Serum albumin optical rotation

Kauzmann, W. and R.B. Simpson. 1953. Kinetics of protein denaturation. in. Optical rotations of serum albumin, fS-lactoglobulin, and pepsin in urea solutions. J Am Chem 75 5154-5157. 

The CD of bilirubin-complexes with serum albumin from a variety of species are very different under identical conditions. In several complexes, the observed CD spectrum is far more complicated than what is observed in the case of HSA. Other rotational strength mechanisms, such as coupling with protein groups, were suggested as additional sources of optical activity [239]. 

The initial decrease in optical rotation found in aqueous solutions of /3-lactoglobulin and ovalbumin is not, however, sufficient to differentiate globular proteins from simpler synthetic polypeptides in their transition behavior, for neither ribonuclease nor human serum albumin appear to exhibit it. The specific rotation of ribonuclease in water-2-chloroethanol mixtures becomes steadily less levorotatory as the proportion of nonpolar solvent increases (Weber and Tanford, 1959). In the case of human serum albumin Bresler (1958) and Bresler el al. (1959) find that only progre.ssive increases in specific rotation occur as the concentration of 2-chloroethanol is increased and that this change is accompanied by a steady rise in viscosity and the corresponding axial ratios characteristic of the formation of rodlike particles. If these proteins do have some initial helical content in water, as can be argued from their optical rotatory dispersion, then it appears that hydrophobic forces are not required for the stability of these regions. 

They confirm that 0.1 M potassium chloride lowers the pH at which the expansion occurs and yet show that it does not greatly affect the isomerization near pH 4. Since the isomerization is undetectable at 589 m i, the wavelength used by Kauzmann, the optical rotation at this point will not parallel changes as indicated by other techniques. The volume contraction which continues in 0.15 M potassium chloride near pH 4 may therefore reflect a slight unfolding during isomerization, a possibility that is in harmony both with the concomitant exposure of about 20% of the tyrosyl residues in bovine serum albumin as measured by difference spectroscopy and the small loss in helical content. 

The denaturation of 3-lactoglobulin by nonpolar agents and the conformational effects of disulfide cleavage upon serum albumin and ribonu-clease demonstrate that sources of stability in addition to helical segments may be required for the rigid compact shape of globular proteins. A number of further studies upon the respective contributions of secondary and tertiary structure to this stability may be selected to illustrate the role that optical rotation, used as a method solely for determining secondary structure, can play in conjunction with other methods of structural assessment. 

J8. Jirgensons, B., Further studies on characterization of human serum albumin by means of optical rotation. J. Am. Chem. Soc. 77, 2289-2292 (1955). 

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