Biopolymers, measuring

Figure 2.6 illustrates the approximate resonance frequencies for a number of nuclides at a particular value of B0. On this scale, the range of chemical shifts for each nuclide is smaller than the width of the vertical line denoting the nuclide. It is clear that the frequency range is a continuum, so the H resonance of TMS can in principle serve as the reference for all nuclei. In a modern NMR spectrometer all frequencies are derived from a single source, so all chemical shifts can be determined relative to TMS (or relative to DSS in aqueous solutions). IUPAC has formally recommended that chemical shifts of 2H, 13C, 15N, and 3,P in biopolymers measured in aqueous solution be reported relative to the H resonance of DSS.55 It seems likely that this recommendation will be broadened to use TMS as the reference for all nuclides in nonaqueous solutions. 

Lee J, Tripathi A (2005) Intrinsic viscosity of polymers and biopolymers measured by microchip. Anal Chem 77 7137-7147... 

Table 2.4 Molar extinction coefficients of typical biopolymers measured at 2 = 0.6THz obtained with powdered samples dispersed in a polyethylene matrix [21].

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Anotlier teclmique used for stmctural inference is dielectric dispersion in tlie frequency [25] or time [26] domains. The biopolymer under investigation must have a pennanent dipole moment p. It is first dissolved in a dielectrically inert solvent, e.g. octanol, which may be considered to bear some resemblance to a biological lipid membrane, and tlien tlie complex impedance i +j( is measured over a range of frequencies / typically from a... 

Mechanical Properties. Although wool has a compHcated hierarchical stmcture (see Fig. 1), the mechanical properties of the fiber are largely understood in terms of a two-phase composite model (27—29). In these models, water-impenetrable crystalline regions (generally associated with the intermediate filaments) oriented parallel to the fiber axis are embedded in a water-sensitive matrix to form a semicrystalline biopolymer. The parallel arrangement of these filaments produces a fiber that is highly anisotropic. Whereas the longitudinal modulus of the fiber decreases by a factor of 3 from dry to wet, the torsional modulus, a measure of the matrix stiffness, decreases by a factor of 10 (30). 

Chirooptical properties give more subtle information on the conformational behavior of biopolymers and peptides in solution. In early experiments, optical rotation and optical rotatory dispersion (ORD) have been recognized as valuable techniques, followed more recently by significant progress and refinements in the equipment which have resulted in the routine measurements of applied circular dichroism (CD). 

The amount of water boimd to the proteins and polysaccharides depends primarily on the ratio of water to the biopolymer in the investigated system The two extreme cases are the dry biopolymer (water content tend to zero) and highly diluted aqueous solutions of the biopolymers. The dry biopolymer undergoes hydration if is exposed to the water vapor of increased vapor pressure. The extent of hydration can be determined y measuring the... 

Small an jlitude oscillatory shear measurements have been widely used to follow the gelation process of biopolymers [9] because of their non-destroying character and the great variability of procedures, which allow simultaneous investigations of various features of a single sample. 

Additional evidence for silica nucleation on biopolymer macromolecules was furnished by experiments in which solutions of proteins were studied by dynamic light scattering. As an illustration, Figure 3.6 shows the relative intensity of light scattering versus the diameter of the scattering particles in solution with 1 wt.% of bovine serum albumin. Curve 1 presents the initial state where the protein was not yet treated with silica precursor. The measured... 

There have likewise been early successes in addressing measurements of biopolymers, particularly their molecular weights, and to a more limited extent... 

Molecular weights of both synthetic and naturally occurring polymers have been obtained by LS almost as a routine measurement. For reasons which are not wholly evident but which probably have historical and medical connotations, sedimentation in the ultracentrifuge seems to be somewhat preferred by workers in the field of biopolymers, although this technique offers no advantages over LS. Indeed LS can frequently provide additional information and is less time consuming. 

---