Coherence double-quantum

Although the natural abundance of nitrogen-15 [14390-96-6] leads to lower sensitivity than for carbon-13, this nucleus has attracted considerable interest in the area of polypeptide and protein stmcture deterrnination. Uniform enrichment of is achieved by growing protein synthesi2ing cells in media where is the only nitrogen source. reverse shift correlation via double quantum coherence permits the... 

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

Double-quantum coherence Coherence between states that are separated by magnetic quantum numbers of 2. This coherence cannot be detected directly, but must be converted to single-quantum coherence before detection. 

Fig. 9.10 (A) Definition of the dihedral angle d between the spin-pairs ij and k,l. (B) Cross-correlated relaxation can be detected by differences in intensities of the multiplet components of double quantum coherences...

A novel tool is a symmetry-based 29Si dipolar recoupling method (SR264n) [123] for small dipolar interactions that has been initially applied in zeolite structural studies by Brouwer et al. [124], One of the advantages of the new method over INADEQUATE is that the latter misses auto-correlations of symmetry-related double-quantum coherences. The SR26411 method provides such information on auto-correlation which allows to identification of all four connectivities of a tetrahedral Si position. 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

Here, we present the example of the trans hydrogen bond coupling between the C of the acceptor and the N of the donor h/(N, C ) that is measured by excitation of double-quantum and zero-quantum coherence between the HN and the C nuclei [12] in a protein. Thus, the double-quantum coherence is split by h /(N, C )+ /(N, H) while the zero-quantum coher-... 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

Fig. 11.16 The pulse sequence used to monitor the evolution of carboncarbon double-quantum coherence over a single rotor period in the presence of the proton-carbon heteronuclear dipolar coupling (a). The evolution of the double-quantum coherence between the Cl 4 and Cl 5 carbons in the retinal of bacteriorhodopsin in the ground state (b). The observed evolution is consistent with a C14-C15 torsion angle of 164° (reproduced with permission from Ref. [172]).

The same sample was subsequently used to measure /nhch dipole-dipole CCR. In this case the pulse sequence proposed by Yang and Kay [43] was applied. The experiment is also based on an NH(CO)CA experiment. The zero and double quantum coherences result in two 2-dimensional (2D) datasets and the 2D spectra obtained (black and red cross peaks in Fig. 4) result after pairwise adding and subtracting the measured 2D datasets. The signals are detected at the frequency and split by the JcaHa coupling. The black and red cross peaks are shifted by Jnhhq . Also in this case, the CCR-rate can directly be obtained from the intensities of the individual peaks ... 

R.P. Mason, G. Cha, G.H. Gorrie, E.E. Babcock, P.P. Antich, Glutathione in whole blood A novel determination using double quantum coherence transfer proton NMR spectroscopy, FEBS Lett. 318 (1993) 30-34. 

Double-quantum coherence (DQC) was used to measure interspin distances in eight doubly labelled derivatives of T4 lysozyme.25 Distances between 20 and 47 A with distance distributions of 1.0 to 2.8 A were measured. For some variants two conformations were detected with distances that differed by 5 to 5.4 A. It was shown that a few measurements of large distances provided significant constraints on protein structure. 

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