The absorption spectra, magnetic moments and the binding of iron in some haemoproteins

Formation of aryl and aryldiazenyl complexes in reactions of arylhydrazines and aryldiazenes with a synthetic model compound of haemoprotein

The anaerobic reaction of chelated protohaemin, a synthetic model compound of ferrihaemoglobin, with phenyldiazene produced a compound with the visible-absorption spectrum of a ferrihaemochrome. The compound reacted with CN-, which is a ligand of both ferric and ferrous porphyrins, to produce the complex of the synthetic ferrihaemoglobin with CN-. Though the spectrum of the compound formed by the addition of phenyldiazene to chelated protohaemin is characteristic of a ferric porphyrin complex, this compound reacted with both toluene-p-sulphonylmethyl isocyanide and CO, which are strong ligands of ferrous porphyrins, to produce the corresponding ferrous complexes. These ligand-binding reactions indicated that the complex of chelated protohaem with phenyldiazene can behave either as a complex of a ferric porphyrin with phenyldiazenyl anion (C6H5N = N-) or a complex of a ferrous porphyrin with phenyldiazenyl radical (C6H5N = N.). Para substituents on phenyldiazene were without effect on the formation of 4-substituted phenyldiazenyl complexes with chelated protohaem. Ortho substituents resulted in less-stable complexes. The phenyl complex of chelated protohaem was prepared by the aerobic reaction of phenylhydrazine with chelated protohaemin, and its structure was confirmed by its n.m.r. spectrum. The ligand-binding properties, n.m.r. spectrum and absorption spectrum of this complex differed from those of the phenyldiazenyl complex. The phenyl complex also was produced when the phenyldiazenyl complex was exposed to O2.

Spin-state equilibrium in the model complexes of azide hemoprotein

Addition of NaN3 to ferric protohemin biscoordinated with 1-methylimidazole (1-MeIm) or 2-methylimidazole (2-MeIm) in (CH3)2SO resulted in sizeable visible absorption changes, corresponding to the formation of the mixed ligand complexes, hemin X N-3 X 1-MeIm and hemin X N-3 X 2-MeIm. The visible absorption spectrum of the 1-MeIm complex was closely similar to those of azide hemoproteins, while the 2-MeIm derivative exhibited intensified 500 and 625 nm bands and depressed 540 and 570 nm peaks. The iron-bound N-3 of the model complexes exhibited two infrared stretching bands, which were assigned to the high- and low-spin peaks. The intensity of the high-spin infrared peaks increased at higher temperature. From the analyses of the infrared spectral changes, the thermodynamic values of the thermal spin equilibria were determined to be delta H = -3920 cal/mol and delta S = -11.1 e.u. for hemin X N-3 X 1-MeIm and delta H = -2150 cal/mol and delta S = 7.9 e.u. for hemin X N-3 X 2-MeIm. The thermodynamic values of the 1-MeIm complex are similar to the reported values for azide metmyoglobin, suggesting that the contribution from the nonbonded porphyrin-globin contacts to the spin equilibrium is small in azide metmyoglobin. Comparison of the delta H and delta S values among model systems indicates that delta H and delta S compensation similar to that observed in hemoprotein also holds in the models. This may suggest an underlying common denominator for the spin-equilibrium mechanisms in hemins and hemoproteins.

X-ray absorption studies of intermediates in peroxidase activity

The structures of the enzyme-substrate compounds of peroxidases and catalase determined by X-ray absorption spectroscopy are presented. The valence state of the iron in Compounds I and II is determined from the edge to be higher than Fe+3. A short Fe-Ne (proximal histidine) distance is observed in all forms except Compound II, forcing the Fe-Np average distance to be long, a result which differentiates the peroxidases from the oxygen transport hemoproteins and plays a pivotal role in the mechanism. A correlation is shown between the ratio of peaks in the low k (ligand field indicator ratio) region, the Fe-Np (heme pyrrole nitrogen) average distance, and the magnetic susceptibility, which provides a sensitive indicator of spin state. The mechanism of H2O2 reduction is shown by analysis of the structural changes observed in the intermediates. Possible relationship of these compounds to that of the peroxidatic form of cytochrome oxidase is suggested by these results.

