Polypeptides, optical rotation

Tanford (1968) reviewed early studies of protein denaturation and concluded that high concentrations of Gdm-HCl and, in some cases, urea are capable of unfolding proteins that lack disulfide cross-links to random coils. This conclusion was largely based on intrinsic viscosity data, but optical rotation and optical rotatory dispersion (ORD) [reviewed by Urnes and Doty (1961) ] were also cited as providing supporting evidence. By these same lines of evidence, heat- and acid-unfolded proteins were held to be less completely unfolded, with some residual secondary and tertiary structure. As noted in Section II, a polypeptide chain can behave hydrodynamically as random coil and yet possess local order. Similarly, the optical rotation and ORD criteria used for a random coil by Tanford and others are not capable of excluding local order in largely unfolded polypeptides and proteins. The ability to measure the ORD, and especially the CD spectra, of unfolded polypeptides and proteins in the far UV provides much more incisive information about the conformation of proteins, folded and unfolded. The CD spectra of many unfolded proteins have been reported, but there have been few systematic studies. 

Optical Rotation and the Conformation of Polypeptides and Proteins Peter Urnes and Paul Doty... 

To answer this question, Goodman and his team synthesised by classic techniques a series of polypeptides of y-methyl-L-glutamate ranging from 3 to 11 units all composed of the same enanthiomorphic form of the amino-acid residues (73). The specific optical rotation of a polymeric molecule involving asymmetrical centres is enhanced by its... 

An increase in the specific rotation of low-molecular weight polypeptides may be caused also by their intermolecular association (75). Such an effect should be concentration dependent, and indeed in dioxane the specific rotations of the penta- and hexamer were found to increase with their concentration, and ultracentrif-ugation studies (77) proved indeed that these oligomers are associated. However, the specific optical rotations of the dimer, trimer and tetra-mer, as well as of the hepta- and nonamer were concentration independent, and hence the abnormally high specific rotation of the hepta- and nonamers indicates that they are probably helical. 

To avoid any complications caused by the intermolecular association, Goodman (78) reinvestigated the optical rotation of the peptides in dimethyl formamide, since in this medium the specific rotation is independent of concentration. From the latter study it was concluded that at 25° C the spontaneous helix formation of poly-y-methyl-L-glutamate in dimethyl formamide is occurring at the critical range of 7—9 units. Extension of these studies (73 b, 79) led to a better understanding of temperature and solvent effects upon the helix-coil transition of oligomeric polypeptides. 

When the most common spiropyran SP-4 was mixed in poly(y-benzylglutamate) to prepare a film, a reversible change in optical rotation was observed upon 365 nm light irradiation and visible light irradiation [49], The largest Aa before and after UV irradiation at sodium D-line (589 nm) was 1.225° for 4 mol% concentration. It was supposed that the change might have arisen from the chiral interaction between the chiral polypeptide environment and the colored merocyanine. Indeed, an induced CD was observed. 

When azobenzenes are attached to polypeptides, photochromic reactions of azobenzenes can induce the change in helical properties of the polypeptides, which may be detected by CD spectrum as well as optical rotation. For 4-phenylazophen-ylamine-condensed poly(y-glutamic acid) A-9 containing up to 80 mol% of 4-phenylazophenylamide side chain, UV irradiation in organic solvents, such as... 

Polymer molecules in solution can be found in many different geometric conformations, and there exist a variety of experimental methods (e.g., viscosity, light scattering, optical rotation) for obtaining information about these conformations. In this experiment, the measurement of optical rotation will be used to study a special type of conformational change that occurs in many polypeptides. 

Optical rotation and chain conformation in polypeptides and proteins have their common ground in the peptide bond, for it is the spatial disposition of this generic chemical group that gives rise to the principal rotatory characteristics of these snbstances and at the same time defines their secondary structure. Changes in the mutual orientation of peptide groups in the polymeric backbone are thus tantamount to conformational changes, so that the special sensitivity of optical rotatory power to steric form renders it uniquely suited to the study of conformation. 

The polypeptides in Table I all exhibit simple dispersion in the random coil, so that their optical rotation is given by the simple Drude equation at any wavelength within the range of its validity. 

The synthetic polypeptide most completely studied in the solid state by optical rotation is poly-n-alanine, which Elliott et al. (1957b, 1958) treat as though it were an L-polypeptide with opposite helical sense in order to make straightforward comparison of the results with other studies. The 6o for the pure L-polymer is —475, and for two helical DL-copolymers, —500 and —505 extrapolated values for the meso-polypeptide lead to a fio of — 560. These compare favorably with Pasman s result of about —425 in 30% dichloroacetic acid in chloroform, a solvent in which the conformation is helical (I asman, 1961a). The [m ] values were found to be less than those obtained for solution, a result probably attributable to environmental changes, yet of meso-poly-DL-alanine is -f 70° at 578 m/u, which is... 

Helix-coil transitions have been followed by optical rotation for almost all high molecular weight polypeptides hitherto discussed, usually in the course of delineating the conditions under which the helical and disordered... 

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