Elucidation of the mode of binding of oxygen to iron in oxyhemoglobin by in frared spectroscopy

Storage conditions determine the characteristics of red blood cell derived extracellular vesicles

Extracellular vesicles (EVs) are released during the storage of red blood cell (RBC) concentrates and might play adverse or beneficial roles throughout the utilization of blood products (transfusion). Knowledge of EV release associated factors and mechanism amends blood product management. In the present work the impact of storage time and medium (blood preserving additive vs isotonic phosphate buffer) on the composition, size, and concentration of EVs was studied using attenuated total reflection infrared (ATR-IR) spectroscopy, microfluidic resistive pulse sensing (MRPS) and freeze-fraction combined transmission electron micrography (FF-TEM). The spectroscopic protein-to-lipid ratio based on amide and the C-H stretching band intensity ratio indicated the formation of various vesicle subpopulations depending on storage conditions. After short storage, nanoparticles with high relative protein content were detected. Spectral analysis also suggested differences in lipid and protein composition, too. The fingerprint region (from 1300 to 1000 cm^-1) of the IR spectra furnishes additional information about the biomolecular composition of RBC-derived EVs (REVs) such as adenosine triphosphate (ATP), lactose, glucose, and oxidized hemoglobin. The difference between the vesicle subpopulations reveals the complexity of the REV formation mechanism. IR spectroscopy, as a quick, cost-effective, and label-free technique provides valuable novel biochemical insight and might be used complementary to traditional omics approaches on EVs.

Biomimetic O2 adsorption in an iron metal-organic framework for air separation

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications. Here, we report an iron metal–organic framework, Fe-BTTri (Fe₃[(Fe₄Cl)₃(BTTri)₈]₂·18CH₃OH, H₃BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), that binds O₂ in a manner similar to hemoglobin and therefore results in highly selective O₂ binding. As confirmed by gas adsorption studies and Mössbauer and infrared spectroscopy data, the exposed iron sites in the framework reversibly adsorb substantial amounts of O₂ at low temperatures by converting between high-spin, square-pyramidal Fe(ii) centers in the activated material to low-spin, octahedral Fe(iii)–superoxide sites upon gas binding. This change in both oxidation state and spin state observed in Fe-BTTri leads to selective and readily reversible O₂ binding, with the highest reported O₂/N₂ selectivity for any iron-based framework.

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications.

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO₂}⁸ active sites. The electronic structure of these sites has been the subject of much debate. This study utilizes Fe K-edge X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO₂}⁸ model compound, [(pfp)Fe(1-MeIm)(O₂)] (pfp = meso-tetra(α,α,α,α-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO₂), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO₂ and oxyhemoglobin are compared with the data for low-spin Fe^II and Fe^III [Fe(tpp)(Im)₂]^0/+ (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO₂ is similar to Fe^II, while oxyhemoglobin is qualitatively similar to Fe^III, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO₂ and oxyhemoglobin is due to a distal histidine H bond to O₂ and the less hydrophobic environment in the protein, which lead to more backbonding into the O₂ A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO₂ is dominantly Fe^II with 6-8% Fe^III character, while oxyhemoglobin has a very mixed wave function that has 50-77% Fe^III character and a partially polarized Fe-O₂ π-bond.

Time-resolved SERS study of the oxygen reduction reaction in ionic liquid electrolytes for non-aqueous lithium–oxygen cells

We use the Raman active bands of O₂˙⁻ to probe its changing Lewis basicity through its interaction with various ionic liquid electrolytes at the electrode surface.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Iron L-edge X-ray absorption spectroscopy of oxy-picket fence porphyrin: experimental insight into Fe-O2 bonding

The electronic structure of the Fe-O(2) center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multideterminant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O(2) heme complex [Fe(pfp)(1-MeIm)O(2)] (pfp ("picket fence porphyrin") = meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of σ-donor, π-donor, and π-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O(2)] is further compared to those of [Fe(II)(pfp)(1-MeIm)(2)], [Fe(II)(pfp)], and [Fe(III)(tpp)(ImH)(2)]Cl (tpp = meso-tetraphenylporphyrin) which have Fe(II)S = 0, Fe(II)S = 1, and Fe(III)S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O(2) pfp site is experimentally determined to have both significant σ-donation and a strong π-interaction of the O(2) with the iron, with the latter having implications with respect to the spin polarization of the ground state.