Correlations among parameters that describe low-spin ferric heme complexes

The g values from low-spin ferric hemes can be related through the t2g hole model to rhombic (V/lambda) and tetragonal (delta/lambda) ligand field components and to the lowest Kramer's doublet energy (E/lambda). The latter is also a measure of unpaired electron sharing among the iron 3d (t2g) orbitals. For a series of ligands (X), there is a monotonic increase in myoglobin complex (Mb . X) [E/lambda] values with nonheme hexacoordinate metal complex (M . X6) [eg-t2gPg] orbital separations. As the aqueous solution pKa values of the sulfurous or nitrogenous ligands in model heme complexes increase, values of V/lambda and delta/lambda increase linearly, but those of [E/lambda] decrease linearly. The greater the electron-acceptor ability of the ligand, as suggested by its position in the spectrochemical series or its pKa, the more the unpaired electron sharing among the heme t2g orbitals increases. The rate of change of [E/lambda] with V/lambda and the pKa is different with sulfurous and nitrogenous ligands, and the magnitude of both rates increases with two sulfurs less than sulfur and nitrogen less than two nitrogens bound to the heme. The maximum magnitude of this rate with V/lambda for cytochrome P-450 is four times less than that for myoglobin, which may explain, in part, the differences in ligand binding between these two hemeproteins. The perturbation of [E/lambda], V/lambda, and delta/lambda induced by strain of iron-ligand bonds is quantitated for several hemeproteins and heme models. In addition, energy level comparisons suggest that the largest-magnitude g value falls approximately along the iron-chlorin ring normal. This suggestion implies that the electron distribution of the iron at the catalytic sites of cytochrome P-450 and certain chlorin-containing enzymes is in some way similar, but distinct from that at the transport site of myoglobin.

Electron addition to the (FeO2) unit of oxyhaemoglobin Glycera

Exposure of glassy solutions containing the monomeric fraction of the oxyhaemoglobin of the polychaete annelid Glycera dibranchiata to 60Co gamma-rays at 77 K resulted in electron addition to the (FeO2) moiety. The form of the g tensor components obtained from the ESR spectrum indicates that the spin-density on oxygen is much greater than that observed for similar paramagnetic centres formed in haemoglobin A or myoglobin. A major difference between these monomer haem units and normal haem units is that the distal histidine (E7 58) is replaced by leucine. We therefore postulate that the oxygen in the (FeO2)- units formed in haemoglobin A and myoglobin is hydrogen-bonded to the NH group of the distal histidine, whilst that of the (FeO2)- units in haemoglobin Glycera are not hydrogen-bonded. However, on annealing to approx. 160 K the spectrum changed irreversibly into one resembling those for (FeO2)- units in haemoglobin A and myoglobin. We postulate that this is caused by hydrogen-bonding to a water molecule in the haem pocket. Exposure of the polymeric fractions of haemoglobin Glycera to gamma-rays gave an (FeO2)- unit with an ESR spectrum remarkably similar to that obtained from oxymyoglobin. The X-ray structure of this protein is unknown but we suggest that our results could indicate the presence of a distal histidine in this material.

Cytochrome P-450 induction by clofibrate. Purification and properties of a hepatic cytochrome P-450 relatively specific for the 12- and 11-hydroxylation of dodecanoic acid (lauric acid)

Hypolipidaemic drugs induce peroxisomal proliferation in the liver and many induce the formation of the hepatic endoplasmic reticulum in general and the formation of cytochrome P-450 in particular. We have induced the formation of rat liver microsomal cytochrome P-450 by the administration of the hypolipidaemic drug clofibrate, isolated the endoplasmic reticulum, solubilized the cytochrome P-450 from these membranes and subdivided the cytochrome P-450 into four fractions by the use of hydrophobic, anionic, cationic and adsorption chromatography. One of these fractions (cytochrome P-450 fraction 1) was highly purified to a specific content of 17nmol of cytochrome P-450/mg of protein and the protein was active in a reconstituted enzyme system towards the 12- and 11-hydroxylation of the fatty acid, dodecanoic (lauric) acid, with preferential activity towards the 12-hydroxy metabolite. This reconstituted activity was absolutely dependent on NADPH, NADPH-cytochrome P-450 reductase and cytochrome P-450, indicating the role of the mixed-function oxidase system in the metabolism of lauric acid. Another fraction of the haemoprotein (cytochrome P-450 fraction 2) preferentially formed 11-hydroxylauric acid, whereas a third fraction (cytochrome P-450 fraction 3) exhibited only trace laurate oxidase activity and was similar to the phenobarbitone form of the haemoprotein in that these last two cytochromes rapidly turned-over the drug benzphetamine. The molecular weights and spectral properties of these cytochrome P-450 fractions are reported, along with the phenobarbitone-induced form of the enzyme and the nature of the cytochrome(s) induced by clofibrate pretreatment are discussed in the terms of possible haemoprotein heterogeneity.

