Positive Circular Dichroism Band Research Articles

On the solution conformation of bradykinin and certain fragments.

A circular dichroism (CD) study of [D-Pro2]- and [D-Pro3]-bradykinin, selected peptide fragments, and the model compound. N-acetyl-L-phenylalaninamide, support our previous conclusion (Biochemistry 12, 3780, 1973) that the positive 221-nm CD band of bradykinin is a composite of bands due to two chromophores, the 217-nm band characteristic of the Phe residues overlying the 223-nm band of the N-terminal sequence, Arg-Pro-Pro. The results also indicate that the 223-nm band of Arg-Pro-Pro is associated with the configuration of the Pro-Pro sequence, Arg-D-Pro-Pro and Arg-Pro-D-Pro virtually being diastereoisomers. Accordingly, the conformation of Arg-Pro-Pro was probed in further detail. Upon increasing the temperature from about 27 to 65 degrees C, Arg-Pro-Pro undergoes a conformational transition characterized by large positive values of deltaHdegrees and deltaSdegrees, which is interpreted to mean that the structure of water and, thus, solute-solvent interactions play a dominant role in determining the conformation of the peptide 13C nuclear magnetic resonance spectroscopy indicates that the effect of lowering the pH on the CD of Arg-Pro-Pro is explicable in terms of hydrogen-bond formation between the carboxyl group and Pro2 carbonyl oxygen at acid pH with concomitant cis to trans isomerization.

Evidence from circular dichroism for the binding of hydrogen ions and calcium ions by poly (L-proline).

AbstractThe positive circular dichroism band observed near 228 nm with poly(L‐proline) responds in a similar fashion to HCl and CaCl2. The spectra in the HCl solutions are compatible with a simple binding equation and a pK near −2 for the dissociation of a proton from a protonated peptide bond in poly(L‐proline). The data obtained in CaCl2 is susceptible to the same analysis, suggesting a pK near −1.5 for the dissociation of a calcium ion from its complex with poly(L‐proline).

A natural CD label to probe the structure of the purple membrane from [formula omitted] by means of exciton coupling effects

Coupling between retinal chromophores on adjacent bacteriorhodopsin molecules in the hexagonal surface lattice of the purple membrane from Halobacterium halobium R 1 leads to exciton circular dichroism (CD) spectra in the 567 nm absorption band. Uncoupling by solubilization of bacteriorhodopsin in Triton X-100 results in a loss of this couplet. In dimethylsulfoxide (DMSO)/water mixtures, both exciton peaks disappear and a positive CD band develops at 460 nm. The change is reversible. It is suggested that DMSO increases the protein mobility in the membrane. Bleaching causes the concerted disappearance of both exciton peaks around 567 nm, and the appearance of optical activity in the 412 nm band. Because of its strong dependence on geometry, this CD effect appears to be a sensitive probe to study changes in protein mobility and in protein-protein interactions.

Estimation of the circular dichroism exhibited by statistical coils of poly(L‐alanine) and unionized poly(L‐lysine) in water

AbstractThe circular dichroism of Ac‐(Ala)x‐OMe and H‐Lys‐(Lys)x‐OH with x = 1, 2, 3, and 4 has been measured in aqueous solutions. The oligomers with x = 4 show similar circular dichroism spectra in water when the lysyl amino groups are protonated, and they respond in similar fashion to heating and to sodium perchlorate. Both oligomers at 15°C exhibit a positive circular dichroism band at 217–218 nm, which is eliminated by the isothermal addition of 4 M sodium perchlorate or by heating. The positive circular dichroism of the lysine oligomer is also eliminated when the pH is elevated to deprotonate the amino groups. Positive circular dichroism is still observed for Ac‐(Ala)4‐OMe at elevated pH. Circular dichroism spectra have been estimated for poly(L‐alanine) and poly(L‐lysine) as statistical coils under the above conditions, based on the trends established with the oligomers. Poly(L‐lysine) and poly(L‐alanine) are predicted to exhibit similar circular dichroism behavior in aqueous solution so long as the lysyl amino groups are protonated. The circular dichroism of the statistical coil of poly(L‐lysine), but not poly(L‐alanine), is predicted to change when the pH is elevated sufficiently to deprotonate the lysyl amino groups. These results suggest that the unionized lysyl side chains participate in interactions that are not available to poly(L‐alanine). Hydrophobic interactions may occur between the unionized lysyl side chains. Protonation of the lysyl amino groups is proposed to disrupt these interactions, causing poly(L‐alanine) and protonated poly(L‐lysine) to have similar circular dichroism properties.

