Resonance Raman spectra of iron(III)-, copper(II)-, cobalt(III)-, and manganese(III)-transferrins and of bis(2,4,6-trichlorophenolato)diimidazolecopper(II) monohydrate, a possible model for copper(II) binding to transferrins

Insights into Plasmon-Induced Dimerization of Rhodanine-A Surface-Enhanced Raman Scattering Study

Plasmon-mediated chemical reactions (PMCRs) have attracted considerable interest in recent times. The PMCR initiated by hot carriers is known to be influenced by the type of metals and the excitation wavelength. Herein, we have carried out the surface-enhanced Raman scattering (SERS) investigation of rhodanine (Rd), an important pharmacologically active heterocyclic compound, adsorbed on silver and gold nanoparticles (AgNP and AuNP) using 514.5 and 632.8 nm lasers. The prominent Raman band at 1566 cm^-1 observed in the SERS spectra is attributed to the characteristic ν(C═C) stretching vibration of the Rd dimer and not of Rd tautomers. The chemical transformation of Rd to Rd dimer on metal surfaces is plausibly triggered by the indirect transfer of energetic hot electrons generated during the non-radiative decay of plasmon. The mechanism involved in the dimerization of Rd via the indirect transfer of hot electrons is also presented. The effect of wavelength on the dimerization of Rd is also observed on the AgNP surface, which indicates that the dimerization occurs more efficiently on the AgNP surface with excitation at 514.5 nm wavelength.

Electron paramagnetic spectrum of dimanganic human serum transferrin

An EPR signal for Mn(III) bound to the metal transport protein transferrin has been detected for the first time. The temperature dependence and simulations of the EPR signal are consistent with the Mn(III) centers being six-coordinate in an elongated tetragonal environment. Thus, the incorporation of Mn(III) within the Tf active site does not vastly alter the coordination number or active site geometry relative to native Fe(III)₂-Tf. This parallel mode EPR signal for Mn(III)₂-Tf could prove valuable for future studies aimed at determining the physiological relevance of Mn(III)₂-Tf.

Supercapattery Based on Binder-Free Co3(PO4)2·8H2O Multilayer Nano/Microflakes on Nickel Foam

A binder-free cobalt phosphate hydrate (Co₃(PO₄)₂·8H₂O) multilayer nano/microflake structure is synthesized on nickel foam (NF) via a facile hydrothermal process. Four different concentrations (2.5, 5, 10, and 20 mM) of Co²⁺ and PO₄^-3 were used to obtain different mass loading of cobalt phosphate on the nickel foam. The Co₃(PO₄)₂·8H₂O modified NF electrode (2.5 mM) shows a maximum specific capacity of 868.3 C g^-1 (capacitance of 1578.7 F g^-1) at a current density of 5 mA cm^-2 and remains as high as 566.3 C g^-1 (1029.5 F g^-1) at 50 mA cm^-2 in 1 M NaOH. A supercapattery assembled using Co₃(PO₄)₂·8H₂O/NF as the positive electrode and activated carbon/NF as the negative electrode delivers a gravimetric capacitance of 111.2 F g^-1 (volumetric capacitance of 4.44 F cm^-3). Furthermore, the device offers a high specific energy of 29.29 Wh kg^-1 (energy density of 1.17 mWh cm^-3) and a specific power of 4687 W kg^-1 (power density of 187.5 mW cm^-3).

Mechanical homeostasis of a DOPA-enriched biological coating from mussels in response to metal variation

Protein-metal coordination interactions were recently found to function as crucial mechanical cross-links in certain biological materials. Mussels, for example, use Fe ions from the local environment coordinated to DOPA-rich proteins to stiffen the protective cuticle of their anchoring byssal attachment threads. Bioavailability of metal ions in ocean habitats varies significantly owing to natural and anthropogenic inputs on both short and geological spatio-temporal scales leading to large variations in byssal thread metal composition; however, it is not clear how or if this affects thread performance. Here, we demonstrate that in natural environments mussels can opportunistically replace Fe ions in the DOPA coordination complex with V and Al. In vitro removal of the native DOPA-metal complexes with ethylenediaminetetraacetic acid and replacement with either Fe or V does not lead to statistically significant changes in cuticle performance, indicating that each metal ion is equally sufficient as a DOPA cross-linking agent, able to account for nearly 85% of the stiffness and hardness of the material. Notably, replacement with Al ions also leads to full recovery of stiffness, but only 82% recovery of hardness. These findings have important implications for the adaptability of this biological material in a dynamically changing and unpredictable habitat.