Spectral properties of myeloperoxidase and its ligand complexes

The effects of ligands with various field strengths on the optical absorption spectrum of myeloperoxidase have been investigated. As is the case with other hemoproteins, the Soret peak in the optical absorption spectra at 77 K moves to longer wavelengths when strong-field ligands are present, whereas binding of such ligands as chloride and fluoride, which stabilize the high-spin state, shows the opposite effect. With a ligand of intermediate field strength, such as azide, the optical spectrum is not affected at room temperature, but lowering of the temperature results in the formation of the low-spin form of the enzyme. Similarly, in native myeloperoxidase a spin state equilibrium is found in which the low-spin state is favoured at high ionic strength and displays corresponding changes in the optical spectra. From the ligand- and the temperature-induced changes in the optical spectra of the ferric enzyme it is concluded that the band at 620-630 nm is an alpha band of the low-spin heme iron species, whereas the bands at 500 and 690 nm are probably 'charge-transfer' bands of the heme with the iron in the high-spin state.

Structure and heme environment of beef liver catalase at 2.5 A resolution

Most of the amino acid side chains of beef liver catalase were clearly identifiable in the 2.5 A resolution electron-density map, and the results are in good agreement with the sequence [Schroeder, W. A., Shelton, J. R., Shelton, J. B., Roberson, B. & Apell, G. (1969) Arch. Biochem. Biophys. 131, 653-655]. The tertiary structure of one subunit consists of a large antiparallel beta-pleated sheet domain with helical insertions, followed by a smaller domain containing four alpha-helices. The heme group is buried at least 20 A below the molecular surface and is accessible by a channel lined with hydrophobic residues. The proximal ligand is tyrosine-357, while histidine-74 and asparagine-147 re the important residues on the distal side of the heme. The inhibitor 3-amino-1,2,4-triazole, which has been shown to covalently bond to histidine-74, can be built into the heme cavity with its N(2) atom coordinated to the heme iron.

A non-equilibrium state of deoxyhaemoglobin. Temperature-dependence and oxygen binding

After reduction of human methaemoglobin by solvated electrons a non-equilibrium low-spin state of deoxyhaemoglobin is formed which has the characteristic haemochrome spectrum. This haemochrome state is ascribed to a weakly 6-coordinated structure of the haem, which is stabilised by the protonated distal histidine. Oxygen binding is not inhibited by the presence of the weak interaction in the haemochrome state. From the pH dependence of the biphasic behaviour of the oxygen binding a pK of about 8.8 is obtained which is ascribed to the deprotonation of the distal histidine which is in the proximity of a negative ion. A model is proposed to explain the complex spin-equilibria observed in methaemoglobin. The enthalpy of activation of the decay of the haemochrome state is about 53 kJ x mol(-1) and increases to 90 kJ x mol(-1) in the presence of 1 M methanol, indicating a strong interaction between methanol and haemoglobin. Around pH 8.4 the rate constant of the binding of oxygen to the haemochrome state is so high that it may well be diffusion controlled.

Solution studies on heme proteins. Circular dichroism and optical rotation of Glycera dibranchiata hemoglobins

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of several liganded derivatives of the monomer and polymer hemoglobin components of the marine annelid, Glycera dibranchiata were measured over the wavelength range 650--195 nm. The differences observed between the monomer and polymer components for the heme dichroic bands in the visible, Soret and ultraviolet wavelength regions seem to result from changes in the heme environment, geometry and coordination state of the central heme iron in these proteins. Within the Soret region, the liganded derivatives of the monomer hemoglobin exhibit predominantly negative circular dichroic bands. The heme band at 260 nm is also absent for the monomer hemoglobin. The ORD and CD spectra in the far-ultraviolet, peptide absorbing region suggest also differences in the alpha-helix content of the monomer and polymer hemoglobins. The values for the single-chain G. dibranchiata hemoglobin are in the expected range (about 70% alpha-helix) as predicted by the X-ray structure of this protein. The lower estimates of the alpha-helix content for the polymer hemoglobin (approx. 50%), may reflect the differences in amino acid composition, primary structure and polypeptide chain foldings. Changes in oxidation state and ligand binding appears to have no pronounced effect on the helicity of either the monomer or polymer hemoglobins. The removal of the heme moiety from the monomer hemoglobin did result in a major decrease in its helix content similar to the loss of heme from myoglobin.

