Closely coupled nuclei

The pulse sequence, as a variant of the spin echo experiment, also refocuses the spread of frequencies caused by field inhomogeneity, so that some improvement in resolution is obtained. The inset at the lower right of Figure 6-18 shows the normal ID spectra of H-4 and H-5 at the top (Figure 6-18c and e) and the unrotated projection of the 2D J-resolved spectra at the bottom [Figure 6-18d and f, extracted from the projected spectrum (Figure 6-18a) at the top of the 2D display]. The much higher resolution of the 2D resonances is clearly evident. Thus, the procedure is an effective way to measure J accurately, particularly when J is poorly resolved in the ID spectrum. The experiment fails for closely coupled nuclei (second-order spectra). 

There are some recent data suggesting that attenuation of transcription may occur in eukaryotes. If this is true, however, the mechanism must be completely different, because the compartmentalization in eukaryotes would not allow for the close coupling of transcription (in the nucleus) and translation (in the cytoplasm). 

We turn now to transcription in eukaryotes, a much more complex process than in prokaryotes. In eukaryotes, transcription and translation take place in different cellular compartments transcription takes place in the membrane-bounded nucleus, whereas translation takes place outside the nucleus in the cytoplasm. In prokaryotes, the two processes are closely coupled (Figure 28.15). Indeed, the translation of bacterial mRNA begins while the transcript is still being... 

Figure 28.15. Transcription and Translation. These two processes are closely coupled in prokaryotes, whereas they are spacially and temporally separate in eukaryotes. (A) In prokaryotes, the primary transcript serves as mRNA and is used immediately as the template for protein synthesis. (B) In eukaryotes, mRNA precursors are processed and spliced in the nucleus before being transported to the cytosol for translation into protein. [After J. Darnell, H. Lodish, and D. Baltimore. Molecular Cell Biology, 2d ed. (Scientific American Books, 1990), p. 230.]...

Second-order spectra are characterized by peak spacings that do not correspond to coupling constants, by nonbinomial intensities, by chemical shifts that are not at the midpoints of resonance multiplets, and by multiplicities that do not follow the n + 1 rules. (See Figures 4-1, 4-2, and 4-3.) Even when the spectrum has the appearance of being first order, it may not be. Lines can coincide in such a way that the spectrum assumes a simpler appearance than seems consistent with the actual spectral parameters (a situation called deceptive simplicity). For example, in the ABX spectrum, the X nucleus is coupled to two nuclei (A and B) that are themselves closely coupled. When is extremely small, the A and B... 

Eukaryotic chromosomes are located within a nucleus bounded by a nuclear membrane and, since proteins are synthesized in the cytoplasm, the sites of transcription and translation are physically separated. Consequently these two processes are not as closely coupled as they are in bacteria. 

However, a p orbital which represents a charge density concentrated along one axis, gives rise to a coupling. Because of the slight penetration of d and / orbits into regions close to the nucleus, it... 

The actual curve, however, is somewhat modified by Coulomb interaction between the electron or positron and the nucleus. This is allowed for by multiplication with a dimensionless function F(Z p), which leads to a correction factor / for the total decay rate, and it is the product ft that is used for purposes of comparing measured lifetimes with theory. The most rapid decays, with ft = 103 to 104 s, are known as super alio wed . These include 0+ to 0+ decays having A//, 2 = 2 and ft is found experimentally to be close to 3000 s, giving the coupling constant for the Fermi interaction... 

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