HCCH experiments

The same idea applies in the quantitative T-HCCH experiment, which can be applied to aliphatic side chains or to sugars to measure the pseudorotation phase (Fig. 7.24). 

Fig. 7.27 a Transferred NOESY, b cross and c reference traces from the transferred rcCH CH-HCCH experiment that display the dependence of the rate of the transferred cross-correlated relaxation... 

CH2 group was used to examine side chain dynamics in proteins. The rates were measured in a modified 2D HCCH experiment from the differential intensities of cross-peak multiplet components. A significant difference in the... 

The CH-CH DD/DD cross-correlated relaxation in protein side-chains can be easily measured in HCCH experiment. For larger proteins the resolution of the experiment may not be sufficient to extract the relaxation parameters. Car-lomagno et al proposed a modification of HBHA(CBCACO)NH sequence for the measurement of C H-C H DD/DD cross-correlated relaxation in the spectra... 

Figure 19. Selected regions from the slice of the HCP, H(CA)P-CC-TOCSY and HCCH experiments, showing the resonance assignments for one of the P-containing chain-ends of the polymer formed of diphenylphosphinyl radical initiated polymerization of styrene. (Reproducedfrom reference 19. Copyright 2001 American Chemical Society.)...

Kay LE, Xu G-Y, Singer AU, Muhandiram DR, Forman-Kay JD. A gradient-enhanced HCCH-TOCSY experiment for recording side-chain 3H and 13C correlations in H20 samples of proteins. J Magn Reson 1993 101B 333-337. 

A more sensitive alternative is to use HCCH-TOCSY experiments [63-65], which transfer magnetization through the much larger 13C-13C coupling constants. In addition, these coupling constants are almost independent of the secondary structure and very uniform... 

The TROSY effect was also used in a relayed HCCH-COSY experiment to correlate adenine H2/H8 resonances in uniformly 13C-labeled RNA molecules [49], and significant sensitivity over the existing HCCH-TOCSY version was reported. Magnetization is transferred simultaneously in an out-and-back manner from H2 and H8 to the three aromatic carbon spins, C4, C5 and C6, establishing thus the connectivity within the adenine base spin system. 

Photolysis of H3NBH3 with 121.5 nm radiation yields imidoborane, HBNH, which has been of theoretical interest Spectral shifts observed for several isotopic species containing °B, N, and D show clearly that the spectrum is due to HNBH which is isoelectronic with HBO, HCN and HCCH. From the spectrum of the isolated species two of the and one of the tr-type vibration frequencies for a linear molecule have been obtained. The location of the missing S (B-H stretch) frequency has been calculated. A comparison of observed and calculated frequencies for HBNH is given in Table 7. Another isolated product observed in these experiments is identified as HNB. This radical may be generated by photodissociation of HNBH subsequent to its formation. In this respect the photolysis mechanism would be similar to the formation of C2H from acetylene. 

The INEPT-COS-INADEQUATE (Figure 8 B), which is in terms of sensitivity equivalent to the 1,1-ADEQUATE,37 outperforms (relative sensitivity of 0.325) the INEPT-INADEQUATE experiments only for HiCCHm (m = 0, 1, 2, 3) segments. Particularly encouraging theoretical predictions for HCCH and HCC segments were obtained for the COS-DEPT pulse sequence35 (not shown) with double the relative sensitivity (0.65) for HCCH segments and the same sensitivity (0.325) for HCC moieties. These results, however, were not confirmed experimentally by the authors. 

Fig. 35. Experimental HCCH-COSY spectra of the fully C-labeled protein rhodniin with two heteronuclear Hartmann-Hahn transfer steps. The spectra were acquired at a spectrometer frequency of 600 MHz employing DIPST2 with = 4.8 kHz (A) and MGS-2 with = 3.78 kHz (B). In the experiments the heteronuclear Hartmann-Hahn mixing periods had a duration of 6 ms. The same plot levels were used for both spectra. (Adapted from Schwendinger et al., 1994, courtesy of Academic Press.)...

In fully C-labeled proteins, the side chains of amino acids can be assigned using CCH or HCCH-TOCSY-type experiments (Fesik and Zuiderweg, 1990 Fesik et al., 1990 Bax et al., 1990b Kay et al., 1993). The three-dimensional HCACO-TOCSY experiment (Kay et al., 1992) is an extension of these experiments that uses the carbonyl chemical shifts to separate overlapping resonances. 

Exclusive TACSY (E.TACSY) mixing sequences (see Section X.E) for aliphatic-selective C- C Hartmann-Hahn transfer without perturbing the polarization of carbonyl C spins (Schmidt et al., 1993 Weisemann et al., 1994 Abramovich et al., 1995) have been used in HCCH-E.COSY-type experiments (see Fig. 39) for the measurement of V(C, H ) coupling constants (see Fig. 40). 

Fig. 40. Expansion of the experimental C -H cross-peak in the soft HCCH-E.TACSY spectrum of C-labeled alanine. The experiment was acquired using the pulse sequence shown in Fig. 39. The E.TACSY mixing time was 15.2 ms, corresponding to eight ETA-1 cycles with a maximum rf amplitude of = 15.19 kFlz. The vector C indicates the...

The transfer of an H atom from one site to another, as in the HCN — NCH isomerization, can be viewed as a special type of internal rotation. A hindered internal rotor treatment of such motions was found [148, 149] to yield an increase in the reactant state density by a factor of 3 to 4 for both HCN and HCCH at the thresholds for CH bond dissociations. Furthermore, for HCN, where the dissociation energy is well known, the resulting low pressure limit rate coefficients were found to be in much improved agreement with experiment. This study also provided a simple general formula for estimating the effect of such corrections for arbitrary isomerizations (Eq. (2.31) in [149]). Illustrative calculations suggested that such effects may be important even in larger molecules. 

Spectra were recorded on Bruker AMX-500 and DMX-750 NMR spectrometers at 30 C. All triple resonance experiments and the HCCH-TOCSY experiment were performed on a single sample of 15N, l3C-ubiquitin in 90% H2O/10% D20 buffer. HCCH-TOCSY spectra were obtained using the pulse sequence described by Bax et al. (1990) and utilized a 27 ms DIPSI-3 mixing sequence. The HCCH-TOCSY data sets were composed of 92 complex points in the... 

In a different approach,61 3/Hcch values (and also VCH and 2/Hch) have been computed by DFT methods for exocyclic hydroxymethyl groups on aldopyranosyl rings, and the values compared with experiment and correlated with the 0-5-C-5-C-6-0-6 torsion angle co, according to the equations ... 

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