Structural features and bioactivities of the chitosan

Fourier transform infrared (FT-IR) spectroscopic studies (3500-600 cm⁻¹) showed some different bands of chitosan. The absorption at 3439 cm⁻¹ is stretching vibration of -OH and -NH₂ bonds, indicating the association of the hydrogen-bond between them. The bands at 1659, 1599 and 1321 cm⁻¹ are attributable to the peaks of stretching vibrations of amide I (ν_(CO)), II (δ_(N-H)), and the peak of stretching and bending vibrations of III (ν_(C-N)) (δ_(N-H)). The chitosan showed strong free radical scavenging activities. Pretreatment with chitosan significantly prevented the decrease of antioxidant enzymes activities and the increase of p-JNK at 3 h after renal ischemia and reduced renal tubular epithelial cell apoptosis.     

Remarkable chiral and luminescent properties of novel Yb(III) and Eu(III) complexes containing BINAPO ligand

Lanthanide complexes are of great importance for their prospective applications in wide range of science and technology. Chiral lanthanide complexes can constitute stereo-discriminating probes in biological media, owing to the luminescent properties of the rare-earth ions. Sensitized emission with narrow bandwidth, having fast radiation rate and high emission quantum efficiency are the main perspective for synthesizing the complexes. Attention has been given on remarkable chirality with high dissymmetry factors (g = Δε_ext/ε_max) of the complexes. For this purpose, beta-diketonato ligands with chiral BINAPO (1,1′-binapthyl phosphine oxide) ligand were chosen to achieve the goal. The complexes [Ln(TFN)₃(S-BINAPO)](TFN = 4,4,4-trifluoro-1(2-napthyl)-1,3-butanedione), [Ln(HFT)₃(S-BINAPO)] (HFT = 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)-1,3-hexanedione) and [Ln(HFA)₃(S-BINAPO)](hfa = hexafluoroacetylacetonate) (where Ln = Yb, Eu) were synthesized. The complex, [Eu(TFN)₃(S-BINAPO)] gives strong red emission at 615 nm with narrow emission band (<10 nm) when excited by 465 nm light with quantum efficiency 86%. The dissymmetry factors (g = Δε_ext/ε_max) corresponding to the ⁷F₁ → ⁵D₀ transition at 590 nm is 0.091 for [Eu(TFN)₃(S-BINAPO)] and for [Yb(hfa)₃(S-BINAPO)](hfa = hexafluoroacetylacetonate) corresponding to the ²F_7/2 → ²F_5/2 transitions is 0.12, are among the largest values for both Eu and Yb complexes to date, respectively. The Eu complexes, [Eu(HFT)₃(S-BINAPO)] and [Eu(TFN)₃(S-BINAPO)] are found to be spontaneously emissive, showing bright red emission, when placed in sunlight or even in the laboratory when light is switched on.     

Effect of dioxygen on copper(II) binding to α-synuclein

Using the fluorescent amino acid tryptophan (Trp), we have characterized the copper(II) binding of F4W α-synuclein in the presence and absence of dioxygen at neutral pH. Variations in Trp fluorescence indicate that copper(II) binding is enhanced by the presence of dioxygen, with the apparent dissociation constant (K_d(app)) changing from 100 nM (anaerobic) to 10 nM (aerobic). To investigate the possible role of methionine oxidation, complementary work focused on synthetic peptide models of the N-terminal Cu(II)-α-syn site, MDV(F/W) and M^∗DV(F/W), where M^∗ = methionine sulfoxide. Furthermore, we employed circular dichroism (CD) spectroscopy to demonstrate that the phenyl-to-indole (F→W) substitution does not alter copper(II) binding properties and to confirm the 1:1 metal-peptide binding stoichiometry. CD comparisons also revealed that Met1 oxidation does not affect the copper-peptide conformation and further suggested the possible existence of a Cu^II-Trp/Phe (cation-π) interaction.     

Vibrational and structural mapping of [Os(bpy)3] and [Os(phen)3]

