¹⁵N{¹H} NOE Experiments 

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The backbone ¹H-¹⁵N heteronuclear NOE provides information about the motion of individual N-H bond vectors. Those that undergo motion faster than the overall tumbling of the molecules (i.e. in th epico- to nanosecond time scale) show a decreased NOE intensity relative to the average observed for the majority of the residues. Thus, for instance, decreased values are usually found at both N- and C-terminal ends of the protein.

For heteronuclear NOE cross-relaxation rates, R_S(I_z ^__> S_z), a single refocused reverse 2D INEPT experiment is used in which proton saturation is achieved during the relaxation T delay prior to the starting 90 ¹⁵N pulse. Two different spectra are usually recorded in an interleaved manner with and without proton saturation during 3-4 seconds. A long recycle delay is needed in order to ensure complete relaxation of water magnetization at the beginning of each scan

Values of steady-state NOEs are established from the ratio of peak intensities according to:

NOE=(I_saturated/I_equilibrium)-1

R_s(I_z-->S_z) is extracted from combined NOE and R_s(S_z) measurements according to:

NOE=-9.86*(R_S(I_z ^__> S_z)/R_s(S_z))

Several approaches to measure ¹⁵N{¹H} NOE have been proposed:

89BIO8972 , 90BIO4394 , 92BIO5269 , 92JMR221-100 , 94JMRA121-111 , 94BIO5984 , and 95BIO2408 .

Others in 93JACS12593 , 93JMRB314-102 , 94JMRB45-105 .

Recently, TROSY-based approaches show improved sensitivity and resolution for large biomolecules ( 00JMR423-143 ).

Modified sequences to overcome solvent-saturation transfer by using jump-and-return pulses ( 95JMRB83-107 ).

Measurement of heteronuclear NOEs in the presence of H(N)-water proton chemical exchange ( 01JMR138-153 ).

Practical aspects of the 2D 15N {1H} NOE Experiment ( 02JB23-23 )

Mean heteronuclear NOE values of some proteins:

0.76 at 600 MHz (0.66 at 500 MHz) for CDK inhibitor p19^INK4d (correlation time of 13.6 ns, 166 residues)

0.79 at 750 MHz (0.74 at 500 MHz) for alpha-sarcin (150 residues) ( 02JB301-24) (correlation time of 7.54 ns, 150 residues)