Room-temperature magnetic properties of oxy- and carbonmonoxyhemoglobin

The magnetic susceptibility and the density of human oxy-(HbO(2)) and carbonmonoxyhemoglobin (HbCO) solutions of various concentrations have been measured at room temperature, with pure water used as a calibrant. Solutions of unstripped and stripped HbO(2) at pH 7.2 in unbuffered water solvent were always found to be less diamagnetic than pure water, whereas solutions of HbCO in identical conditions were always found to be more diamagnetic than pure water. After correcting for concentration-dependent density changes and assuming the HbCO samples to be fully diamagnetic, the paramagnetic reduction of the diamagnetic susceptibility of HbO(2) corresponds to a molar susceptibility per heme (chi(M) (heme)) of 2460 +/- 600 x 10(-6) cgs/mol.

A single assay for multiple storage-sensitive red blood cell characteristics by means of infrared spectroscopy

BACKGROUND

To maintain a high quality of red blood cells (RBCs), RBC characteristics must be followed during storage under blood bank conditions. By means of infrared (IR) spectroscopy, several characteristics can be measured simultaneously.

STUDY DESIGN AND METHODS

IR spectra were acquired for samples from RBCs that were collected and stored according to Dutch blood bank procedures for a period of up to 50 days. Spectra of the soluble cell components were acquired separately after hypotonic lysis of the cells, followed by centrifugation. Characteristic vibrational bands were analyzed with respect to storage time-dependent changes in peak position and in intensity.

RESULTS

A decrease in corresponding peak intensities indicates that RBCs lose protein and lipid during storage. Changes in protein secondary structure during storage are largely confined to integral membrane proteins and membrane-associated proteins. A concurrent decrease in lipid packing density probably reflects the gradual change in cellular shape from discoidal to globular. By integration over a narrow range, storage-dependent changes in intracellular adenosine triphosphate (ATP) and glucose levels could be estimated. ATP levels decrease during storage, but stay above the required 75% of the initial level after 35 days of storage. Glucose concentrations stay well above 5 mmol/L over the entire storage period.

CONCLUSION

IR spectroscopy is a promising technique to follow structural and metabolic changes in RBCs during storage under blood bank conditions. Several variables can be determined rapidly in a single measurement.

Phytol-modified heme in mesoporous silica: Conjugates as models of hemoproteins

A ferriprotoporphyrin, hemin (Fe(3+)), modified with 3,7,11,15-tetramethyl-2-hexadecen-1-ol, phytol, was adsorbed in nano-spaces of about 4 nm in diameter in mesoporous silica (FSM; folded-sheet mesoporous material) forming a phytol-modified hemin (Fe(3+))-FSM nano-conjugate. The properties and the structure of the conjugate were studied by UV-visible light absorption, IR absorption spectroscopy, and a nitrogen adsorption isotherm. Although the hemin without phytol could not be adsorbed to the mesoporous silica, modification with phytol imparted preferential adsorption properties. The conjugate was not only stable but also had a peroxidase-like activity in a 0.1% hydrogen peroxide solution, while free hemin in the solution was easily destroyed. The hemin (Fe(3+)) in the FSM was reduced to heme (Fe(2+)) by hydrazine. The phytol-modified heme (Fe(2+))-FSM conjugate formed an O(2)-heme complex with a superoxide type structure, resembling oxyhemoglobin or oxymyoglobin, which has not been previously observed for free heme in solution. The addition of carbon monoxide or nitrogen monoxide to the phytol-modified heme (Fe(2+))-FSM conjugate caused the formation of CO- or NO-heme complex in the nano-spaces of the FSM. These properties are attributed not only to the Fe-complex but also to the cooperative functions of the heme with mesoporous silica, resembling properties of a natural heme-protein conjugate; hemoglobin or peroxidase. These results are an elegant example of biomimetic nano-technology.

