Constant-time experiments

RA Chylla, IE Markley. Improved frequency resolution m multidimensional constant-time experiments by multidimensional Bayesian analysis. I Biomol NMR 3 515-533, 1993. 

Constant-time experiment An NMR experiment in which the total duration of <1 is kept constant. 

Figure 19 Schematic effect of the STAR operator on 2JCH and 3,/CH couplings. The vicinal component of magnetization in the long-range response that is two-bond coupled to a protonated carbon experiences modulation, which serves as a pseudo-evolution for this coupling. In contrast, the vicinal component of magnetization in the long-range response that is three-bond coupled to a protonated carbon does not exhibit a F, skew. Homonuclear modulation during the evolution period f, is still present, as the full experiment is not a constant-time experiment.

For those purposes, the authors used constant-time version of the sensitivity-enhanced HMBC sequence,79 combined with a two-step low-pass J filter. Constant-time experiments have no coupling structures in the carbon dimension making it easy to identify the centre of signals in... 

The HMQC approach has also been employed in the measurements of 3/(H3 P) and 3/(C4 P) coupling constants using a 2D 31P spin-echo difference constant-time experiment [44] to increase sensitivity by a factor of 1.5-2.4 in a 17 kDa DNA-protein complex. 

For the measurement of cross-correlated relaxation rates, there are mainly three methods that have been used in practice. In the /-resolved constant time experiment, the multiplet Hnes exhibiting differential relaxation are resolved by the f couplings, and the line width is translated into intensity in a constant time experiment (Fig. 7.19a,d). In the J-resolved real time experiment the line width of each multiplet line is measured instead (Fig. 7.19b, d). This experiment has been applied so far only for the measurement of... 

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 ... 

FIGURE 24. ll detected H—29Si correlation spectra of 15 ( II frequency 600 MHz, 5 mm inverse detection broad-band probe, relaxation time 2.0 s, acquisition time 0.38 s, 64 transient for each of 64 increments). (The two non-equivalent CH2 protons are distinguished by the letters a and b.) (a) Phase-sensitive spectmm measured with HSQC constant time experiment described in text (b) magnitude presentation of HMBC spectrum. Reproduced by permission of Academic Press from Reference 212... 

The pulse sequence is shown in Fig. 12.7. This sequence represents a constant time experiment and differs from those that we have seen in that the total evolution time comprises two variable periods. As <, is incremented, one period increases but the other decreases to retain a constant overall evolution time, A. The 180° pulse refocuses chemical shifts at the end of the 1, period, but they then precess as... 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

Variable time experiment Constant-time experiment... 

Utilizing the concept of the constant time experiment noted in the introduction to this section (see Section 3.1) Furihata and Seto next reported the development of two constant time HMBC variants, to which they gave the acronyms... 

Application to two-dimensional spectra started in 1993 when Chylla and Markley " applied BPT to constant time experiments. The results of both simulations and experimental data are quite impressive. The fact that BPT is not only compared to DFT, but also to linear prediction makes the demonstration even more sound. Independently, Routh eta . also tackled the processing of two-dimensional spectra following a discussion of noise statistics. 

The solid-state NMR homonuclear decouphng sequences in the DUMBO family have been shown to be effective at ultra-fast MAS rates of up to 65 kHz. The sequences were apphed to model compounds glycine and [2-13C]-L-alanine as well as to the dipeptide p-L-Asp-L-Ala in windowed and continuous phase-modulated versions. They were shown to achieve especially impressive resolution when implemented in a 2D constant-time experiment. At 65 kHz MAS, H resolution using homonuclear decoupling was found to be similar to that obtained at lower MAS rates, whereas the spectral sensitivity was improved by a factor of 5 over homo-nuclear-decoup led spectra at 10 kHz MAS. 

---