Role of nonhistone chromosomal proteins in determining circular dichroism spectra of chromatin

Complexing of histone proteins, from WI-38 cells with pure DNA from WI-38 cells, causes a marked decrease in the amplitude of the positive ellipticity band and a red shift in circular dichroism spectra in the 250–300 nm region. Total nonhistone chromosomal proteins from WI-38 cells (without histones) cause an analogous effect, but of significantly reduced magnitude. However, the two effects are not additive, because, when DNA is complexed with both histones and nonhistones, the amplitude of the positive ellipticity band has an intermediate value, between the histone-DNA complex and the nonhistone-DNA complex. Removal of certain nonhistone proteins from chromatin of WI-38 cells, by extraction with 0.25–0.35 m NaCl, causes a decrease in the positive circular dichroism band in the 250–300 nm region. Removal of histones and other nonhistone proteins from chromatin by extraction with 0.75 and 1.5 m NaCl causes a strong increase in positive ellipticity. This suggests the existence of modest but definite effects of nonhistone proteins in determining DNA conformation in native chromatin. Taken as a whole, nonhistone chromosomal proteins have a weaker but analogous effect to that of histones, while the nonhistone proteins extractable with 0.25–0.35 m NaCl have an opposite effect.

Studies on interaction between poly(L-lysine58, L-phenylalanine42) and deoxyribonucleic acids.

A random copolymer of 58% L-lysine and 42% L-phenylalanine, poly(Lys58Phe42), was used as a model protein for studying the role of phenylalanine residues in protein-DNA interaction. Complexes between this copolypeptide and DNA, made by direct mixing, were studied by absorbance, circular dichroism (CD), fluorescence, and thermal denaturation. Complex formation results in an increase in absorbance, and an enhancement, red-shift, and broadening of phenylalanine fluorescence. The fluorescence enhancement is opposite to the quenching observed when a tyrosine copolypeptide is bound to DNA (R. M. Santella and H.J. Li (1974), Biopolymers 13, 1909). The positive CD band of DNA near 275 nm is reduced and red-shifted by the binding of the phenylalanine copolypeptide to a greater extent than by the tyrosine copolypeptide. Thermal denaturation of the complexes in 2.5 times 10(-4) M EDTA (pH 8.0) shows three characteristic melting bands. For complexes with calf thymus DNA, free base pairs melt at Tm,I (47-49 degrees) and copolypeptide-bound base pairs show two melting bands (Tm,II at 73-75 degrees, and Tm,III at 88 -90 degrees). Similar thermal denaturation results have been observed for complexes with Micrococcus luteus DNA. The fluorecence intensity of the complexes is greatly increased when the temperature is raised to the Tm,II region. In addition to fluorescence measurements, the effects of increasing temperature on absorption and CD spectra of the complexes were also studied. Stacking interaction between the phenylalanine chromophore and DNA bases, either partial or full intercalation, is implicated by the experimental results. Several mechanisms are proposed to describe the reaction between the copolypeptide and DNA, and thermal denaturation of the complex.

Circular dichroism of histone-bound regions in chromatin.

AbstractNative, NaCl‐treated, trypsin‐treated, and polylysine‐bound nucleohistones were studied in 2.5 × 10−4 M EDTA, pH 8.0, using circular dichroism (CD) and thermal denaturation. Removal of histone I by 0.6 M NaCl has a much smaller effect on both Δε220 and Δε278 than the removal of other histones. This indicates that histone I has less helical content and less conformational effect on the DNA in nucleohistone. By extrapolating to 100% binding by histones other than I, the positive CD band near 275 nm is close to zero. Comparison is also made between the effects of binding by the more basic and the less basic halves of histones by trypsin‐digestion and polylysine‐binding experiments. Trypsin digestion of nucleohistone reduces melting band IV at 82°C much more than melting band III at 72°C. However, the CD changes of Δε278 and Δε220 induced by trypsin digestion are small, unless melting band III is also reduced by the use of a higher trypsin level. This implies that the less basic halves of histones, which stabilize DNA to 72°C (melting band III), have more helical structure and are more responsible for conformational change in DNA than are the more basic halves, which stabilize DNA to 82°C (melting band IV). Polylysine binding to nucleohistone diminishes melting band III but has no effect on melting band IV. This binding affects only slightly the Δε220 of nucleohistone, indicating that polylysine interferes very little with the structure of the less basic halves of bound histones. The implications of these studies with respect to chromatin structure are discussed.