Ligand binding properties of myoglobin reconstituted with iron porphycene: unusual O2 binding selectivity against CO binding

Sperm whale myoglobin, an oxygen storage hemoprotein, was successfully reconstituted with the iron porphycene having two propionates, 2,7-diethyl-3,6,12,17-tetramethyl-13,16-bis(carboxyethyl)porphycenatoiron. The physicochemical properties and ligand bindings of the reconstituted myoglobin were investigated. The ferric reconstituted myoglobin shows the remarkable stability against acid denaturation and only a low-spin characteristic in its EPR spectrum. The Fe(III)/Fe(II) redox potential (-190 mV vs NHE) determined by the spectroelectrochemical measurements was much lower than that of the wild-type. These results can be attributed to the strong coordination of His93 to the porphycene iron, which is induced by the nature of the porphycene ring symmetry. The O2 affinity of the ferrous reconstituted myoglobin is 2600-fold higher than that of the wild-type, mainly due to the decrease in the O2 dissociation rate, whereas the CO affinity is not so significantly enhanced. As a result, the O2 affinity of the reconstituted myoglobin exceeds its CO affinity (M' = K(CO)/K(O2) < 1). The ligand binding studies on H64A mutants support the fact that the slow O2 dissociation of the reconstituted myoglobin is primarily caused by the stabilization of the Fe-O2 sigma-bonding. The IR spectra for the carbon monoxide (CO) complex of the reconstituted myoglobin suggest several structural and/or electrostatic conformations of the Fe-C-O bond, but this is not directly correlated with the CO dissociation rate. The high O2 affinity and the unique characteristics of the myoglobin with the iron porphycene indicate that reconstitution with a synthesized heme is a useful method not only to understand the physiological function of myoglobin but also to create a tailor-made function on the protein.

Crystal structures of myoglobin-ligand complexes at near-atomic resolution

We have used x-ray crystallography to determine the structures of sperm whale myoglobin (Mb) in four different ligation states (unligated, ferric aquomet, oxygenated, and carbonmonoxygenated) to a resolution of better than 1.2 A. Data collection and analysis were performed in as much the same way as possible to reduce model bias in differences between structures. The structural differences among the ligation states are much smaller than previously estimated, with differences of <0.25 A root-mean-square deviation among all atoms. One structural parameter previously thought to vary among the ligation states, the proximal histidine (His-93) azimuthal angle, is nearly identical in all the ferrous complexes, although the tilt of the proximal histidine is different in the unligated form. There are significant differences, however, in the heme geometry, in the position of the heme in the pocket, and in the distal histidine (His-64) conformations. In the CO complex the majority conformation of ligand is at an angle of 18 +/- 3 degrees with respect to the heme plane, with a geometry similar to that seen in encumbered model compounds; this angle is significantly smaller than reported previously by crystallographic studies on monoclinic Mb crystals, but still significantly larger than observed by photoselection. The distal histidine in unligated Mb and in the dioxygenated complex is best described as having two conformations. Two similar conformations are observed in MbCO, in addition to another conformation that has been seen previously in low-pH structures where His-64 is doubly protonated. We suggest that these conformations of the distal histidine correspond to the different conformational substates of MbCO and MbO(2) seen in vibrational spectra. Full-matrix refinement provides uncertainty estimates of important structural parameters. Anisotropic refinement yields information about correlated disorder of atoms; we find that the proximal (F) helix and heme move approximately as rigid bodies, but that the distal (E) helix does not.

Interaction of Derivatized Capped Iron(II) Porphyrin Complexes with CO and O(2)

A series of porphyrins strapped and capped by benzene and amidobenzene rings have been prepared. O(2) and CO bindings to their iron(II) complexes have been examined, and the role of hydrogen-bonding in stabilizing O(2) binding has been measured. Each comparison between the benzene ring (no H-bonding) and the amidobenzene ring analogues showed a free energy gain of approximately 1 kcal/mol (at -45 degrees C) for the amidobenzene derivatives. An X-ray structure analysis was carried out for the ferric (Cl(-)) complex of the benzene-capped porphyrin strapped by two butyl side chains. The crystals were monoclinic, with a = 10.557(3) Å, b = 31.290(5) Å, c = 11.221(3) Å, beta = 104.62(2) degrees, Z = 4, and space group P2(1)/n. The structure was solved by the Patterson method and was refined by full-matrix least-squares procedures to R = 0.040 (R(w) = 0.041) for 3844 reflections with I >/= 3sigma(F(2)).

