Coherence helix

The same is true for the opposite side of the spectral window (—l/2r) as a 180° rotation gives the same result whether it is cw or ccw. If we put this 6-pulse sequence at the center of our PFGSE, it will reverse the sense of the coherence helix for the resonance l/2r away from the center, and it will maintain the sense of the coherence helix for the on-resonance water peak and for peaks 1/r away from the center. The resonance l/2r away will be unwound under the influence of the second gradient, whereas the on-resonance (water) peak will be wound twice as tightly, leading to zero net magnetization when summed over the whole sample (Fig. 8.21). 

The pathway S, S is unaffected by the first gradient (Fig. 8.26) because z magnetization does not precess, so the 13C SQC (Sx) is only twisted by the second gradient and arrives at the FID in a coherence helix that adds to zero over the whole sample. There is no need to subtract it out—it never reaches the receiver. We can add up the twists imparted by the two gradients using the fact that coherence order (p) equals zero for z magnetization ... 

In the case of lx, the central spin echo (r-180-r) leads to Vch evolution for a total time of IIJ, which moves lx to 2Iy Sz and on to - x. The central 180° pulse on H must be taken into account, but we consider it as if it happened at the beginning of the evolution, when we have lx, so it has no effect. Thus, the overall effect of the BIRD element is exactly the same as a 180° pulse on the / axis for the 13C-bound protons, and it has no effect on the 12C-bound protons. This will reverse the sense of the coherence helix in the NMR tube for the 13C-bound protons only, allowing the second gradient to straighten them out while it further scrambles the 12C-bound protons. 

Determination of protein secondary structure has long been a major application of optical spectroscopic studies of biopolymers (Fasman, 1996 Havel, 1996 Mantsch and Chapman, 1996). These efforts have primarily sought to determine the average fractional amount of overall secondary structure, typically represented as helix and sheet contributions, which comprise the extended, coherent structural elements in well-structured proteins. In some cases further interpretations in terms of turns and specific helix and sheet segment types have developed. Only more limited applications of optical spectra to determination of tertiary structure have appeared, and these normally have used fluorescence or near-UV electronic circular dichroism (ECD) of aromatic residues to sense a change in the fold (Haas, 1995 Woody and Dunker, 1996). 

Fig. 15. Coherence transfer efficiencies as a function of delay 2TC for the residues in a-helix and ffisheet in the sequential HNCA-TROSY scheme. Equation (8) is plotted using the following parameters T2 un-trosy = 50 ms, T2 o = 25 ms, 2Ta = 25 ms,...

Fig. 17. Efficiencies of coherence transfer for the residues in a-helix and P-sheet, as a function of delay 4Ta in the iHNCA-TROSY experiment. Equation (9) is plotted...

We saw in Chapter 8 how a selective 180° pulse can be placed between two gradients of the same sign and duration to give a pulsed field gradient spin echo (PFGSE) that not only selects the desired coherence but also destroys any other coherences. First, we use a hard 90° pulse to create coherence on all spins, and then the first gradient twists the coherence into a helix (Fig. 8.21). The selective 180° pulse reverses the direction of twist in... 

It is generally believed that the bases in the centre of the double helix molecule form the pathway for electron transfer. The delocalized p-orbitals in consecutive bases overlap to form a channel for the movement of electrons [7]. The works of Ladik et al. [40] and Bakhshi et al. [41] have led to the assumption that the transmission channels are along the long axis of the DNA molecule [101, 107-109]. Several mechanisms have been given for charge transport in DNA, but the dominant ones are the coherent transport via extended molecular orbitals, and the thermal hopping mechanism. 

Also, the helix structure of the carbohydrate residues would be coherent with the presence of a pocket from where the first wheels of the helix emerge. 

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