Chemical modification and conformation studies on bence jones protein

AbstractA λ‐type Bence Jones protein was chemically modified by carbamaylation of the ε‐amino groups of its lysine side chains and by nitration and reduction of its tyrosine residues. All 30 ε‐amino groups were carbamylated, but only 6 of the 14 tyrosine side chains were nitrated. The gross conformation of the macromolecules (shape) was not altered in these reactions, as tested by intrinsic viscosity and sedimentation coefficient determinations. Also, the immunoprecipitability was not affected by the chemical modification. However, circular dichroism (CD) spectra revealed slight to moderate changes in the conformational details within the macromolecules. The appearance of a positive CD band at 240 nm on nitration was caused by 3‐nitrotyrosine residues, whereas the new band at 300 nm was probably due to change of asymmetric environment of tryptophan chromophores.

Solution studies on heme proteins: Circular dichroism and optical rotation of Lumbricus terrestris and Glycera dibranchiata hemoglobin

The circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of the multi-chain hemoglobin of the earthworm, Lumbricus terrestris, are presented and compared to the spectra of the single chain and four-chain Glycera dibranchiata and human A hemoglobins in both the ferro and ferri heme oxidation states. In Fe 2+ oxy form L. terrestris hemoglobin exhibits positive dichroic bands at 258,412, and 560 nm and negative bands at 288,330, 520, and 578 nm. Except for the presence of a positive CD band at 577 nm, corresponding to the 578 nm band of L. terrestris oxyhemoglobin, human oxyhemoglobin has similar CD spectra in the same near-ultraviolet to visible wavelength region. On the other hand, all the resolved dichroic bands or extrema of G. dibranchiata hemoglobin at 280, 295, 345 and 540 nm are negative, save for the last band at 575 nm, which is positive. These differences seem to result from the changes in the heme environment and the geometry, and coordination state of the central heme iron in these proteins. While the changes in the oxidation state to the Fe 3+ form of these hemoglobins seem to produce significant changes in the position and amplitude of most of the observed CD bands, including a change in the sign of L. terrestris cyanomethemoglobin at 525 and 575 nm, the largely negative character of the CD spectra of the G. dibranchiata cyanomethemoglobin (with negative CD bands shifted to 270, 297, 364, 427, and 548 nm) seems to be preserved. The ORD and CD spectra in the far-ultraviolet, peptide absorbing region are found to be largely unaltered on changes in oxidation states and ligand binding. The CD spectrum of L. terrestris oxyhemoglobin is thus characterized by CD bands centering at 218, 208 and 193 nm, with mean residue ellipticities [ Θ] λ = −15 750, −14 900, and 33 100 deg. · cm 2 · dmole −1, and the ORD spectrum with extrema at 233 and about 200 nm with mean residue rotations, [ m′] λ of −5430 and 33 000 deg. · cm 2 · dmole −1. The corresponding CD and ORD amplitudes of G. dibranchiata oxyhemoglobin are found to be, respectively, [ Θ] λ = −24 350, −23 100, and 61 600, and [m′] λ = −9180, and 48 400 deg. · cm 2 · dmole −1. Estimates of apparent helix content obtained with the recently published ORD and CD reference parameters by Chen et al., based on X-ray structural and solution data of various proteins, indicate rather low helical folding of 33 to 45% for L. terrestris hemoglobin and 70 to 75% helix for G. dibranchiata hemoglobin. The latter estimate is similar to that obtained on human hemoglobin A. The possibility that the low estimates of helix content of L. terrestris hemoglobin are caused by spectral distortions and absorption flattening due to Mie scattering of this high molecular hemoprotein was considered, and ruled out, largely on the basis of light scattering and concommitant ORD and CD measurements as a function of pH. Light scattering measurements at 630 nm confirmed the mol. wt of approx. 3.1 · 10 6 at pH 7.0. Dissociation of the relatively large subunit organization of this protein in the alkaline region, at pH 10.1 to 10.4, to about a tenth to a twelfth of the original molecular weight was found to have relatively little or no effect on ORD and CD spectra. No increases in the amplitudes and significant shifts in the wavelengths of the 233 and 200 nm ORD and 218, 208, and 193 nm CD band were found, suggesting that spectral distortions are of minor importance regarding the ORD and CD spectra, and estimates of helix content of L. terrestris hemoglobin.

