Conformation-dependent chemical shifts

Torsion angles in chain 0.3-0.9nm Conformation dependent chemical shifts (s-NMR)... 

In this paper, we briefly describe some empirical rules found for conformation-dependent chemical shift displacements in conjugated systems. On the basis of these results, we interpret the anomalous chemical shift displacements of the chromophores of Rh and bR, leading to the determination of their conformations. 

Complementary to the analysis of conformation-dependent chemical shifts, two-dimensional experiments that correlate anisotropic interactions may be used to study backbone conformation by MAS NMR. This is possible in particular for dipolar interactions, whose orientations are along the intemuclear vector, and for the carbonyl CSAs, which generally adopt a particular orientation with respect to the peptide plane (Fig. 3a). Under MAS, these interactions may be recoupled during the evolution and/or detection period of a two-dimensional correlation experiment, as discussed earlier. The resulting spectrum will contain cross peaks whose pattern is characteristic of the relative orientation of the two interactions being correlated. An alternative approach is to excite a state of 2QC between two neighbouring nuclei, which then evolves under the influence of two anisotropic interactions. 

Applications of the conformation-dependent chemical shift to the conformational characterization of polypeptides... 

Table 24.4. Conformation-dependent chemical shifts of Ala-residues (ppm from TMS) ...

The structure of silk fibroin from a wild silkworm was examined by using solution and solid state NMR methods. The structural transition of the silk fibroin in aqueous solution was monitored by using solution NMR spectroscopy as a function of temperature. The torsion angles of several Ala and Gly residues in the model peptide, GGAGGGYGGDGG(A)i2GGA-GDGY-GAG, were determined by the conformation-dependent chemical shifts, REDOR and 2D spin-diffusion techniques in the solid state. 

Finally, structural studies of two well-known silk fibroin proteins, Bombyx mori and Sarnia cynthia ricini, have been reported. Solid state NMR methods, such as C 2D spin-diffusion NMR and REDOR were used in addition to the quantitative use of the conformation-dependent chemical shifts, measured by C CP MAS. 

In membrane proteins, however, it should be borne in mind that the seeondary struetures of fully hydrated membrane proteins are far from static under physio-logieal eondition, in spite of proposed models available from diffraetion studies on erystalline samples at lower temperature. Indeed, they are very flexible even in the 2D erystal, beeause they are embedded in flexible lipid bilayers at ambient temperature, undergoing various kinds of molecular motions with correlation times of the order of 10 10 s. In fact, it is notable from Tables 2 and 3 that the spread of the distribution of the conformation-dependent chemical shifts of membrane proteins is much larger than that of the reference polypeptides in the solids " C chemical shifts of the a-helix forms spread over 3 and 2ppm for [l- C]Val- or Pro-, and [3- C]Ala-labeled bR, respectively and NMR peaks from the turned structures located at the loop region of [l- C]Val-bR spread over 3 ppm. It may well be recognized, therefore, that the concept of the conformation-dependent displacement of chemical shifts as predicted under static condition should be modified to some extent in such flexible membrane proteins. 

Assuming no conformation dependent chemical shift effects to occur and using the chemical shift of orthorhombic polyethylene (33 ppm) [6] we can now calculate the chemical shifts of the methine carbon atoms in the three triads of the solid crystalline E-VOH copolymer, respectively 000 (67 ppm), CX)E (70.4 ppm) and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads. The chemical shift values presented above are only meant to yield useful assignments of the several methine carbon NMR signals of E-VOH copolymers. These assignments are necessary because Ovenall [5] did not report dependable estimates for all three types of methines sustained by experimental results. We are aware of chemical shift differences between liquids and solids. Moreover, the choice of orthorhombic polyethylene as a basis for the shift calculations is rather arbitrary but this will only cause the same uncertainty in each of the three shifts. Of more importance is the known sensitivity of substituent-induced shifts towards different conformational equilibria. From results obtained by Cantow [7] for different poly (1,2-dimethylbutane) polymers it can be estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is, however, improbable that the order of the three methine carbon signals will be misjudged. 

A novel approach based on the application of solid-state NMR spectroscopy has been reported that permits the rapid determination of 3D molecular structure with a single uniformly isotope labelled sample. Analogy with the solution NMR spectroscopic investigations is used, which rely on the detection of short distances between hydrogen atoms providing the principal source of information about the 3D fold of the protein. Since the 2D H, H-correlation methods are of limited use for solid-state NMR spectroscopy due to the restricted spectral resolution, the indirect detection and structural analysis of interactions via C, C-correlation spectroscopy is proposed. It has been shown that combined with dihedral-angle constraints, which can be derived from conformation-dependent chemical shifts, the characterisation of the 3D molecular structure from a single protein sample becomes possible. The new approach has been demonstrated on kaliotoxin, a 38-residue peptide. 

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