Analysis of optical activity spectra of turbid biological suspensions

Ultraviolet absorption spectroscopy of peptides

UV absorption spectroscopy is commonly used with peptides for determining concentration and enzyme activity, but high-resolution UV spectra can also provide information on peptide secondary and tertiary structure and association behavior. New developments using temperature- and cation-dependent high-resolution second derivative absorption methods can also provide information concerning peptide dynamics. Data from several low-resolution spectroscopic techniques, including UV absorption, can be combined to generate an overall picture of peptide structure as a function of environmental conditions.

Circular dichroism of peptides

Circular dichroism measures the difference between the absorbance of left- and right-handed circularly polarized light, and can be used to monitor the secondary structure of peptides (far UV) and the tertiary structure of larger polypeptides (near UV). This technique is especially useful for helix-coil transitions and other aspects of structural alterations. Data from several low-resolution spectroscopic techniques, including CD, can be combined to generate an overall picture of peptide structure as a function of environmental conditions.

Giant circular dichroism of high molecular weight chlorophyllide-apomyoglobin complexes

Chlorophyllide a and the apoprotein of myoglobin (Mb) spontaneously form three types of complex. The M (M(r) approximately 3 x 10(5)) and H (M(r) >/= 4 x 10(6)) complexes, but not the L (M(r) approximately 1.7 x 10(4)), display a circular dichroism (CD) spectrum that is highly red-shifted, nonconservative, and very intense-characteristics shared by the CD spectra of reaction center complexes from purple photosynthetic bacteria. At its 710-nm peak, the H complex CD spectrum has a larger magnitude, 0.06 differential absorbance per unit total absorbance, than has been reported for chlorophyll in any medium.

Spectroscopic Analysis of Highly Concentrated Suspensions of Bovine Somatotropin in Sesame Oil

Spectroscopy was employed to analyze the structural and thermal stability of highly concentrated oil suspensions of bovine somatotropin (bST). These methods were then compared with more dilute aqueous solutions (1 and 10 mg/mL). All oil suspensions were opaque, viscous, and highly concentrated in bST (>300 mg/mL) and thus provided unique analytical challenges. Using front surface fluorescence and ATR-FTIR spectroscopy, protein structure and stability could be directly monitored in this environment. Differences were detected in structure between concentrated oil and dilute aqueous formulations. Fluorescence spectroscopy found that bST was highly thermally stabile within oil suspensions, since minimal changes in emission peak maxima and emission intensity were observed with increasing temperature when compared to dilute solutions. It was also observed that the amount of aggregate in a sample had some effect on the fluorescence spectra. As the amount of aggregated protein increased, the emission peak maximum and emission intensity changed. Employing ATR-FTIR, the secondary structure was examined with increasing temperature. The secondary structure of bST was also found to be very thermally stabile since no change in relative amount of helix/random structure is observed up to 70 degrees C while significant losses are observed in aqueous solution. This study demonstrates that conformational stability can be directly analyzed within highly concentrated, opaque environments using slight modifications of conventional methods.

Physical characterization of a reactivatable liposome-bound rhodanese folding intermediate

Recently, we described the formation of a complex between liposomes and the unfolded protein rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1), which could be liberated and efficiently reactivated after treatment of the complex with detergents [Zardeneta, G., & Horowitz, P. M. (1992) Eur. J. Biochem. 210, 831-837]. Previous data suggested that liposome-bound rhodanese was in the form of a folding intermediate. We have characterized in greater detail the nature of the conformation of the bound rhodanese. Physical characterization of the bound rhodanese intermediate was carried out using proteolysis, fluorescence studies with 1,8-anilinonapthalene-8-sulfonic acid, a probe for hydrophobic site exposure, intrinsic fluorescence to determine tryptophan accessibility using the quenchers acrylamide and iodide, and circular dichroism to detect extent of secondary structure. These studies show that the rhodanese intermediates bound to either cardiolipin or phosphatidylserine liposomes are not identical, the former being in a less compact conformation yet having more secondary structure than the latter, an observation which may explain why the reactivation of the former intermediate is more effective. Finally, turbidimetric and proteolytic studies raise the possibility that each rhodanese intermediate binds to several liposomes. This finding suggests that a possible reason for the differential reactivation yields obtained may be due to the fact that unfolded rhodanese has more binding sites for cardiolipin than for phosphatidylserine liposomes. A greater number of binding sites would result in better anchoring of rhodanese's interactive surfaces and thus reduce the likelihood of misfolding.