Structure of milk lactoperoxidase. A study using circular dichroism and difference absorption spectroscopy

Circular dichroism spectra of milk lactoperoxidase, its fluoride and cyanide derivatives, and those of ferrous lactoperoxidase and its carbonyl and cyanide compounds, were recorded in the wavelength region 200-670 nm. All derivatives have split ellipticity bands, suggesting that lactoperoxidase has a narrow heme pocket that prevents ligands forming linear iron-ligand bonds. Difference absorption spectroscopy of the enzyme in the far-ultraviolet region supports the previously held view that the fifth ligand of the heme iron is histidine. The secondary structure of lactoperoxidase, calculated from the far-ultraviolet circular dichroism spectrum, contains 65% beta-structure, 23% alpha-helix and 12% unordered structure. Reduction of lactoperoxidase with dithionite gives two forms, indicating that after reduction some compound arising from dithionite binds in the vicinity of the heme iron.

The interaction of aliphatic analogs of methylene-dioxyphenyl compounds with cytochromes P-450 and P-420

The spectra resulting from the interaction of a series of substituted dioxolanes with microsomal cytochromes P-450 or P-420, as well as purified cytochrome P-450, were measured. With the exception of dioxolane, 4-methyldioxolane and 4-ethyldioxolane, these compounds interacted with ferric cytochrome P-450 to give complexes exhibiting type I optical difference spectra, and, after incubation with NADPH, spectra with peaks at about 430 nm. These complexes, as well as those formed from dioxolanes in the presence of cumene hydroperoxide, inhibit the binding of CO to the cytochrome. Consideration of the known chemistry of dioxolanes, together with recent advances in the understanding of double Soret spectra, lead to a possible explanation for the differences between the spectra of dioxolanes and their aromatic analogs, the methylenedioxyphenyl compounds.

A spin label study of horseradish peroxidase

The topography of the active sites of native horseradish peroxidase and manganic horseradish peroxidase has been studied with the aid of a spin-labeled analog of benzhydroxamic acid (N-(1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxy)-p-aminobenzhydroxamic acid). The optical spectra of complexes between the spin-labeled analog of benzhydroxamic acid and Fe3+ or Mn3+ horseradish peroxidase resembled the spectra of the corresponding enzyme complexes with benzhydroxamic acid. Electron spin resonance (ESR) measurement indicated that at pH 7 the nitroxide moiety of the spin-labeled analog of benzhydroxamic acid became strongly immobilized when this label bound to either ferric or manganic horseradish peroxidase. The titration of horseradish peroxidase with the spin-labeled analog of benzhydroxamic acid revealed a single binding site with association constant Ka approximately 4.7 . 10(5) M-1. Since the interaction of ligands (e.g. F-, CN-) and H2O2 with horseradish peroxidase was found to displace the spin label, it was concluded that the spin label did not indeed bind to the active site of horseradish peroxidase. At alkaline pH values, the high spin iron of native horseradish peroxidase is converted to the low spin form and the binding of the spin-labeled analog of benzhydroxamic acid to horseradish peroxidase is completely inhibited. From the changes in the concentration of both bound and free spin label with pH, the pK value of the acid-alkali transition of horseradish peroxidase was found to be 10.5. The 2Tm value of the bound spin label varied inversely with temperature, reaching a value of 68.25 G at 0 degree C and 46.5 G at 52 degrees C. The dipolar interaction between the iron atom and the free radical accounted for a 12% decrease in the ESR signal intensity of the spin label bound to horseradish peroxidase. From this finding, the minimum distance between the iron atom and nitroxide group and hence a lower limit to the depth of the heme pocket of horseradish peroxidase was estimated to be 22 A.