Lipid microsphere containing lipophilic heme: preparation and oxygen transportation under physiological conditions

Lipophilic heme (1-laurylimidazole-ligated 5,10,15,20-tetrakis(alpha, alpha, alpha, alpha-o- pivalamidophenyl)porphinatoiron(II) complex) is solubilized in lipid (triglyceride) at high concentrations and emulsified with a phospholipid in physiological salt solution, giving a deeply red-colored suspension of lipid microspheres (approx. 250 nm in diameter). The heme forms an oxygen adduct in a similar manner as oxyhemoglobin and the lipid microspheres take up and release oxygen reversibly at 37 degrees C in the aqueous medium. The oxygen-transporting ability is comparable with that of the red blood cell. Intravenous injection of the heme/lipid microsphere solution to rabbits demonstrates that it transports oxygen even in vivo and that it is cleared from the blood stream with a half-life time of approx. 1 h.

Structural transitions upon ligand binding in a cooperative dimeric hemoglobin

Comparison of the 2.4 angstrom resolution crystal structures of dimeric clam hemoglobin in the deoxygenated and carbon-monoxide liganded states shows how radically different the structural basis for cooperative oxygen binding is from that operative in mammalian hemoglobins. Heme groups are in direct communication across a novel subunit interface formed by the E and F helices. The conformational changes at this interface that accompany ligand binding are more dramatic at a tertiary level but more subtle at a quaternary level than those in mammalian hemoglobins. These findings suggest a cooperative mechanism that links ligation at one subunit with potentiation of affinity at the second subunit.

Increase of the Fe effective charge in hemoproteins during oxygenation process

The x-ray absorption near edge structure (XANES) spectra of hemoglobin and myoglobin have been measured at the wiggler beam line of the Frascati Synchrotron Radiation Facility. The energy shifts of the iron absorption jump edge and the chemical shifts of the bound excited state at threshold of 1s core excitations, going from deoxygenated to oxygenated form, are interpreted as evidence of some increase of the positive effective charge on the iron atom upon oxygenation.

Calorimetric studies of oxyhemoglobin dissociation. II. Erythrocytic oxygen depletion by sodium dithionite

Dithionite causes the depletion of dioxygen from suspensions of erythrocytes by reduction of the external dioxygen and not by diffusion into the cell. The molar enthalpy for the reduction shows a small difference with respect to the values found for free hemoglobin; and the normal stoichiometry of 2 moles dithionite/mole dioxygen found there is not observed with erythrocytes. At low hematocrit, the stoichiometry is 2.6:1 and decreases to 1.5:1 at high hematocrit. The change is not due to differences in the hemoglobin saturation or to an inability of dithionite to reduce all dioxygen present at the higher hematocrit. Neither catalase nor peroxidase added to the extracellular volume significantly alters the stoichiometry or the enthalpy of dioxygen reduction by dithionite. Addition of superoxide dismutase, however, restores the normal stoichiometry at high hematocrit and further increases the stoichiometry at low hematocrit. The calorimetrical signal of hydrogen peroxide, clearly seen with free dioxygen, is not present with erythrocytes. In all these cases the total heat evolved is the same.

Active site structures of deoxyhemerythrin and oxyhemerythrin

The physiologically active forms of the nonheme-iron, oxygen-transport protein hemerythrin have been studied by x-ray crystallographic techniques. At 3.9-A resolution, a difference electron-density map between the deoxy form and met form (methemerythrin) of the protein suggests only small differences in the binuclear iron complexes. The coordination of the iron atoms appears to be the same in both the deoxy and met forms, one iron of the complexes being pentacoordinate, the other iron being hexacoordinate. The iron atoms appear to be somewhat farther apart in the deoxy form. A 2.2-A resolution study of oxyhemerythrin shows that dioxygen binds to one iron atom--the pentacoordinate one in the met form of the protein, the same binding site found for azide in azidomethemerythrin.

Resonance Raman studies of Co-O2 and O-O stretching vibrations in oxy-cobalt hemes

