Residual quadrupole splitting

The static deuterium quadrupole coupling constant is (e qQ/h) = 170 kHz for aliphatic carbon-deuterium (C—D) bonds. A change in the residual quadrupole splitting can be caused by two different mechanisms. First, the angle of the molecular fluctuations may increase or decrease, secondly, the molecule may undergo a conformational change which alters the orientation of the C—D bond vector with respect to the bilayer normal. 

Figure 23.9(A) shows the quadrupole echo spectrum of an oriented block of [2,2- H2]Gly labeled silk fibroin fibers when the fiber axis was set parallel to the magnetic field direction. The quadrupole echo spectrum, Fig. 23.9(B), of unaligned [2,2- H2]Gly labeled silk fibroin fiber is also observed as a powder pattern. Both spectra are split into doublets, which give the value of the quadrupole splitting, Avq as 117.8 kHz for the [2,2- H2]Gly site the values are the same in each case. The full rigid-lattice width of about 126 kHz should be observed when the motion is frozen [72]. Thus, it is concluded that the motion of the methylene groups of the Gly residue is almost frozen at room temperature, which is in agreement with the prediction from the intermolecular hydrogen bonding network in the silk fibroin backbone chain with an antiparallel /3-sheet conformation.

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