Minimum chain length required for the observation of a temperature-sensitive, salt-sensitive, positive circular dichroism band in derivatives of L-alanine.

Minimum chain length required for the observation of a temperature-sensitive, salt-sensitive, positive circular dichroism band in derivatives of L-alanine.

Reconstructive denaturation of immunoglobulins by sodium dodecyl sulfate: Circular dichroism studies

Circular dichroism (CD) of two individual specimens of human 7-S myeloma IgG immunoglobulins (one of the κ and the other of the λ antigenic type) was measured in the presence and absence of sodium dodecyl sulfate. In the absence of dodecyl sulfate, the κ-type immunoglobulin had a positive CD band at 200 nm and negative bands at 217 and 236 nm, whereas the λ-type globulin had two positive bands at 190–200 nm, a negative peak at 217 nm, and a shoulder at 225–228 nm. In the presence of sodium dodecyl sulfate (mass ratio 0.6–18 g sodium dodecyl sulfate per 1 g protein) the far ultraviolet CD spectra of both proteins were converted into spectra indicating a moderate α-helix content. In the near ultraviolet zone, the CD spectra of the immunoglobulins differed strongly from each other and the changes induced by sodium dodecyl sulfate were complex. With low concentration of sodium dodecyl sulfate, the CD bands of the κ-type protein at 250–290 nm underwent a positive shift, whereas only the strongly negative bands of the λ-type immunoglobulin at 250–280 nm became more positive but the presumably tryptophanyl bands at 286–300 nm shifted toward the negative direction. At high concentrations of dodecyl sulfate (1.6–4 g and more sodium dodecyl sulfate per 1 g protein), the near ultraviolet spectra of both proteins were similar and were characterized by several negative CD bands at 260–290 nm. The transitions were promoted by acid and they were insensitive to moderate changes of ionic strength. The data indicate that sodium dodecyl sulfate caused a profound change in conformation which cannot be qualified as disorganization. The term “reconstructive denaturation” is proposed for these transitions.

Neutral Salt Effect on the Interaction of Poly-α, l-glutamic Acid with Acridine Orange

Abstract The effect of potassium chloride on the absorption and optical rotatory properties of the poly-α,l-glutamic acid-acridine orange system in an aqueous solution was studied under various conditions. The stability of the helical conformation of poly-α,l-glutamic acid in this system was examined at different potassium chloride concentrations by estimating the helix fraction of the polymer. At pH 4.5, where poly-α,l-glutamic acid exists in the helix, it was found that the added potassium chloride raised the stability of the helical conformation of the polymer in this system. In this case, it was assumed that the added potassium chloride lowers the disturbance of helix formation by the bound acridine orange cation. However, when the solution’s pH was raised, the added potassium chloride lowered the extent of the helix fraction in this system. These phenomena were discussed on the basis of several experimental findings on the circular dichroism of the systems. Furthermore, from the change in the intensities of the circular dichroism with the variation in the potassium chloride concentration, it was presumed that the positive circular dichroism band at about 520 nm was due to the binding of dimeric acridine orange molecules to the polymer.

Effect of Chemical Modification of Lysine Residues on the Conformation of Human Immunoglobulin G

Abstract Human IgG (immunoglobulin G) was chemically modified by citraconic anhydride and the conformational transitions were tested by measuring CD (circular dichroism), hydrodynamic properties, and immunoprecipitation reactions. It was found that mild conversion of the lysine e-amino groups (at pH 7.7 and room temperature) into the negatively charged citraconyl groups caused expansion and loss of the β structure of IgG. The positive CD band at 202 nm became negative, while the weak bands in the near-ultraviolet zone changed only slightly; the intrinsic viscosity increased, the sedimentation coefficient decreased, and the immunoresponse against goat antiserum was lost. The citraconylated IgG was reconverted to native IgG by dialysis against 0.05 m acetate of pH 4.0 for 72 hours. The reversal to the native conformation was achieved also by raising the ionic strength to about 1.1. Carbamylation of 88% of the e-amino groups of IgG did not change the CD spectrum, hydrodynamic properties, or immunoprecipitation reaction. The results indicate that the conformational change on citraconylation was caused by electrostatic repulsion of the introduced carboxylate ions into the macromolecules. Moreover, according to these results the e-amino groups of lysine residues are not essential for the immunoprecipitation reaction, whereas the native rigid conformation of the macromolecules is decisive in this reaction.

