Pulse sequence COLOC

Figure 7.14 Pulse sequence for the HMBCS (heteronuclear multiple-bond correlation, selective) experiment, which uses advantageously a 270° Gaussian pulse for exciting the carbonyl resonances. It is also called the semisoft inverse COLOC. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

To be fair, we must point out that this type of experiment is extremely sensitive to the parameters chosen. Various pulse sequences are available, including the original COLOC (Correlation by means of Long range Coupling) as well as experiments variously referred to as HMBC (Heteronuclear Multiple-Bond Correlation) and HMQC (Heteronuclear Multiple-Quantum Correlation). Depending on the parameters chosen, it is often not possible to suppress correlations due to one-bond coupling ... 

The principal disadvantage of the COLOC sequence lies in the fixed nature of the evolution period. In such a pulse sequence, C-H correlations are diminished or even absent when two- and three-bond H- C couplings are of a magnitude similar to that of H- H couplings within a molecular fragment. This situation occurs quite commonly (Chapter 4). 

The principal disadvantage of the COLOC sequence derives from the fact that it is a jce /-evolution-time experiment (i.e., t is incorporated into the delay time Aj). The major limitation with fixed-/] pulse sequences is that C-H correlations are considerably diminished, or completely absent, when the two- and three-bond coupling con-... 

Several ID and 2D NMR techniques have become standard tools for structure elucidation studies of the resveratrol oligomers. The most commonly employed of these include the one-bond i3C- H HETCOR and HMQC, as well as, the two- and three-bond C- H FLOCK [106], COLOC [67,72,88,106], andHMBC [53,71,78] pulse sequences. Typical 2 Vc-h coupling constant values used for these experiments range between. 7=8-12 Hz. Other commonly used H- H correlation methods include standard COSY [64] and NOESY [67,72,78] pulse sequences however, the most widely employed experiment concerning proton-proton... 

Additional partial structures Ib-f (Table 10) are detected by two-dimensional CH-correlation experiments using pulse sequences adjusted to the much smaller CH coupling constants of C and H nuclei separated by two, three, or more bonds. Such experiments are known as the CH COLOC with detection (Fig. 9) or as the more sensitively H detected and therefore less time-consuming HC HMBC. Contour plots of these experiments (Fig. 9) permit the localization of carbon atoms two or three bonds apart from a certain proton. Thus, the methyl protons with Sh 2.02 in Fig. 9 display eross signals with the earbon nuclei at 8c = 170.5 and 8c = 68.8. 

Figure 15 (A) The basic HSQC (heteronuclear single quantum coherence) pulse sequence. (B) The HSQC spectrum of the aliphatic region of [1] with the C along and the H spectrum along f2- The C- H connectivities are marked for the same molecular fragment as in Figure 14. Other sequences of proto-nated carbons can be determined from the same spectrum while an />bond (n=2,3) C- H shift correlation spectrum such as HMBC, COLOC or FLOCK (see Table 2) can identify non-proto-nated carbons and tie together the molecular fragments into a complete structure.

Figure 5.57. Pulse sequences for heteronuclear shift correlation experiment (A) normal mode (B) employing homonuclear broad-band decoupling (COLOC).

Figure 5.58. The COLOC pulse sequence allows long-range couplings (e.g., between NH and CH proton across the carbonyl group in the peptide) to be observed.

Figure 5.59. H, C-COSY spectrum of the carbonyl carbon atoms of cyclo[-Pro-Phe-D-Trp-Lys-Thr-Gly-] obtained by COLOC pulse sequence. The carbonyl carbons are shown on the vertical axis while the H-NMR assignments of the trans isomer for the various amino acids in the peptide are shown above.

The original COLOC sequence did not attempt to suppress l J(C, H) correlation peaks using a low-pass J filter, which is part of the HMBC experiment (section 5.8.3). The authors who proposed the COLOC sequence [5.170] stated that there would be no practical improvement using additional 1J(C, H) suppression elements [2]. However, several modified COLOC experiments have been published to suppress H(C, H) correlation peaks and spectrum artefacts that are introduced by pulse imperfections. The "BIRD-COLOC" sequence [3] uses two BIRD units to suppress the one-bond IH, correlation peaks [5.182]. The alternative "TANGO-COLOC [5.183] is similar to the "BIRD-COLOC" experiment except that a TANGO unit replaces the first BIRD unit. The S-COLOC sequence [5.94] utilizes the COLOC-TANGO sequence plus an additional 180° pulse to enhance the constant-time procedure. 

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