Circular dichroism studies on cytochrome c peroxidase and cytochrome c-551 of Pseudomonas aeruginosa

Circular dichroism (CD) spectra of ferric, ferrous and ferrous-carbonyl forms of Pseudomonas cytochrome c peroxidase have been recorded in the wave length range 200 to 650 nm. CD spectra in the Soret region show that in the oxidized enzyme the two hemes are degenerate, whereas in the reduced form the hemes are perturbed differently and one of the hemes appears to be non-degenerate. Changes in optical activity upon formation of the carbonylderivative suggest a spin-state conversion and indicate the presence of one high-spin and a low-spin heme. A histidine residue is proposed for the axial ligand of the heme iron. The alpha-helical content of the enzyme is estimated to be 34%. Ligand binding or changes in the oxidation state of the heme iron do not alter the conformation of the protein backbone. The dichroic spectra of oxidized and reduced cytochrome c-551 (P. aeruginosa) are included for comparison. In the visible region the cytochrome exhibits CD spectra similar to those of the peroxidase, whereas in the Soret region the dichroic spectra of the cytochrome are simpler. CD spectra in the far-ultraviolet region show the cytochrome to have a high alpha-helix content.

Crystalline state disorder and hyperfine component line widths in ferric hemoglobin chains

In X-band electron paramagnetic resonance spectra from single crystals of horse ferric hemoglobin, observed line widths at the low- and high-field extrema are 30 and 24 g, and as much as 400 G in the intermediate region. This behavior is similar to that of ferric myoglobin. Due to large anisotropy in the g-tensors, the line width variation can be accounted for on the basis of heme orientation disorder. This disorder is characterized by an angle, determined here by two independent methods. In these computations Gaussian disorder on a sphere is assumed. The disorder angle is found to be constant on the sphere and about 4 degrees for both alpha- and beta- chains. Treatment of crystals with heavy water (buffer) increases the disorder. Since ligand nitrogen hyperfine couplings are available from hemoglobin electron nuclear double resonance, single crystal electron paramagnetic resonance spectra can be simulated by superimposing hyperfine bands, where the line width of the component bands is a variable and the disorder model above is employed. Comparison with observed resonances fixes the hyperfine component line widths. These component line widths from ferric hemoglobin in the crystalline state are found to be smaller than those in frozen solution.

Circular dichroism studies on cytochrome c peroxidase from baker's yeast (Saccharomyces cerevisiae)

Circular dichroism spectra of cytochrome c peroxidase from baker's yeast, those of the reduced enzyme, the carbonyl, cyanide and fluoride derivatives and the hydrogen peroxide compound, Compound I, have been recorded in the wavelength range 200 to 660 nm. All derivatives show negative Soret Cotton effects. The results suggest that the heme group is surrounded by tightly packed amino acid sidechains and that there is a histidine residue bound to the fifth coordination site of the heme iron. The native ferric enzyme is probably pentacoordinated. The circular dichroism spectra of the ligand compounds indicate that the ligands form a nonlinear bond to the heme iron as a result of steric hindrance in the vicinity of the heme. The spectrum of Compound I shows no perturbation of the porphyrin symmetry. The dichroic spectrum of the native enzyme in the far-ultraviolet wave-length region suggests that the secondary structure consists of roughly equal amounts of alpha-helical, beta-structure and unordered structure. After the removal of the heme group no great changes in the secondary structure can be observed.

Complexes of trivalent oxygenated phosphorus compounds with cytochrome P-450 and cytochrome P-420: the origin of double Soret spectra

Trivalent oxygenated phosphorus ligands include alkyl and aryl phosphites, (RO)3P, phosphonites, (RO)2PR, and phosphinites, ROPR2. All such compounds tested, with the exception of triphenyl phosphite, interact with ferrous cytochrome P-450 and its denatured form, cytochrome P-420, to produce complexes having two peaks in the Soret region of their optical difference spectra. Careful evaluation of these spectra indicate that they arise for different reasons for each of the two cytochromes. Clear evidence shows that cytochrome P-450 is not denatured by these ligands. The high affinity of these ligands for heme iron is indicated by small Ks values. The experimental results are used to substantiate a theory of the origin of microsomal double Soret spectra and the nature of the environments available for microsomal cytochromes P-450 and P-420.

