Determination of the secondary structures of proteins by circular dichroism and optical rotatory dispersion

Lipid-protein interactions in high density lipoproteins

Delipidated high density lipoprotein (apo-HDL), isolated apolipoproteins apoA-I and apoA-II, S-carboxymethylated apoA-II, apoC-III, the NH(2)- and COOH-terminal CNBr peptides of apoA-II, and the COOH-terminal CNBr peptide of apoA-I were recombined in vitro with [N-C(3)H(3)-choline]phosphatidylcholine (PC) and [N-(14)CH(3)-choline]sphingomyelin (SPM). The lipid-protein complexes were analyzed by ultracentrifugal flotation, agarose gel chromatography and circular dichroism. ApoHDL, apoA-II, and S-carboxymethylated apoA-II readily recombined with PC or SPM to form particles that were similar in size to native HDL. The COOH- but not the NH(2)-terminal CNBr peptide of apoA-II recombined with lipid. ApoA-I and the COOH-terminal CNBr peptide of apoA-I, however, recombined with PC or SPM to only a limited extent, suggesting that protein-protein interactions between apoA-I and apoA-II are important in the integration of apoA-I into recombined lipoprotein particles. Analysis of the recombined lipid-protein complexes by circular dichroism indicated that there was an increase in helical structure concomitant with lipid-protein binding. The reconstituted particles had many of the physical and chemical properties of the native lipoprotein.

Circular dichrosim of chromaffin granule proteins in situ: analysis of turbidity effects and protein conformation

The circular dichroism spectra for proteins in situ in catecholamine secretory vesicles (chromaffin granules) is presented together with an analysis of protein conformation and turbidity effects on the spectra. The calculational analysis has resolved scattering and absorption effects in the turbid suspension spectra using a coated-sphere scattering model which allows for different materials in its shell and core. The intrinsic conformation of the proteins in situ was estimated by an iterative procedure with various trial protein conformations, for the chromaffin granules both intact and after release of their contents. The resulting average secondary structures (within about 10%) are: (25% alpha helix, 15% beta structure) for the membrane proteins and (15% alpha helix, 5% beta structure) for the soluble contents. The protein conformation did not change with osmotic release of the granule's contents. Consequently, if chromogranins are involved in a catecholamine storage complex, this is not reflected in any detectable change in their average secondary structure.

Membrane optical activity: some facts and fallacies

The circular dichroism of hypothetical, water-filled, spherical shells, 75-3500 nm in radius, with walls 7.5 nm thick, composed of poly(L-lysine) in various conformational proportions, and suspended in water, were computed from the known optical properties of this polypeptide by classical general light-scattering theory (Mie theory). Comparison of the computed curves of circular dichroism spectra with those of diverse membranes reveals large discrepancies below 215 nm and shows that light scattering does not adequately account for the optical activity of membranes containing appreciable proportions of nonhelical conformation. However, turbidity effects can explain the anomalies of membrane optical rotatory dispersion near 233 nm, if not uniquely so. We conclude that the optical activity of neither most soluble proteins nor membrane proteins can provide accurate conformational information when synthetic polypeptides are used as standards and list the reasons for this argument. We also show that present techniques to "correct" membrane optical activity are likely to produce additional artifact.