Comparison of monoclonal and polyclonal antibody-based immunoassays for osteocalcin: a study of sources of variation in assay results

We have compared two assays for osteocalcin (BGP, bone gla protein), one based on a monoclonal antibody, the other based on a polyclonal serum, in an effort to better understand the factors that contribute to the wide variation reported for osteocalcin normal ranges. The two assays compared well using serum samples, with a correlation coefficient of 0.9071 (n = 48). However, the monoclonal antibody assay returns values increased over the polyclonal assay by approximately 1.5-fold. Gel permeation studies indicate that these larger values are due primarily to increased reactivity in the monoclonal assay with high- and low-molecular-weight forms of osteocalcin in plasma; both assays give similar reactions to intact osteocalcin. Analysis of samples from individuals with increased bone resorption due to parathyroid hormone administration reveals that the decrease seen in osteocalcin values after hormone infusion occurs primarily in the fraction that corresponds to intact osteocalcin. During the course of these studies, we re-evaluated the extinction coefficient for osteocalcin, arriving at the value E = 1.33 mg/ml-1, cm-1. We also observed a significant negative interference in both assays (and a commercial assay) caused by hemolysis. This interference is due to proteolysis of osteocalcin by enzymatic activity released from the lysed red cells.

Conformational change and aggregation of concanavalin A at high temperatures

Increase of the beta sheet content and aggregation of concanavalin A (con A) induced at about 60 C were followed with circular dichroism (c.d.) and scattered light intensity (I90) on both metal-complexed and demetallized species. The conversion occurred at a higher temperature for metal-complexed species than for demetallized one. A concentration-independent conversion curve of metal-complexed species, obtained for a concentration range below around 6 microgram/ml (6 x 10(-3) kg m-3) with a midpoint at 57 degrees C, was well described in terms of a conformational equilibrium between two conformers. However, aggregation did exist even at a low concentration of 1 microgram/ml. Aggregation also occurred without the conformational change as found at the initial stage or in Tris buffer, which suggested the absence of direct coupling between the conformational change and the aggregation. Changes of c.d. at 222 nm, expected to represent the main chain conformation, differed from those at 290 nm reflecting the environment of side chain chromophores. Time courses of three properties examined, c.d. at 222 nm, at 290 nm, and I90, always exhibited a lag in the case of metal-complexed species while the lag was not observed in the case of demetallized species, however. Lag became longer in c.d. but it became shorter in I90 as the protein concentration increased.

Structural basis of human erythrocyte glucose transporter function in proteoliposome vesicles: circular dichroism measurements

The secondary structural compositions of the human erythrocyte glucose transporter in proteoliposome vesicles were assessed on the basis of circular dichroism (CD) spectra measured in the absence and in the presence of D-glucose or an inhibitor, cytochalasin B. We designed and used a scattered-light-collecting device, which corrects CD spectra for optical artifacts originating from light scattering. Relative contents of eight types of secondary structure were estimated by using basis spectra generated by the eigenvector method based on CD spectra of 15 proteins of known structure. Results indicate that the glucose transporter is composed of approximately 82% alpha-helices, 10% beta-turns, and 8% other random structure, with no beta-strands. In the presence of an excess of D-glucose, the alpha-helical content is reduced by more than 10% and there is a significant increase in the random structure content. Cytochalasin B does not appear to affect the secondary structural composition of the transporter to any significant degree.

Dependence on lipid structure of the coil-to-helix transition of bovine myelin basic protein

In aqueous solution bovine myelin basic protein has a close-to-random coil structure that is partially transformed to helix on interaction with lipids. Circular dichroism spectra have been used to follow this conformational transition which, with phospholipids, decreases in the order phosphatidylglycerol, phosphatidic acid approximately equal to phospholipids, decreases in the order phosphatidylethanolamine. There appears to be a strong correlation between the extent of alpha-helix formation and the degree of penetration of the hydrophobic region of the bilayer, as assessed by other methods. Cholesterol mixed in bilayers with phosphatidylserine has little effect on the protein secondary structure. Although basic protein binds strongly to cerebroside and to cerebroside sulphate, two of the other major myelin lipids, the intrinsic chirality of these lipids precludes assessment of their effect on the protein conformation. No significant changes in the circular dichroism spectra accompany the protein association with either of the zwitterionic bilayer-forming lipids, phosphatidylethanolamine and phosphatidylcholine. This seems to exclude extensive penetration into bilayers of these lipids and hence to exclude appreciable hydrophobic interactions; on the other hand, it is argued that little evidence exists for ionic attractions to these lipids. The optical activity of peptides derived from the basic protein by cleavage at the 42-43 and 88-89 peptides bonds (with cathepsin D) and at the 115-116 bond (with a skatole derivative) has also been measured in an attempt to locate the helix-forming regions within the primary structure.