Strong evidence suggests that the stretching vibration of the bound oxygen can be perturbed by an accidentally degenerate porphyrin ring mode, resulting in two split frequencies. In the Co(II)(TpivPP) (pyridine) (18)O(2) complex, we demonstrate that the nu((18)O-(18)O) mode, after being shifted from its nu((16)O-(16)O) value at 1,156 cm(-1), undergoes a resonance interaction with the 1,080 cm(-1) porphyrin mode, giving rise to two lines at 1,067 and 1,089 cm(-1). In the O(2) complex of Co(II) mesoporphyrin IX-substituted sperm whale myoglobin, we observed a dramatic intensity increase at 1,132 cm(-1) upon (16)O(2) --> (18)O(2) substitution, which is due to the reappearance of the 1,132-cm(-1) porphyrin mode after the removal of resonance conditions. A decrease in O(2) binding affinity, caused by the proximal base tension, corresponds to an increase in the Co-O(2) stretching frequency. The nu(Co-O(2)) at 527 cm(-1) for the low affinity Co(II)(TpivPP)(1,2-Me(2)Im) O(2) complex is 11 cm(-1) higher than the 516-cm(-1) value for the high affinity complex (with N-MeIm replacing 1,2-Me(2)Im). However, in the corresponding iron complexes the reverse behavior is observed, i.e., the nu(Fe-O(2)) decreases for the (1,2-Me(2)Im) complex. There is a 24-cm(-1) difference in the Co-O(2) stretching frequencies between Co(II)(TpivPP)(N-MeIm)O(2) (at 516 cm(-1)) and oxy meso CoMb (at 540 cm(-1)), suggesting a protein induced distortion of the Co-O-O linkage. However, the values for nu(Fe-O(2)) are nearly identical between Fe(II)(TpivPP)(N-MeIm)O(2) (at 571 cm(-1)) and oxy Mb (at 573 cm(-1)), indicating that O(2) binds to myoglobin in the same manner as in the sterically unhindered "picket fence" complex. Evidence is presented that suggests the presence of two dioxygen stretching frequencies due to two different conformers in each of the N-MeIm and 1,2-Me(2)Im complex of oxy Co(II)(TpivPP).

Evidence for hydrogen bonding of bound dioxygen to the distal histidine of oxycobalt myoglobin and haemoglobin

The origin of the differences in oxygen binding energy in various haemoglobins and myoglobins has long been debated. Perutz proposed that the haem-coordinated histidine (proximal histidine) strains the haem iron in low affinity globins but relaxes it in high affinity globins. The existence of such tension in T-structure deoxyhaemoglobin (deoxyHb) was recently confirmed by electron paramagnetic resonance (EPR), resonance Raman and NRM spectroscopy. Although its contribution to the free energy of cooperativity is insignificant in the deoxy state, the tension at the haem is considered to be approximately 1 kcal mol-1 for the ligated form in which the haem iron moves into the porphyrin plane. The remaining free energy is probably stored in other parts of the molecule. Therefore, a study of the stabilization mechanisms of the oxygenated form became increasingly important. A hydrogen bond between the bound oxygen and the distal histidine has been proposed by Pauling; this would be expected to stabilize the oxy form of the protein and could contribute to the regulation of the oxygen affinity through the oxygen dissociation rate. A series of EPR and functional studies on various cobalt-substituted monomeric haemoglobins and myoglobins suggested the presence of such hydrogen bonding and it has recently been established in crystals of oxy iron myoglobin (oxyFeMb) and in oxyhaemoglobin. Here we present resonance Raman spectra of the oxy forms of cobalt--porphyrin-substituted myoglobin and haemoglobin (CoMb and CoHb) recorded in buffered H2O and D2O solutions at 406.7 nm excitation. Only the Raman lines corresponding to the O-O stretching mode of the bound oxygen, appearing near 1,130 cm-1, are shifted (2-5 cm-1) replacement of H2O by D2O; no other vibrations, including the Co--O2 stretching mode, exhibit any frequency shifts. This indicated that the bound oxygen in oxyCoMb and in both subunits of oxyCoHb interacts with the adjacent exchangeable proton, and confirms the formation of a hydrogen bond between the bound oxygen and the distal histidine.

Nature of the iron-oxygen bond and control of oxygen affinity of the haem by the structure of the globin in haemoglobin

Spectroscopic and chemical evidence speak in favour of the iron-oxygen bond being polar. X-ray analysis shows that the oxygen molecule is inclined at an angle of about 115 degrees to the haem plane. Cooperative binding of oxygen by haemoglobin is attributable to an equilibrium between two alternative structures that differ in oxygen affinity by the equivalent of 3-3.5 kcal/mol. The author has proposed that in the low-affinity structure the globin opposes the movement of the iron atom from its pentacoordinated pyramidal geometry in the haem of deoxyhaemoglobin to its hexacoordinated planar geometry in the haem of oxyhaemoglobin, while in the high-affinity structure this restraint is absent. Recent evidence supporting this mechanism is described.