Structural changes between [Os^IIL₃]²⁺ and [Os^IIIL₃]³⁺ (L: 2,2′-bipyridine; 1,10-phenanthroline) and molecular and electronic structures of the Os^III complexes [Os^III(bpy)₃]³⁺ and [Os^III(phen)₃]³⁺ are discussed in this paper. Mid-infrared spectra in the ν(bpy) and ν(phen) ring stretching region for [Os^II(bpy)₃](PF₆)₂, [Os^III(bpy)₃](PF₆)₃, [Os^II(phen)₃](PF₆)₂, and [Os^III(phen)₃](PF₆)₃ are compared, as are X-ray crystal structures. Absorption spectra in the UV region for [Os^III(bpy)₃](PF₆)₃ and [Os^III(phen)₃](PF₆)₃ are dominated by very intense absorptions (ε = 40 000-50 000 M⁻¹ cm⁻¹) due to bpy and phen intra-ligand π → π^∗ transitions. In the visible region, relatively narrow bands with vibronic progressions of ∼1500 cm⁻¹ appear, and have been assigned to bpy or phen-based, spin-orbit coupling enhanced, ¹π → ³π^∗ electronic transitions. Also present in the visible region are ligand-to-metal charge transfer bands (LMCT) arising from π(bpy) → t_2g(Os^III) or π(phen) → t_2g(Os^III) transitions. In the near infrared, two broad absorption features appear for oxidized forms [Os^III(bpy)₃](PF₆)₃ and [Os^III(phen)₃](PF₆)₃ arising from dπ-dπ interconfigurational bands characteristic of dπ⁵Os^III. They are observed at 4580 and 5090 cm⁻¹ for [Os^III(bpy)₃](PF₆)₃ and at 4400 and 4990 cm⁻¹ for [Os^III(phen)₃](PF₆)₃. The bpy and phen infrared vibrational bands shift to higher energy upon oxidation of Os(II) to Os(III). In the cation structure in [Os^III(bpy)₃](PF₆)₃, the Os^III atom resides at a distorted octahedral site, as judged by ∠N-Os-N, which varies from 78.78(22)° to 96.61(22)°. Os-N bond lengths are also in general longer for [Os^III(bpy)₃](PF₆)₃ compared to [Os^II(bpy)₃](PF₆)₂ (0.010 Å), and for [Os^III(phen)₃](PF₆)₃ compared to [Os^II(phen)₃](PF₆)₂ (0.014 Å). Structural changes in the ligands between oxidation states are discussed as originating from a combination of dπ(Os^II) → π^∗ (bpy or phen) backbonding and charge redistribution on the ligands as calculated by natural population analysis.     

Mixed-valency with cyanides as terminal ligands: Diruthenium(III,II) complexes with the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine bridge and variable co-ligands (CN vs. bpy or NH3)

New diruthenium complexes (PPN)₄[(NC)₄Ru(μ-bptz)Ru(CN)₄], (PPN)₄1, and [(bpy)₂Ru(μ-bptz)Ru(CN)₄], 2, (PPN⁺ = bis(triphenylphospine)iminium; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy = 2,2′-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant K_c = 10^7.0 of the mixed-valent species [(NC)₄Ru(μ-bptz)Ru(CN)₄]³⁻ as obtained by oxidation of 1⁴⁻ in CH₃CN is much lower than the K_c = 10^15.0 previously detected for [(H₃N)₄Ru(bptz)Ru(NH₃)₄]⁵⁺, reflecting the competition between CN⁻ and bptz for the π-electron density of the metals. Comparison with several other bptz-bridged diruthenium(II,III) complexes reveals an approximate correlation between K_c and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ_max = 624 nm, the main IVCT (intervalence charge transfer) band of 1³⁻ was detected by spectroelectrochemistry at λ_max = 1695 nm (in CH₃CN; ε = 3200 M⁻¹ cm⁻¹). The experimental band width at half-height, Δν_1/2 = 2700 cm⁻¹, is slightly smaller than the theoretical value Δν_1/2 = 3660 cm⁻¹, calculated from the Hush approximation for Class II mixed-valent species. In agreement with comparatively moderate metal-metal coupling, the mixed-valent intermediate 1³⁻ was found to be EPR silent even at 4 K. The unsymmetrical mixed-valent complex [(bpy)₂Ru^II(μ-bptz)Ru^III(CN)₄]⁺, obtained in situ by bromine oxidation of 2 in CH₃CN/H₂O, displays a broad NIR absorption originating from an IVCT transition at λ_max = 1075 nm (ε ≈ 1000 M⁻¹ cm⁻¹, Δν_1/2 ≈ 4000 cm⁻¹). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)₄]²⁻ was measured in H₂O by laser flash photolysis; the obtained value of τ = 19.6 ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz = 2,2′-bipyrazine) in analogous tetracyanoruthenium complexes.     

