Carbohydrates segmental motion

Steady-state fluorescence anisotropy of 10 pM of Calcofluor in the presence of 5 pM of ai -acid glycoprotein = 435 nm and Xqx 300 nm) was performed at different temperatures. A Perrin plot representation (Fig. 8.21a.) yields a rotational correlation time equal to 7.5 ns at 20 °C. This value is lower than that (16 ns) expected for a i-acid glycoprotein and thus indicates that calcofluor displays segmental motions independent of the global rotation of the protein. Thus, two motions contribute to the depolarization process, the local motion of the carbohydrate residues and the global rotation of the protein, i.e., a fraction of the total depolarization is lost due to the segmental motion, and the remaining polarization decays as a result of the rotational diffusion of the protein. 

The red-edge excitation spectra and anisotropy measurements yield information on the dynamics of the fluorophore and of its environment. At high concentration of calcofluor compared to that of a i-acid glycoprotein, a red-edge excitation shift is observed (Fig. 8.23a). This shift indicates that the microenvironment of the fluorophore exhibits restricted motions. Therefore, the carbohydrate residues in the vicinity of calcofluor are rigid and do not show any segmental motions. 

Owing to the complexity of the internal motion of carbohydrate molecules, the elucidation of their conformational properties by MO calculations requires a lessening of dimensionality to manageable proportions. Several small acyclic molecules have therefore been used as models for ab initio or semiempirical MO studies on the structural segments of carbohydrates. On the whole, calculations reproduce all of the main structural trends and conformational preferences observed experimentally in the crystal structures of carbohydrates and in solution. 

---