Reference measurement procedures for the iron saturation in human transferrin based on IDMS and Raman scattering

Two reference measurement procedures are presented here that allow the determination of the iron saturation in human transferrin, based on different molecular properties. The results, directly derived from the number of ions bound to the protein molecule, are traceable to the SI. Up to now, the iron saturation has only been deduced indirectly from the amount-of-substance ratio of serum iron to transferrin in serum. Interlaboratory tests have shown the need for more accurate methods, as the results from many participant test samples for both parameters do not lie within the acceptable range of deviation given by relevant guidelines when different methods or kits are applied. Using isotope dilution, an HPLC ICP-MS procedure was developed in compliance with the requirements of a primary reference measurement procedure. In this manner, the iron saturation was measured with an associated relative expanded measurement uncertainty of 4%. Based on the results, a straightforward Raman procedure was evolved, which allows the determination of the iron saturation in transferrin with an associated relative expanded uncertainty of 7%.

Evidence for a dual role of an active site histidine in α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase

The previously reported crystal structures of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) show a five-coordinate Zn(II)(His)(3)(Asp)(OH(2)) active site. The water ligand is H-bonded to a conserved His228 residue adjacent to the metal center in ACMSD from Pseudomonas fluorescens (PfACMSD). Site-directed mutagenesis of His228 to tyrosine and glycine in this study results in a complete or significant loss of activity. Metal analysis shows that H228Y and H228G contain iron rather than zinc, indicating that this residue plays a role in the metal selectivity of the protein. As-isolated H228Y displays a blue color, which is not seen in wild-type ACMSD. Quinone staining and resonance Raman analyses indicate that the blue color originates from Fe(III)-tyrosinate ligand-to-metal charge transfer. Co(II)-substituted H228Y ACMSD is brown in color and exhibits an electron paramagnetic resonance spectrum showing a high-spin Co(II) center with a well-resolved (59)Co (I = 7/2) eight-line hyperfine splitting pattern. The X-ray crystal structures of as-isolated Fe-H228Y (2.8 Å) and Co-substituted (2.4 Å) and Zn-substituted H228Y (2.0 Å resolution) support the spectroscopic assignment of metal ligation of the Tyr228 residue. The crystal structure of Zn-H228G (2.6 Å) was also determined. These four structures show that the water ligand present in WT Zn-ACMSD is either missing (Fe-H228Y, Co-H228Y, and Zn-H228G) or disrupted (Zn-H228Y) in response to the His228 mutation. Together, these results highlight the importance of His228 for PfACMSD's metal specificity as well as maintaining a water molecule as a ligand of the metal center. His228 is thus proposed to play a role in activating the metal-bound water ligand for subsequent nucleophilic attack on the substrate.

Inhibitory effect of copper(II) on zinc(II)-induced aggregation of amyloid beta-peptide

Aggregation of amyloid beta-peptide (Abeta), a key pathological event in Alzheimer's disease, has been shown in vitro to be profoundly promoted by Zn(II). This fact suggests that some factors in the normal brain protect Abeta from the Zn(II)-induced aggregation. We demonstrate for the first time that Cu(II) effectively inhibits the Abeta aggregation by competing with Zn(II) for histidine residues. The Raman spectrum of a metal-Abeta complex in the presence of both Zn(II) and Cu(II) shows that the cross-linking of Abeta through binding of Zn(II) to the N(tau) atom of histidine is prevented by chelation of Cu(II) by the N(pi) atom of histidine and nearby amide nitrogens. The inhibitory effect is strongest at a Cu/Abeta molar ratio of around 4. Above this ratio, Cu(II) itself promotes the Abeta aggregation by binding to the phenolate oxygen of Tyr10. These results emphasize the importance of regulation of Cu(II) levels to inhibit Abeta aggregation, and are consistent with an altered metal homeostasis in Alzheimer's disease.