Alkaline transition of pseudoazurin Met16X mutant proteins: protein stability influenced by the substitution of Met16 in the second sphere coordination

Several blue copper proteins are known to change the active site structure at alkaline pH (alkaline transition). Spectroscopic studies of Met16Phe, Met16Tyr, Met16Trp, and Met16Val pseudoazurin variants were performed to investigate the second sphere role through alkaline transition. The visible electronic absorption and resonance Raman spectra of Met16Phe, Met16Tyr, and Met16Trp variants showed the increasing of axial component at pH 11 like wild-type PAz. The visible electronic absorption and far-UV CD spectra of Met16Val demonstrated that the destabilization of the protein structure was triggered at pH > 11. Resonance Raman (RR) spectra of PAz showed that the intensity-weighted averaged Cu–S(Cys) stretching frequency was shifted to higher frequency region at pH 11. The higher frequency shift of Cu–S(Cys) bond is implied the stronger Cu–S(Cys) bond at alkaline transition pH 11. The visible electronic absorption and far-UV CD spectra of Met16X PAz revealed that the Met16Val variant is denatured at pH > 11, but Met16Phe, Met16Tyr, and Met16Trp mutant proteins are not denatured even at pH > 11. These observations suggest that Met16 is important to maintain the protein structure through the possible weak interaction between methionine –SCH₃ part and coordinated histidine imidazole moiety. The introduction of π–π interaction in the second coordination sphere may be contributed to the enhancement of protein structure stability.     

Syntheses, structures and vibrational spectroscopy of some unusual silver(I) (pseudo-) halide/unidentate nitrogen base polymers

The meagre (structurally defined) array of 1:2 silver(I) (pseudo-)halide:unidentate nitrogen base adducts is augmented by the single-crystal X-ray structural characterization of the 1:2 silver(I) thiocyanate:piperidine (‘pip’) adduct. It is of the one-dimensional ‘castellated polymer’ type previously recorded for the chloride: ?Ag(pip)₂(μ-SCN)Ag(pip)₂? a single bridging atom (S) linking successive silver atoms. By contrast, in its copper(I) counterpart, also a one-dimensional polymer, the thiocyanate bridges as end-bound SN-ambidentate: ?CuSCNCuSCN? A study of the 1:1 silver(I) bromide:quinoline (‘quin’) adduct is recorded, as the 0.25 quin solvate, isomorphous with its previous reported ‘saddle polymer’ chloride counterpart.Recrystallization of 1:1 silver(I) iodide:tris(2,4,6-trimethoxyphenyl)phosphine (‘tmpp’) mixtures from py and quinoline (‘quin’)/acetonitrile solutions has yielded crystalline materials which have also been characterized by X-ray studies. In both cases the products are salts, the cation in each being the linearly coordinated silver(I) species [Ag(tmpp)₂]⁺, while the anions are, respectively, the discrete [Ag₅I₇(py)₂]²⁻ species, based on the already known but unsolvated [Cu₅I₇]²⁻ discrete, and the polymeric, arrays, and polymeric . The detailed stereochemistry of the [Ag(tmpp)₂]⁺ cation is a remarkably constant feature of all structures, as is its tendency to close-pack in sheets normal to their P-Ag-P axes.The far-IR spectra of the above species and of several related complexes have been recorded and assigned. The vibrational modes of the single stranded polymeric AgX chains in [XAg(pip)₂]_(∞|∞) (X = Cl, SCN) are discussed, and the assignments ν(AgX) = 155, 190 cm⁻¹ (X = Cl) and 208 cm⁻¹ (X = SCN) are made. The ν(AgX) and ν(AgN) modes in the cubane tetramers [XAg(pip)]₄ (X = Br, I) are assigned and discussed in relation to the assignments for the polymeric AgX:pip (1:2) complexes, and those for the polymeric [XAg(quin)]_(∞|∞) (X = Cl, Br) compounds. The far-IR spectra of [Ag(tmpp)₂]₂[Ag₅I₇(py)₂] and its corresponding 2-methylpyridine complex show a single strong band at about 420 cm⁻¹ which is assigned to the coordinated tmpp ligand in [Ag(tmpp)₂]⁺, and a partially resolved triplet at about 90, 110 and 140 cm⁻¹ which is assigned to the ν(AgI) modes of the [Ag₅I₇L₂]²⁻ anion. An analysis of this pattern is given using a model which has been used previously to account for unexpectedly simple ν(CuI) spectra for oligomeric iodocuprate(I) species.     

Resonance Raman spectroscopic studies of hydroperoxo derivatives of cobalt-substituted myoglobin

Recent progress in generating and stabilizing reactive heme protein enzymatic intermediates by cryoradiolytic reduction has prompted application of a range of spectroscopic approaches to effectively interrogate these species. The impressive potential of resonance Raman spectroscopy for characterizing such samples has been recently demonstrated in a number of studies of peroxo- and hydroperoxo-intermediates. While it is anticipated that this approach can be productively applied to the wide range of heme proteins whose reaction cycles naturally involve these peroxo- and hydroperoxo-intermediates, one limitation that sometimes arises is the lack of enhancement of the key intraligand ν(O-O) stretching mode in the native systems. The present work was undertaken to explore the utility of cobalt substitution to enhance both the ν(Co-O) and ν(O-O) modes of the CoOOH fragments of hydroperoxo forms of heme proteins bearing a trans-axial histidine linkage. Thus, having recently completed RR studies of hydroperoxo myoglobin, attention is now turned to its cobalt-substituted analogue. Spectra are acquired for samples prepared with ¹⁶O₂ and ¹⁸O₂ to reveal the ν(M-O) and ν(O-O) modes, the latter indeed being observed only for the cobalt-substituted proteins. In addition, spectra of samples prepared in deuterated solvents were also acquired, providing definitive evidence for the presence of the hydroperoxo-species.     

