Q.3. (b) Explain spin labeling & Hyperfine splitting in ESR spectrometry and its application in study of macromolecules
Ans.3.(b) Spin labeling: Site-directed spin labeling (SDSL) is a technique for investigating the structure and local dynamics of proteins using electron spin resonance. The theory of SDSL is based on the specific reaction of spin labels with amino acids. A spin label's built-in protein structure can be detected by EPR spectroscopy. SDSL is also a useful tool in examinations of the protein folding process. A spin label (SL) is an organic molecule which possesses an unpaired electron, usually on a nitrogen atom, and the ability to bind to another molecule. Spin labels are normally used as tools for probing proteins or biological membrane-local dynamics using EPR spectroscopy. The site-directed spin labeling (SDSL) technique allows one to monitor a specific region within a protein. In protein structure examinations, amino acid-specific SLs can be used. The goal of spin labeling is somewhat similar to that of isotopic substitution in NMR spectroscopy. There one replaces an atom lacking a nuclear spin (and so is NMR-silent) with an isotope having a spin I≠0 (and so is NMR-active).
Application: This technique is useful for tracking the chemical environment around an atom when full substitution with an NMR-active isotope is not feasible. Recently, spin-labelling has also been used to probe chemical local environment in NMR itself, in a technique known as Paramagnetic Relaxation Enhancement (PRE).
Hyperfine splitting: There is no phenomenon which parallels the NMR chemical shift in ESR studies. However, the ESR spectrum exhibits hyperfine splitting which is caused by the interactions between the spinning electrons and adjacent spinning magnetic nuclei. When a single electron interacts with one nucleus, the no. of splittings will be equal to 2l+1, where l is the spin quantum no. of nucleus. In general, if a single electron interacts magnetically with an equivalent nucleus, the electron signal is split up into a 2nl+1 multiplet. For e.g. Hydrogen atom having one proton and one electron (I= ½ for the proton). In the absence of a magnetic field, the single electron of spin (s = 1/2) gives rise to a doubly degenerate spin energy state. When a magnetic field is applied, the degeneracy is removed and two energy levels, one corresponds to ms= -1/2 aligned with the field and the other corresponding to the ms= +1/2 aligned opposing the field, will be obtained.
Applications:
Astrophysics
As the hyperfine splitting is very small, the transition frequencies usually are not optical, but in the range of radio- or microwave frequencies. Hyperfine structure gives the 21 cm line observed in H I regions in interstellar medium. Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record. In radio astronomy, heterodyne receivers are widely used in detection of the electromagnetic signals from celestial objects. The separations among various components of a hyperfine structure are usually small enough to fit into the receiver's IF band. Because optical depth varies with frequency, strength ratios among the hyperfine components differ from that of their intrinsic intensities. From this we can derive the object's physical parameters.
Nuclear technology
The AVLIS process uses the hyperfine splitting between optical transitions in uranium-235 and uranium-238 to selectively photo-ionize only the uranium-235 atoms and then separate the ionized particles from the non-ionized ones. Precisely tuned dye lasers are used as the sources of the necessary exact wavelength radiation.
Use in defining the SI second and meter The hyperfine structure transition can be used to make a microwave notch filter with very high stability, repeatability and Q factor, which can thus be used as a basis for very precise atomic clocks. Typically, the hyperfine structure transition frequency of a particular isotope of caesium or rubidium atoms is used as a basis for these clocks. Due to the accuracy of hyperfine structure transition-based atomic clocks, they are now used as the basis for the definition of the second. One second is now defined to be exactly 9,192,631,770 cycles of the hyperfine structure transition frequency of caesium-133 atoms. Since 1983, the meter is defined by declaring the speed of light in a vacuum to be exactly 299,792,458 metres per second.
Precision tests of quantum electrodynamics
The hyperfine splitting in hydrogen and in muonium have been used to measure the value of the fine structure constant α. Comparison with measurements of α in other physical systems provides a stringent test of QED.
Qubit in ion-trap quantum computing
The hyperfine states of a trapped ion are commonly used for storing qubits in ion-trap quantum computing. They have the advantage of having very long lifetimes, experimentally exceeding ~10 min (compared to ~1 s for metastable electronic levels). The frequency associated with the states' energy separation is in the microwave region, making it possible to drive hyperfine transitions using microwave radiation. However, at present no emitter is available that can be focused to address a particular ion from a sequence. Instead, a pair of laser pulses can be used to drive the transition, by having their frequency difference (detuning) equal to the required transition's frequency. This is essentially a stimulated Raman transition.
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