Electron paramagnetic resonance and difference ultraviolet studies of Mn2+ binding to serum transferrin

Serum transferrin is the mammalian protein whose normal function is to transport ferric ions through the blood among sites of absorption, storage, and utilization. It has two specific metal-binding sites that bind a variety of metal ions in addition to ferric ion. The macroscopic equilibrium constant for the binding of the first equivalent of Mn2+ to apotransferrin has been determined by electron paramagnetic resonance spectroscopy (EPR) to be logKM1 = 4.06 +/- 0.13 at pH 7.4 in 0.1 M N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (Hepes). An equilibrium constant for nonspecific binding of Mn2+ to apotransferrin of logKns = 2.93 +/- 0.13 has also been obtained by using EPR. Binding of Mn2+ to apotransferrin and to both C- and N-terminal nonferric transferrin has also been studied by difference UV spectroscopy. The second stepwise macroscopic equilibrium constant for the formation of Mn2Tf is logKM2 = 2.96 +/- 0.13. The site-specific microconstants for Mn2+ binding are logkN = 3.13 +/- 0.09 for the N-terminal site and logkC = 3.80 +/- 0.09 for the C-terminal site. There does not appear to be any significant cooperativity between the two sites with respect to metal binding. An equilibrium model for the speciation of Mn2+ in serum has been developed which estimates that almost 90% of Mn2+ is bound to serum proteins, but only approximately 1% is bound to transferrin. The weak binding of Mn2+ to apotransferrin and the obvious inability of transferrin to compete with albumin indicates that the appearance of Mn-transferrin as a major serum species in vivo must involve oxidation of the metal to form the much more stable Mn(3+)-transferrin complex. The computer model confirms that albumin has a sufficient binding affinity to complex most of the Mn(II) in serum in competition with the common low molecular weight ligands in serum. However, there is insufficient data to rule out the possibility that some other protein, such as alpha 2-macroglobulin, may compete with albumin for Mn(II).

Evidence for retention of biological activity of a non-heme iron enzyme adsorbed on a silver colloid: a surface-enhanced resonance Raman scattering study

The structure and catalytic properties of the enzyme (E) chlorocatechol dioxygenase (CCD) adsorbed on a citrate-reduced silver colloid are analyzed by surface-enhanced resonance Raman spectroscopy (SERRS). This is the first SERRS study of a non-heme metalloenzyme. It is demonstrated that the native conformation of CCD is retained in the adsorbed state by comparison of resonance Raman scattering (RRS) from CCD in solution with SERRS from CCD adsorbed on the silver colloid. Both spectra show clear evidence of vibrational bands typical of iron-tyrosinate proteins. Furthermore, it is demonstrated that adsorbed CCD retains 60-85% of its enzymatic activity in the reaction of catechol substrate (S) with O2 to give the dioxygenated product (P) cis,cis-muconate. This is accomplished by enzyme assays of Ag-adsorbed CCD and comparison of the SERRS of Ag-adsorbed enzyme-substrate (ES) complex under anaerobic conditions with that of Ag-adsorbed ES in the presence of dioxygen. The SERRS difference spectrum, ES(aerobic)--ES(anaerobic), shows clear evidence for the appearance of the vibrational modes of adsorbed product. The analogous SERR difference spectroscopy experiment is also carried out for the enzyme-inhibitor (EI) complex of CCD with tetrachlorocatechol (TCC). Slow turnover of CCD-TCC is observed by SERRS on exposure to dioxygen which is consistent with the slow rate of turnover of TCC by CCD in solution.

Spectroscopic analysis of the unfolding of transition metal-ion complexes of human lactoferrin and transferrin

1. Human lactoferrin and transferrin are capable of binding several transition metal ions [Fe(III), Cu(II), Mn(III), Co(III)] into specific binding sites in the presence of bicarbonate. 2. Increased conformational stability and increased resistance to protein unfolding is observed for these metal-ion complexes compared to the apoprotein form of these proteins. 3. Mn(III)-lactoferrin and transferrin complexes exhibit steeper denaturation transitions than the Co(III) complexes of these proteins suggesting greater cooperativity in the unfolding process. 4. The incorporation of Fe(III) into the specific metal binding sites offers the greatest resistance to thermal unfolding when compared to the other transition metal ions studied. 5. Non-coincidence of unfolding transitions is observed, with fluorescence transition midpoints being lower than those determined by absorbance measurements. 6. Fully denatured proteins in the presence of urea and alkyl ureas exhibit fluorescence wavelength maxima at 355-356 nm indicative of tryptophan exposure upon protein unfolding.