A circular dichroism study of Escherichia coli Initiation Factor-1 binding to polynucleotides

Binding of Escherichia coli Initiation Factor-1 protein to the nucleic acid lattice induces alterations in the secondary structures of a variety of purine and pyrimidine containing polynucleotides in both the single and double stranded conformations, as assessed by circular dichroism spectroscopy. The helical hairpin form of poly(U), the single-stranded stacked form of poly(C), and the duplex poly(A) x poly(U) (in the presence of Mg2+) are stoichiometrically converted by Initiation Factor-1 (IF-1) to structures spectrally indistinguishable from their partially or completely thermally denatured forms. By contrast, the binding of IF-1 to double stranded poly(C), single- and double-stranded poly(A) elicited spectral responses which were interpreted in terms of diminished base-base interaction, not equivalent to that induced by thermal means. Stoichiometric endpoints of 3-5 nucleotide residues/IF-1 were determined for polynucleotide structures in those cases where light scattering artifacts at low nucleotide residue to protein ratios were absent. In the absence of Mg2+ IF-1 was unable to elicit a conformation alteration effect in poly(A) x poly(U), while for poly(U) much less of an effect was observed than in the presence of this divalent ion. The functional significance of these results is briefly considered.

The effects of pH and temperature on the circular dichroism of human erythrocyte membranes

The effects of pH and temperature on the structure of human erythrocyte membranes were studied by circular dichroism (CD). The results obtained demonstrate that the membrane CD spectra undergo significant changes when the pH of the solution deviates from its native pH range of 7 to 8. Spectral changes in the acidic pH region include drastic reductions and slight shifts in the CD signal which may reflect a decrease in alpha-helical content of the proteins and/or an increase in optical artifacts, both of which are irreversible. In the alkaline pH region, dramatic increases in ellipticity and blue-shifts in the spectra are observed between pH 8 and 10. In addition, the spectra more closely resemble those reported for membrane samples where the spectral distortions have been removed. The changes in the alkaline region are demonstrated to be only partially reversible and may be due to conformational alterations in the membrane proteins and/or to a reduction in optical distortions. Thermal stability studies reinforce the irreversible behavior of the membrane samples.

Metal ion induced conformational transitions of prothrombin and prothrombin fragment 1

Circular dichroism experiments indicate that prothrombin fragment 1 undergoes essentially the same secondary structural change whether in the presence of Ca(2+), Mg(2+), or Mn(2+). Titration with any of these metal ions results in a sigmoidal titration curve indicative of cooperative binding. Mg(2+) and Ca(2+) have nearly identical transition midpoints, while that for Mn(2+) is an order of magnitude less. These results correlate well with the results of previous metal ion intrinsic fluorescence quenching experiments. Fragment 1 has previously been shown to undergo a second transition corresponding to dimerization at high calcium concentrations. The present circular dichroism experiments show that this transition does not result in a gross alteration of secondary structure in the fragment 1 molecule. Studies with prothrombin, similar to those with fragment 1, indicate a similar metal ion dependent conformational change but of smaller magnitude. As apparently only the fragment 1 portion of the molecule undergoes the transition, it would appear that the covalently linked fragment 1 is constrained from attaining the same conformation as the purified entity. This suggests that caution must be used in interpreting the results of metal ion binding studies using fragment 1 as an analogue for prothrombin.

Effects of light adaptation on the purple membrane structure of Halobacterium halobium

Absorption, circular dichroism and optical rotatory dispersion of the bacteriorhodopsin containing purple membrane form Halobacterium halobium were studied in regard to the structural stability of this membrane during the photoisomerization of the retinal of the bacteriorhodopsin from the 13-cis to the all-trans configuration. The following conclusions were reached: (a) the macromolecular structure (protein-protein interaction which may result in the possible exciton interaction of the retinal pi-pi* (NV1) transition moments and protein-lipid interaction) are not significantly altered, (b) possibilities of delocalized conformation changes of the apoprotein involving secondary and/or tertiary structure can be ruled out, (c) localized secondary structure conformation changes of the apoprotein must be limited to the involvement of no more than one or two amino acid residues and localized tertiary structure conformation changes of the apoprotein must be limited to a very short segment of the protein chain containing only a few aromatic amino acid residues, and (d) the interaction between the apoprotein and retinal seems to be relatively more pronounced when the retinal is in the all-trans form than the 13-cis from and also the apoprotein seems to impose a more pronounced dissymmetric constraint on the retinal in the all-trans form than in the 13-cis form.