Resonance Raman investigation of dioxygen bonding in oxycobaltmyoglobin and oxycobalthemoglobin: structural implication of splittings of the bound O--O stretching vibration

Splittings related to the stretching vibration of bound dioxygen in hemoproteins have been detected by resonance Raman spectroscopy. With excitation at 406.7 nm we observe three isotope-sensitive lines in oxycobaltmyoglobin (oxyCoMb) [or in oxycobalthemoglobin A (oxyCoHbA)] at 1103 (1107), 1137 (1137), and 1153 (1152) cm-1, of which the most intense one appears at 1137 cm-1. The first two frequencies arise from resonance interaction between a v(O--O) mode at approximately 1122 cm-1 and an accidentally degenerate porphyrin ring mode at 1123 (1121) cm-1, whereas the third one represents an "unperturbed" v(O--O) vibration from a different species. These two v(O--O) modes at approximately 1122 and approximately 1153 cm-1 shift to approximately 1066 and approximately 1096 cm-1, respectively, upon 16O2 leads to 18O2 substitution. The same resonance interaction may also occur in oxyFeMb (probably also in oxyFeHb(a), because it exhibits an intensity increase at 1125 cm-1 upon 16O2 leads to 18O2 substitution, although the v(O--O) vibrations have not been observed directly. Concomitant enhancement is observed in the v(Co--O) vibration at 539 (537( cm-1, which is considerably lower than the v(Fe--O) frequency at approximately 570 cm-1 in oxyFeMb and oxyFeHbA. The Co--O bond is longer and weaker than the Fe--O bond. Enhancement of both v(O--O) and v(Co--O) indicates the existence of a charge-transfer transition underlying the Soret band, which may be assigned as pi*(pi g*O2/xz) leads to sigma*(dz2Co/pi g*). The presence of two v(O--O) vibrations (at approximately 1122 and approximately 1152 cm-1) but only one v(Co--O) mode at approximately 538 cm-1) means that the two species in oxyCoMB or oxyCoHbA have the same Co--O bond lengths but different O--O bond lengths. The bound dioxygen in a bent end-on configuration may have two allowed orientations, which differ in the extent of sp2(N epsilon) leads to pi*(O2) donation from distal histidine.

Stereochemical mechanism of oxygen transport by haemoglobin

Spectroscopic and chemical evidence speak in favour of the iron-oxygen bond being polar. X-ray analysis shows that the oxygen molecule is inclined at an angle of about 115 degrees to the haem plane. Cooperative binding of oxygen by haemoglobin is due to an equilibrium between two alternative structures, which differ in oxygen affinity by the equivalent of 3-3.5 kcal/mol. I proposed that in the low affinity structure the globin opposes the movement of the iron atom from its five-coordinated pyramidal geometry in the haem of deoxyhaemoglobin to its six-coordinated planar geometry in the haem of oxyhaemoglobin, while in the high affinity structure this restraint is absent. Recent evidence supporting this mechanism is described.

Potential health effects of chlorine dioxide as a disinfectant in potable water supplies

Chlorination of potable water supplies high in organics may yield carcinogenic compounds such as trihalomethanes. Chlorine dioxide has been proposed as an alternative disinfectant to chlorine. However, chlorine dioxide is a strong oxidant that forms significant amounts of chlorite when added to potable water supplies, and chlorite is similar to nitrite in its molecular structure and may be similar in its mechanism of methemoglobin production. Nitrites and chlorites are thought to act synergistically to produce MetHb. Neonates and persons with G-*-PD deficiency are likely to be unusually susceptible to MetHb formation from these compounds because their red cells lack the metabolic machinery to adequately protect against oxidant stress. Since male blacks represent the largest population in the U.S. to be G-6PD deficient, Black male neonates may represent the group at highest risk to the use of chlorine dioxide as a disinfectant in the nations water supplies.

Electron capture by oxyhaemoglobin: an e.s.r. study

Exposure of aqueous glasses containing oxyhaemoglobin to 60 Co γ rays at 77 K gave two similar paramagnetic centres whose electron spin resonance (e. s. r.) spectra resembled those of low-spin ferric derivatives. These were shown to be formed in the α and β chains by electron capture. The use of oxygen labelled with 17 O showed the presence of two inequiva­lent oxygen atoms and it is shown that the unpaired electron has consider­able spin density on the dioxygen ligand as well as on iron. When warmed above 77 K two new paramagnetic centres were formed, possibly as a result of protonation, before the formation of normal high-spin methaemoglobin, presumably by loss of HO 2 ¯ . Oxymyoglobin gave comparable centres.