Binding of thiols to microsomal cytochrome P-450

Lipophilic thiol compounds interact spectrally with liver microsomes from phenobarbital-pretreated rats by formation of unusual optical difference spectra with peaks at 378, 471, 522 and 593 nm in the oxidized state. The binding kinetics were biphasic. The EPR spectrum of cytochrome P-450 was slightly modified but the magnitude of the low-spin signal was unchanged. n-Octanethiol competitively displaced metyrapone and n-octane from the active site of cytochrome P-450. Other thiols behaved similarly with variations in the magnitude and the affinity of the binding process. Tertiary thiols caused the formation of the high-spin cytochrome P-450 substrate complex, and model studies with myoglobin revealed that steric hindrance prevented the liganding of the tertiary thiol group to the ferric cytochrome P-450. Addition of thiols to dithionite reduced microsomes resulted in relatively small spectral changes with maxima at 449 nm typical for ligand complexes of the ferrous cytochrome. It was concluded that lipophilic thiols can be bound as ligands by at least two species of oxidized cytochrome P-450 which represent, however, not more than about one fifth of the total cytochrome P-450 content in liver microsomes from phenobarbital-pretreated rats.

Methionine sulfoxide cytochrome c

Cytochrome c has been chemically modified by methylene blue mediated photooxidation. It is established that the methionine residues of the protein have been specifically converted to methionine sulfoxide residues. No oxidation of any other amino acid residues or the cysteine thioether bridges of the molecule occurs during the photooxidation reaction. The absorbance spectrum of methionine sulfoxide ferricytochrome c at neutrality is similar to that of the unmodified protein except for an increase in the extinction coefficient of the Soret absorbance band and for the complete loss of the ligand sensitive 695 nm absorbance band in the spectrum of the derivative. The protein remains in the low spin configuration which implies the retention of two strong field ligands. Spin state sensitive spectral titrations and model studies of heme peptides indicate that the sixth ligand is definitely not provided by a lysine residue but may be methionine-80 sulfoxide coordinated via its sulfur atom. Circular dichroism spectra indicate that the heme crevice of methionine sulfoxide ferri- and ferrocytochrome c is weakened relative to native cytochrome c. The redox potential of methionine sulfoxide cytochrome c is 184 mV which is markedly diminished from the 260 mV redox potential of native cytochrome c. The modified protein is equivalent to native cytochrome c as a substrate for cytochrome oxidase and is not autoxidizable at neutral pH but is virtually inactive with succinate-cytochrome c reductase. These results indicate that the major role of the methionine-80 in cytochrome c is to preserve a closed hydrophobic heme crevice which is essential for the maintainance of the necessary redox potential.

Proton magnetic resonance studies of peroxidases from turnip and horseradish

Proton NMR spectra at 270 MHz have been measured for horseradish peroxidase and turnip peroxidase isoenzymes (P1, P2, P3 and P7) in both their high spin ferric native states and as the low spin ferric cyanide complexes. Resonances of amino acids near the heme have been identified and used to investigate variations in the structure of the heme crevice amongst the enzymes. Ligand proton resonances have been resolved in spectra of the cyanide complexes of the peroxidases and these provide information on the heme electronic structure. The electronic structure of the heme and the tertiary structure of the heme crevice are essentially the same in the acidic turnip isoenzymes, P1, P2 and, to a lesser extent, P3 but differ in the basic turnip enzyme, P7. The heme electronic structure and nature of the iron ligands in peroxidases are discussed. Further evidence is presented for histidine as the proximal ligand. A heme-linked ionizable group with a pK of 6.5 has been detected by NMR in the cyanide complex of horseradish peroxidase.

Circular dichroism of soybean leghemoglobin

Circular dichroic (CD) spectra of soybean leghemoglobin, and some of its liganded derivatives were measured over the wavelength range of 650 to 200 nm. The heme-related circular dichroic bands in the visible, Soret and ultraviolet wavelength regions exhibit Cotton effects characteristic of each of the compounds examined. The positions of the dichroic bands vary with ligand substitutions and the oxidation state of the iron. All leghemoglobin derivatives, except the apoprotein, exhibit negative circular dichroic bands in the region of Soret absorption. In this region the optical activity of compounds with high-spin moments is greater than that of compounds with low or intermediate spin moments. The ellipticity of the heme band at about 260 nm is also altered by ligand binding and spin state. The dichroic spectra in the far-ultraviolet region indicated a high extent of alpha-helical structure (about 70%) in the native leghemoglobin and its liganded derivatives. The helicality of the apoprotein seems to diminish suggesting a decrease caused by the removal of the heme.