Characteristics in tyrosine coordinations of four hemoglobins M probed by resonance Raman spectroscopy

Resonance Raman spectra of four hemoglobins (Hbs) M with tyrosinate ligand, that is, Hb M Saskatoon (beta distal His----Tyr), Hb M Hyde Park (beta proximal His----Tyr), Hb M Boston (alpha distal His----Tyr), and Hb M Iwate (alpha proximal His----Tyr), were investigated in order to elucidate structural origins for distinctly facile reducibility of the abnormal subunit of Hb M Saskatoon in comparison with other Hbs M. All of the Hbs M exhibited the fingerprint bands for the Fe-tyrosinate proteins around 1600, 1500, and 1270 cm-1. However, Hb M Saskatoon had the lowest Fe-tyrosinate stretching frequency and was the only one to display the Raman spectral pattern of a six-coordinate heme for the abnormal beta subunit; the others displayed the patterns of a five-coordinate heme. The absorption intensity of Hb M Saskatoon at 600 nm indicated a transition with a midpoint pH at 5.2, whereas that of Hb M Boston was independent of pH from 7.2 to 4.8. The fingerprint bands for the tyrosinate coordination as well as the Fe-tyrosinate stretching band disappeared for Hb M Saskatoon at pH 5.0, and the resultant Raman spectrum resembled that of metHb A, while those bands were clearly observed for Hb M Boston at pH 5.0 and for two Hbs M at pH 10.0. These observations suggest that the unusual characteristics of the heme in the abnormal beta chain of Hb M Saskatoon result from the weak Fe-tyrosinate bond, which allows weak coordination of the proximal histidine, giving rise to the six-coordinate high-spin state at pH 7.(ABSTRACT TRUNCATED AT 250 WORDS)

The "manganese(III)-containing" purple acid phosphatase from sweet potatoes is an iron enzyme

An improved purification of the purple acid phosphatase from sweet potatoes has been developed, and the properties of the enzyme have been reexamined. Contrary to previous reports, (e.g., Y. Sugiura, et al., J. Biol. Chem., 256, 10664-10670 (1981) ), the enzyme contains two moles of iron and insignificant amounts of manganese. The specific activity of the iron-containing preparations is ca. 14 times higher than that reported previously for the purported "Mn(III)" enzyme. The sweet potato purple acid phosphatase does indeed bind manganese, but it can be removed by dialysis with no changes in specific activity or spectral properties.

Iron(III)-adriamycin and Iron(III)-daunorubicin complexes: physicochemical characteristics, interaction with DNA, and antitumor activity

Fe(III) complexes of two anthracyclines, adriamycin and daunorubicin, have been studied. Using potentiometric and spectroscopic measurements, we have shown that adriamycin and daunorubicin form two well-defined species with Fe(III), which can be formulated as respectively Fe(HAd)3 and Fe(HDr)3. In these formulas, HAd and HDr stand for adriamycin and daunorubicin in which the 1,4-dihydroxy-anthraquinone moiety is half-deprotonated. Both complexes are six-membered chelates. The stability constant is beta = (2.5 +/- 0.5) X 10(28) for both complexes. Interaction with DNA has been studied showing that, despite strong coordination to Fe(III), anthracyclines are able to intercalate between DNA bases pairs, releasing the metal. These complexes display antitumor activity against P 388 leukemia that compares with that of the free drug. Fe(HAd)3, unlike adriamycin, does not catalyze the flow of electrons from NADH to molecular oxygen through NADH dehydrogenase. Moreover, it is shown that the triferric adriamycin compound so called "quelamycin" is in fact a mixture of Fe(HAd)3 and polymeric ferric hydroxide.

The identification of protected tyrosine residues in iron-ovotransferrin

Tetranitromethane reacts with essentially all 21 tyrosine residues of iron-free ovotransferrin. In iron-ovotransferrin, 7 mol of tyrosine/mol of protein are unreactive. Peptides containing the unreactive tyrosine residues were isolated from digests of nitrated iron-ovotransferrin. By comparing the structures of the peptides with the amino acid sequence of ovotransferrin it is found that there are ten protected residues occupying positions 42, 82, 92, 188, 319, 415, 431, 521 and 524 in the polypeptide chain. The problem of identifying the tyrosine residues that form bonds with the metal atoms is discussed.

