Secondary shift

An example of the use of chemical shifts to delineate residual secondary structure is given in Figure 3 for the molten globule state of apomyo-globin (Eliezer et al., 1998 Eliezer et al., 2000). Combined use of 13C , H , 13C, and 13CO secondary shifts gives a more precise definition of secondary structure boundaries than use of 13C shifts alone (Eliezer et al., 2000). 

The main reason is that the transfer of an electron from the outside of the molecule toward the center induces important secondary shifts in the other orbitals as well. A first indication of these shifts can be obtained from a standard Mulliken population analysis, as shown in Table II. In each case, the actual shifts are much smal-... 

After the assignment process is complete, various types of NMR data can be used to build up a picture of the secondary structure of the peptide (based on coupling constants, chemical shifts, and NOEs) and then, if required, a full 3D structure can be determined. Figure 6 shows some of this information for Vcl.l. As noted earlier, deviations from random coil chemical shifts provide an indication of secondary structure, and in the case of Vcl.l. a region of negative aH secondary shifts provides a first indication that the helix typically present in a-conotoxins is indeed present in Vcl.l (Fig. 6A). This conclusion is supported by the CSI, coupling, and NOE data in Fig. 6B. Finally, the 3D structure is shown in various representations in the lower panels of the figure (58). 

Fig. 6. Structural analysis of Vc 1.1 by NMR. (A) The aH secondary shifts. (B) Structural information from NOEs, chemical shift index (CSI), amide exchange, and scalar coupling, are summarized (adapted from ref. 58). (C) A stereoview of the 20 low-energy structures. Some residues are numbered for orientation. (D) Ribbon representation of the peptide the cysteine residues are in yellow. The N-terminus and C-terminus are marked N and C, respectively. (E) The surface of the peptide with selected residues labeled.

Figure 9 NMR data showing the cis/trans isomerisation of an alanine mutant of SFTI-1. (a) A comparison of the secondary shifts of native SFTI-1 with the mutant in which Aspl4 is replaced with an Ala. The trans isomer is very similar to the native peptide, whereas the cis isomer has differences around Prol3. (b) TOCSY spectra of the Aspl4 mutant highlighting the second conformation that occurs as a result of cis/trans isomerisation around Prol3. The cis conformation is not observed in the native peptide.96...

Figure 2. Primary and secondary shift convertors—HTSR loop for hydrogen plant...

A practical problem with the N NMR literature arises from the wide range of secondary shift references which have been employed, making it necessary to correct shift values quoted directly against the secondary shift reference before the spectra from different studies can be compared. A common secondary standard is desirable, even if... 

H shifts compete effectively with capture by solvent. For the parent 2-nor-bornyl cation the kc/ku ratio is ca. 1 in formic acid, ca. 2 in acetic acid, and ca. 6 in aqueous acetone50, 516. The decomposition of 2-norbornanediazonium ions in aqueous sodium hydroxide, however, proceeds with less than 4% 6,2-H shift528). Tertiary, secondary shifts, e.g. (749) - (756)505), occur more readily than secondary, secondary shifts do. Steric effects also play some role there is more 6,2—H... 

The starting point for many investigations on peptides and proteins is the famous paper of Spera and Bax, who revealed a clear distinction between the secondary shifts of a and carbons in a-helical or p-sheet environments... 

The UV spectra of peroxy radicals are very similar to each other, as evident from the HCFC-derived peroxy radicals illustrated in Figures 4 to 11. All the spectra are broad and featureless with Anuu = 200 to 250 nm and involves orbitals localized on the O—O moiety and because the upper state is dissociative. The variations that occur in the position of arise from changes in the structure of the alkyl group, such as the substitution of halogen atoms, that effect secondary shifts in the energies of the oxygen orbitals. 

It should be mentioned that so-called secondary shifts can be of importance, particularly when working at high resolution and using monochroiiiated radiation. All the chemical shifts discussed so far have been primary shifts. That is. (he binding energy for the core level of the atom in the particular electronic en-... 

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