with helium.
Jörg Wrachtrup sees not one but several future fields of application for his high-resolution quantum sensors. “It is conceivable that, in future, it will be possible to detect individual proteins that have undergone a noticeable change in the early stage of a disease and which have so far been overlooked.”
Furthermore, Wrachtrup is collaborating with an industrial company on a slightly larger quantum sensor which could be used in the future to detect the weak magnetic fields of the brain. “We call this sensor the brain reader.
We hope it will help us to decipher how the brain works – and it would be a good complement to the conventional electrical devices derived from the EEG” – the electroencephalogram.
For the brain reader, Wrachtrup is already working with his industrial partner on a holder and a casing so that the device is easy to wear and to operate on a day-to-day basis. To reach this point, however, it will take at least another ten years of research.