Resonance Raman studies on protocatechuate 3,4-dioxygenase-inhibitor complexes

Resonance Raman spectra of a number of protocatechuate 3,4-dioxygenase-inhibitor complexes were studied by use of the available lines of an argon and a krypton laser. Three types of inhibitors were investigated-hydroxybenzoates, dicarboxylates, and 4-nitrocatechol. The hydroxybenzoate study shows that the hydroxy group in 3-hydroxybenzoate does not coordinate to the active site iron, in agreement with earlier suggestions, and confirms the coordination of the hydroxy group in the isomeric 4-hydroxybenzoate. The dicarboxylate study demonstrates that both glutarate and terephthalate perturb the active-site environment, shifting the charge-transfer interaction to lower energy. The pH dependence of terephthalate binding as well as the spectral similarities of the dicarboxylate complexes to the ESO2 intermediate provides further evidence for the suggestion that this intermediate is a tightly bound enzyme-product complex. The 4-nitrocatechol study indicates that, unlike the substrate catechols, 4-nitrocatechol does not bind to the iron; a binding configuration wherein the acidic phenolate group interacts with the carboxylate binding site has been suggested by others. Finally the spectra of the 4-hydroxybenzoate and terephthalate complexes demonstrate the presence of two tyrosines coordinated to the active-site iron as suggested by others; these tyrosines have different vCO's and excitation profiles.

Characterization of transferrin metal-binding sites by diffusion-enhanced energy transfer

The distance from the protein surface to ferric or manganic ions in the two specific metal-binding sites of human serum transferrin has been estimated by measuring energy transfer from freely diffusing terbium chelaters in aqueous solution to transferrin-bound metal ions. In addition, both monoferric forms of the protein were studied, as well as the diferric complex formed by using oxalate instead of (bi)carbonate as the auxiliary anion in binding of iron(III) to transferrin. Second-order rate constants for energy transfer between electrically neutral terbium(III)--N-(2-hydroxy-ethyl)ethylenediaminetriacetate and the FeA, FeB, and Fe2 forms of transferrin were 0.9 X 10(5) M-1 S-1, 1.4 X 10(5) M-1 S-1, and 2.6 X 10(5) M-1 S-1, respectively (based on iron concentraton). For the Fe2 species, substitution of oxalate for (bi)carbonate has the effect of decreasing the accessibility of both electrically neutral and negatively charged terbium chelates to the protein-bound iron chromophores. Theoretical considerations of the effect of acceptor location in the protein on energy transfer suggest that the iron chromophores are not on the surface of the protein but are less than 1.7 nm below the surface. The use of diterbium transferrin as energy donor to a small cobalt chelate in solution or to diferric transferrin corroborates these results.

Studies on human lactoferrin by electron paramagnetic resonance, fluorescence, and resonance Raman spectroscopy

Investigations of metal-substituted human lactoferrins by fluorescence, resonance Raman, and electron paramagnetic resonance (EPR) spectroscopy confirm the close similarity between lactoferrin and serum transferrin. As in the case of Fe(III)- and Cu(II)-transferrin, a significant quenching of apolactoferrin's intrinsic fluorescence is caused by the interaction of Fe(III), Cu(II), Cr(III), Mn(III), and Co(III) with specific metal binding sites. Laser excitation of these same metal-lactoferrins produces resonance Raman spectral features at ca. 1605, 1505, 1275, and 1175 cm-1. These bands are characteristic of tyrosinate coordination to the metal ions as has been observed previously for serum transferins and permit the principal absorption band (lambda max between 400 and 465 nm) in each of the metal-lactoferrins to be assigned to charge transfer between the metal ion and tyrosinate ligands. Furthermore, as in serum transferrin the two metal binding sites in lactoferrin can be distinguished by EPR spectroscopy, particularly with the Cr(III)-substituted protein. Only one of the two sites in lactoferrin allows displacement of Cr(III) by Fe(III). Lactoferrin is known to differ from serum transferrin in its enhanced affinity for iron. This is supported by kinetic studies which show that the rate of uptake of Fe(III) from Fe(III)--citrate is 10 times faster for apolactoferrin than for apotransferrin. Furthermore, the more pronounced conformational change which occurs upon metal binding to lactoferrin is corroborated by the production of additional EPR-detectable Cu(II) binding sites in Mn(III)-lactoferrin. The lower pH required for iron removal from lactoferrin causes some permanent change in the protein as judged by altered rates of Fe(III) uptake and altered EPR spectra in the presence of Cu(II). Thus, the common method of producing apolactoferrin by extensive dialysis against citric acid (pH 2) appears to have an adverse effect on the protein.