Magnetic Circular Dichroism of Molecules in Dense Media: Benzene

This paper presents some calculated results on the vibrational structuring of the magnetic circular dichroism of the benzene (1)A(1g)"(1)B(2u) transition. The good agreement between the experimental and calculated relative intensities of the B-bands (positive magnetic circular dichroism bands) indicates that these bands are mainly induced by the e(2g) 1596 cm(-1) mode. The unfavorable results obtained by comparison of the theoretical results with experimental results for the A-bands (negative magnetic circular dichroism bands) is presumably due to neglect of the contribution from the e(2g) 3056 cm(-1) mode, from hot bands, and from the transitions of normal modes other than the A(1g) 992 cm(-1) mode. It should be noticed that while the e(2g) 608 cm(-1) mode plays a dominating role in the ultraviolet spectrum of benzene, in the magnetic circular dichroism of benzene, the e(2g) 1596 cm(-1) mode is, if not more important, at least as important as the e(2g) 608 cm(-1) mode.

Circular dichroism of κ- and λ-type Bence-Jones proteins at various pH values and temperatures

Comparison of the near ultraviolet circular dichroism (CD) spectra of κ- and λ-type Bence-Jones proteins showed differences which, in some cases, were correlated with the proteins' antigenic type. The far ultraviolet (190–250 nm) CD spectra showed that both antigenic types had a negative band at 216–218 nm indicative of β structure. A consistent finding on 5 κ and 7 λ samples was that the positive CD band near 200 nm was resolved into a single band (200–204 nm) for the κ type proteins and into two positive bands (at 190–195 nm and at 197–204 nm) for the λ-type proteins. The positive ellipticity near 200 nm was progressively decreased as the pH was lowered and was replaced by a negative trough at pH 2. Increasing the temperature (25–60 °C) also caused a progressive decrease in the positive ellipticity near 200 nm. The results indicated that acid and heating to 60 °C produced an unfolding of the native structure which was partially reversible. CD spectra showed that heating to 95 °C resulted in irreversible conformational changes which led to an increase in the amount of random chain structure. Physical and immunological characterization of one sample indicated that it was a half ( M r 11 500) of a λ-type Bence-Jones protein corresponding to the constant portion. The changes in the far ultraviolet CD spectrum at various temperatures for this sample were similar to those obtained with intact Bence-Jones proteins. CD studies between 25 and 60 °C were also performed on a κ-type Bence-Jones protein corresponding to the variable half, and it was found that the degree of unfolding was greater for the variable half than for the constant half.

Investigation of xanthine oxidase. Magnetic circular dichroism studies.

The magnetic circular dichroism and circular dichroism spectra of xanthine oxidase are reported from 200–700 nm. The magnetic circular dichroism spectrum exhibits weak bands at 520 and 375 nm which are assigned to the iron‐sulfur and molybdenum chromophores, respectively. The tryptophan chromophore gives a strong positive magnetic circular dichroism band at 292 nm and from the intensity of this band in xanthine oxidase, the presence of a total of 23–24 residues per molecule has been calculated.

Effect of temperature upon the conformations of carboxypeptidase A (Anson), A γLeu, A γVal, and A α+β

1. 1. The conformations of several bovine carboxypeptidase A (peptidyl- l-amino acid hydrolase, EC 3.4.2.1) preparations were compared using the circular dichroism (CD) spectra from 200 to 330 nm. Carboxypeptidase A γ Val, A γ Leu and A α+β (70% A α Leu and 30% A β Val) have nearly identical CD spectra. After considering the origin of the various CD bands, it is suggested that these species have similar backbone conformations and similar environments for their tryptophanyl side chains. 2. 2. The CD spectrum of carboxypeptidase A (Anson) differes in one respect from that of the homogeneous species (A γ Val and A γ Leu). In the Anson preparation the broad, positive CD band between 255 and 270 nm is about twice as intense as that in either carboxypeptidase A γ Val or A γ Leu. The intensification of this CD band in the Anson preparation may possibly be due to an impurity. Alternatively the conformation of the carboxypeptidase A γ molecules may be altered during their isolation from the Anson preparation. 3. 3. The effect of temperature upon the conformations of carboxypeptidase A γ Val, A γ Leu, A α+β and A (Anson) was examined by recording CD spectra between 24 and −196°. The far ultraviolet CD bands of these carboxypeptidase preparations are not altered by cooling, which suggests that the peptide backbone conformation is relatively rigid. In contrast, the CD fine structure arising from the tryptophanyl side chains is greatly intensified upon cooling the various preparations. This behavior results because the tryptophanyl side chains of carboxypeptidase A exist in multiple conformations at 24°. Cooling shifts more side chains into the conformer having the lowest energy. Apparently some side chains of carboxypeptidase A possess appreciable motility at room temperature.

