Internal flexibility

A.4. Rotational dynamics application of the Lipari-Szabo approach to describe internal flexibility of macromolecular chelates... 

Bismuto et alF compared the phosphorescence from both tuna and sperm whale apomyoglobin. The emission occurs from a tryptophan in the A helix. The temperature dependence of lifetime and the position of the 0-0 vibrational band differ as a function of temperature for the two proteins. The authors interpreted their results to indicate that the microenvironment of the tryptophan in sperm whale apomyoglobin possesses a higher degree of internal flexibility than that in the tuna protein. 

Table 4. The R Rfj=p-iatio and parameter C in Eq. (14) for various polymer architectures. The C-parameter gives information on the internal flexibility (C=0 inflexible particles, C= 0.2 flexible chain behavior) ...

The internal flexibility of oligosaccharides is a major obstacle to interpretation of experimental data. To deduce three-dimensional structure, one must, therefore, be able to correctly model internal flexibility. Various methods and results for conformational energy calculations for oligosaccharides have recently been reviewed (9-13). Therefore, no attempt will be made here to describe such efforts to calculate conformational energy surfaces. 

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Fig. 2. a. Intensity correlation of the scattered light from a PMMA sample in acetone recorded with a Malvern autocorrelator with 96 channels. At channel 80, a time shift device is introduced, by which the last 12 channels are shifted by 256 times the delay time of the first channel. These last channels were used for a determination of the base line, b Plot of In (G2 - 1 )m against the channel number (= time) for the same sample as in a. The deviation from the straight line arises from the internal flexibility of the PMMA chain... 

Internal Flexibility of Macromolecular Gd(lll) Complexes. The Lipari-Szabo Approach... 

Gd(III) chelates, the relatively low relaxivity is the consequence of the flexibility of the linker group between the Gd(III) chelate and the rigid dendrimer molecule (slow water exchange is also limitative). Internal flexibility has been also proved for certain non-covalently bound Gd(III) chelate - protein adducts. The tr value determined for MP-2269 bound to bovine serum albumin is 1.0 ns, one order of magnitude lower than the rotational correlation time of the protein molecule [50]. 

Equation 37 has been used in an attempt130 to describe internal flexibility of the three hydroxymethyl groups of sucrose molecule in DzO solutions. The experimental data showed that the contribution of the overall motion to the spectral-density function of the hydroxymethyl group is similar to that of the ring carbons of sucrose. However, the presence of rapid internal motions about the three exocyclic bonds reduces the spectral density amplitudes. On the basis of the calculated order parameters in conjunction with model calculations, it was suggested130 that internal motions may be described as torsional librations. 

The thyroid hormones, exemplified by thyroxine, provide another case where NMR relaxation time measurements give an insight into internal flexibility, and perhaps mode of action, of pharmaceutically important molecules. Synthetic thyroxine, 5, is widely used for the treatment of thyroid disorders, and indeed was the second most widely prescribed drug in the United States in 1998. 

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