Insensitive nuclei enhanced by polarization transfer

Insensitive nuclei enhanced by polarization transfer (INEPT) is a signal resolution enhancement method used in NMR spectroscopy which involves the transfer of nuclear spin polarization from spins with large Boltzmann population differences to nuclear spins of interest with low Boltzmann population differences.^[1] INEPT uses J-coupling for the polarization transfer in contrast to Nuclear Overhauser Effect (NOE) which arises from dipolar cross-relaxation.

Background

The sensitivity of NMR signal detection depends on the gyromagnetic ratio (γ) of the nucleus. In general, the signal intensity produced from a nucleus with a gyromagnetic ratio of γ is proportional to γ³ because the magnetic moment, the Boltzmann populations, and the nuclear precession all increase in proportion to the gyromagnetic ratio γ. For example, the gyromagnetic ratio of ¹³C is 4 times lower than the proton, so the signal intensity it produced will be 64 times lower than that of a proton. However, noise also increases as the square root of the frequency, the sensitivity therefore becomes roughly proportional to γ^5/2.^[2] A ¹³C nucleus would be 32 times less sensitive than a proton, and ¹⁵N around 300 times less sensitive. Sensitivity enhancement techniques are therefore desirable when recording an NMR signal from an insensitive nucleus.

The sensitivity can be enhanced artificially by increasing the Boltzmann factors. One method may be through nuclear Overhauser effect (NOE), for example, for ¹³C signal, the signal-to-noise ratio can be improved three-fold when the attached protons are saturated. However, for NOE, a negative value of K, the ratio of gyromagnetic ratios of the nuclei, may result in a reduction in signal intensity. For ¹⁵N (Nitrogen-15 NMR spectroscopy) which has a negative gyromagnetic ratio, if the dipolar relaxation has to compete with other mechanisms, the observed ¹⁵N signal can be near zero.^[2] Alternative methods are therefore necessary for nuclei with a negative gyromagnetic ratio, and one such method using the INEPT pulse sequence was proposed by Ray Freeman in 1979.^[1]

Signal enhancement via the INEPT technique

Graphical representation of the INEPT NMR pulse sequence. INEPT is utilized often to improve ^¹⁵N resolution because it can accommodate negative gyromagnetic ratios, increases Boltzmann polarization, and decreases T₁ relaxation.^[3]

The INEPT signal enhancement has two sources:

The spin population effect increases the signal by a factor of K = ratio of gyromagnetic ratios γ_I/γ_S of the nuclei, where γ_I and γ_S are the gyromagnetic ratio of the proton (the I spins) and the low-sensitivity nuclei (the S spins) respectively.
Nuclei with higher magnetogyric ratio generally relax more quickly. Since the rate at which the INEPT transfer can be repeated is limited by the relaxation of these spins (rather than the low sensitivity spins), then the experiment can be repeated more frequently, increasing the signal-to-noise ratio.

As a result, INEPT can enhance the NMR signal by a factor larger than K, while the maximum enhancement via NOE is by a factor of 1+K/2.^[1] Unlike NOE, in INEPT, no penalty is incurred by a negative gyromagnetic ratio, it is therefore a useful method for enhancing the signal from nuclei with negative gyromagnetic ratio such as ¹⁵N or ²⁹Si. The ¹⁵N signal may be enhanced by a factor of 10 via INEPT.

References

1 2 3 Gareth A. Morris, Ray Freeman (1979). "Enhancement of Nuclear Magnetic Resonance Signals by Polarization Transfer". Journal of the American Chemical Society. 101 (3): 760–762. doi:10.1021/ja00497a058.
1 2 Ray Freeman (1987). A Handbook of Nuclear Magnetic Resonance (2 ed.). Longman. p. 178. ISBN 0-582-25184-2.
↑ M H Levitt (2008). Spin Dynamics. John Wiley & Sons Ltd. ISBN 0470511176.

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