Spectroscopic properties of the hemes of human adult hemoglobin and its subunits

The circular dichroism (CD) and difference spectra of human adult hemoglobin (Hb A) and its subunits in the deoxy-, oxy- and CO-forms are presented. All four absorption bands of the hemes were optically active in the hemoglobins. The positions of the CD bands observed roughly agreed with those of the corresponding absorption bands. Generally, the α-subunit had more positive ellipticity than the β-subunit, and the CD spectrum of Hb A could be roughly composed by taking the arithmetic mean of those of the subunits. Similarly, the difference spectra of the subunits, between oxy- or CO- and deoxy-forms, differed from each other. These facts suggest that the local milieu around the hemes was not the same in the subunits. Human fetal hemoglobin showed the same CD spectrum as Hb A. In the visible region, two additional bands around 520 and 580 mμ were observed in the CD spectra, but these bands were indiscernible in the absorption spectra. A positive CD band around 580 mμ of the deoxy-form appeared in the α-subunit and Hb A but not in the β-subunit. On the basis of the difference spectra, the difference in the absorption of the deoxy-hemes in the separated subunits and in the tetrameric Hb A was confirmed. In the ultraviolet region, the CD peak positions of a band around 310 mμ differed markedly from each other in the subunits, while those of a band around 260 mμ did not. The CD band around 260 mμ of the deoxy-forms was probably degenerated. A negative CD band around 280 mμ was assigned to the aromatic amino acid residues of the protein moiety, and the band was indiscernible in the α-subunit.

Circular dichroism of acridine orange bound to DNA

AbstractThe circular dichroism (CD) spectra of DNA–acridine orange (DNA–AO) complex in the visible region were measured at DNA phosphate‐to‐dye ratios (P/D) from 1 to 550. The CD spectrum of DNA–AO complex in the P/D ratio between 1 and approximately 40 consists of four components, i.e., positive CD bands centered at 510 and 480 mμ, and negative CD bands at 497 and 468 mμ. The CD bands at 510 and 468 mμ are optimum at P/D = 4, and the change of ε1 − εr with P/D suggests that both of them are induced from the interaction between dye molecules bound to adjacent DNA binding sites, each of which is composed of four nucleotides. This is supported by the fact that the values of ε1 − εr for both decrease with increasing temperature or increasing methylene blue concentration added to the complex. The negative Cotton effect at, 497 mμ is most favored at larger P/D ratio (∼8), and the suggested assignment is to the interaction between two dye molecules bound with an empty site between them. A positive Cotton effect at 480 mμ is observed at P/D ratio of less than 4 and is optimum at 1. Above P/D ratio of 40, the CD spectrum of the complex can not be resolved into its components and even at sufficiently high P/D ratio (550) the complex shows a small Cotton effect.

Mercurial-induced circular dichroism changes of phycoerythrin and phycocyanin

1. 1. The effect of a mercurial on circular dichroism (CD) of B-phycoerythrin, R-phycoerythrin and C-phycocyanin has been investigated. 2. 2. B-phycoerythrin exhibits a double CD band centered at the longest wavelength absorption. Upon treatment with a mercurial, the double CD band appears to reverse its sign resulting in a different double CD band shifted toward shorter wavelengths. 3. 3. R-phycoerythrin, having three positive CD bands, lacks the double CD band both before and after mercurial treatment. 4. 4. C-phycocyanin from Anacystis nidulans shows two positive CD bands. The longer wavelength CD band at pH 5 is much greater than the shorter wavelength band. At pH 7, where both bands are of about the same intensity and are well separated, the longer wavelength CD band shifts to a shorter wavelength in the presence of a mercurial. 5. 5. Subsequent addition of mercaptoethanol brings about a partial regeneration of the longest wavelength positive CD band in the case of both R-phycoerythrin and C-phycocyanin but not with mercurial-treated B-phycoerythrin which produces a double CD band of opposite sign. 6. 6. These CD changes are discussed in connection with absorption spectral changes as well as with the dissociation and association aspects of these chromoproteins.