An experimental method correcting for absorption flattening and scattering in suspensions of absorbing particles: circular dichroism and absorption spectra of hemoglobin in situ in red blood cells

An experimental approach to interpretation of the anomalous absorption and circular dichroism (CD) spectra of hemoglobin in situ in red blood cells is reported. Absorption flattening effects have been overcome by use of high cell concentratons in very short light path cuvettes. Differential scattering contributions to circular dichroism have been resolved using a CD instrument capable of variable detection geometry. Scattering effects have also been resolved using media of high refractive index to match that of the red blood cell. The results are in agreement with a parellel calculational analysis of red blood cell CD spectra, which predicted the relative magnitudes of the flattening and differential scattering CD contributions. An experimental absorption spectrum has been obtained for hemoglobin in the red blood cell with scattering and flattening eliminated. This quantitatively simulates the spectrum of a hemoglobin solution. The methods described should be widely applicable to conformational studies of macromolecules in their native environment.

Circular dichroism studies of ethidium bromide binding to the isolated nucleolus

Circular dichroism in the 300-360 nm region and fluorescence induced by intercaltating binding of ethidum bromide to both DNA and RNA components were studied in isolated HeLa nucleoli. Both DNA and RNA compoents contribute to the induced dichroic elliticity. Digestion of nucleoli by RNase or DNase shows that most of the induced ellipticity comes from the DNA component. In nucleoli with an RNA/DNA = 0.8/1.0 the RNA component gives only 20% of the total ellipticity when measured at an ethidium bromide/DNA = 0.25. Spectro-fluorometric titration shows that ethidium bromide intercalates mostly into DNA in nucleoli. Both circular dichroism and fluorescence studies indicate that both DNA and RNA components in isolated nucleoli are less accessible to intercalating binding by ethidium bromide when compared to purified nucleolar DNA, DNA in chromatin or purified ribosomal RNA. Circular dichroic measurements of intercalating binding of ethidium bromide to to nucleoli may be used to study changes in nucleoli under different physiological or pathological conditions.

Heterogeneity in the conformation of different protein fractions from the human erythrocyte membrane

We have isolated 5 families of proteins from human red blood cell membranes and characterized their secondary structure by ultraviolet circular dichroism measurements. The protein families were prepared by selective solubilization from ghosts under nondenaturing conditions. We find that the intact ghost has a mean alpha-helix fraction of 0.37, whereas a low-ionic-strength extract (bands 1, 2, 5, "spectrin") has a substantially higher helix fraction, 0.55. Further extraction of the ghosts with para-chloromercuribenzoate yields bands 2.1, 4.1, 4.2, and 6; their helix content is only 0.17. Finally, the major intrinsic protein, band 3, was solubilized by a non-ionic detergent. Its helix fraction is 0.38.

Influence of ouabain and dihydroouabain on the circular dichroism of cardiac plasmalemmal microsomes

1. The influence of ouabain on the tertiary structure of cardiac plasmalemmal proteins was investigated by means of circular dichroism measurement. Purified plasmalemmal microsomes were obtained by sucrose gradient centrifugation. The CD-spectra of the membranal proteins were shifted to the red and the amplitudes were smaller than those of the same proteins after solubilization. 2. Ouabain induced an increase of the ellipticity bands at 210 and 222 nm of about 50% above the level yielded with microsomes after sonication. At 222 nm ouabain exhibited the half maximum effect at a concentration of 5 X 10(-9) M. The effect could, however, only be exerted if the inside of the microsomes was exposed to ouabain by sonication, thus reflecting the inside-out nature of the plasmalemmal microsomes. 3. The high specificity of the ouabain effect was underlined by the following experiments: a) Dihydroouabain, a much less cardioactive derivative of ouabain proved to be ineffective in corresponding concentrations, b) ouabain had no influence upon the CD spectrum of microsomes derived from cardiac sarcoplasmic reticulum, c) a detergent-like action of ouabain underlying the observed effect can be excluded since highly active tensides, i.e. desoxycholate and dodecylsulfate, only influence the CD spectra at concentrations exceeding 10(-3) M, d) electronmicrographs of microsomes exposed to ouabain demonstrated no alteration of either the appearance or size of the microsomes. 4. The magnitude of the observed ouabain effect indicates that a large portion of the membrane-bound proteins is involved. The number of binding sites and their isolated structural alteration induced by ouabain are not sufficient to account quantitatively for the enhanced amplitudes of the CD spctra. This suggests that ouabain evokes structural changes of membrane proteins different from actual binding sites. It seems, therefore highly improbable that changes of the Na-K-ATPase present in the plasmalemmal microsomes are responsible for the observed effect.