Dinuclear terephthalato-bridged copper(II) complexes: Syntheses, spectral and structural characterization, and magnetic properties

Herein, we report the syntheses, spectral and structural characterization, and magnetic behavior of four new dinuclear terephthalato-bridged copper(II) complexes with formulae [Cu₂(trpn)₂(μ-tp)](ClO₄)₂ · 2H₂O (1), [Cu₂(aepn)₂(μ-tp)(ClO₄)₂] (2), [Cu₂(Medpt)₂(μ-tp)(H₂O)₂](ClO₄)₂ (3) and [Cu₂(Et₂dien)₂(μ-tp)(H₂O)](ClO₄)₂ (4) where tp = terephthalate dianion, trpn = tris(3-aminopropyl)-amin, aepn = N-(2-aminoethyl)-1,3-propanediamine, Medpt = 3,3′-diamino-N-methyldipropylmine and Et₂dien = N,N-diethyldiethylenetriamine. The structures of these complexes consist of two μ-tp bridging Cu(II) centers in a bis(monodentate) bonding fashion. The coordination geometry of the Cu(II) ions in these compounds may be described as close to square-based pyramid (SP) with severe significant distortion towards trigonal bipyramid (TBP) stereochemistry in 1. The visible spectra of the complexes in aqueous solutions are in complete agreement with the assigned X-ray geometry around the Cu(II) centers. Also, the solid infrared spectral data for the stretching frequencies of the tp-carboxalato groups, the ν(COO⁻) reveals the existence of bis(monodentate) coordination mode for the bridged terephthalate ligand. The susceptibility measurements at variable temperature over the range 2-300 K are reported. Despite the same bonding mode of the tp bridging ligand, there has been observed slight antiferromagnetic coupling for the compounds 1 and 4 with J values of −0.5 and −2.9 cm³ K mol⁻¹, respectively, and very weak ferromagnetic coupling for 2 and 3 with J values of 0.8 and 10.1 cm³ K mol⁻¹, respectively. The magnetic results are discussed in relation to other related μ-terephthalato dinuclear Cu(II) published compounds.     

Structural, spectroscopic and redox studies of a new ruthenium(III) complex with an imidazole-rich tripodal ligand

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[Ru^III(L)(Cl)₂]ClO₄ complex (1). The crystal structure of 1 shows Ru^III in a distorted octahedral geometry, in which two chloride ions, cis-positioned to each other, are coordinated besides the four nitrogen atoms from the tetradentate ligand L. The cyclic voltammogram of 1 exhibits three redox processes at −67, +73 and +200 mV versus SCE, which are attributed to the Ru^III/Ru^II couple in the cis-[Ru^III(L)(Cl)₂]⁺, cis-[Ru^II(L)(H₂O)(Cl)]⁺ and cis-[Ru^II(L)(H₂O)₂]²⁺, respectively. After chemical reduction (Zn(Hg) or Eu^II) only the cis-[Ru^II(L)(H₂O)₂]²⁺ species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (ε=1.4×10³ mol⁻¹ L cm⁻¹), 335 nm (ε=7.9×10³ mol⁻¹ L cm⁻¹), 301 nm (ε=6.7×10³ mol⁻¹ L cm⁻¹) and 264 nm (ε=9.9×10³ mol⁻¹ L cm⁻¹), in water solution (CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ with CF₃COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the Ru^III center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the Ru^II center to the pyridine ligand. The lability of the water ligands in the cis-[Ru^II(L)(H₂O)₂]²⁺ species has been investigated using the auxiliary ligand pyrazine. Reactions in the presence of stoichiometric and excess of pyrazine yield the same species, cis-[Ru^II(L)(H₂O)(pz)]²⁺, which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (ε=5.1×10³ mol⁻¹ L cm⁻¹) and 394 nm (ε=4.2×10³ mol⁻¹ L cm⁻¹). Experiments performed with a large excess of pyrazine gave a specific rate constant k₁=(2.8±0.5)×10⁻² M⁻¹ s⁻¹, at 25 °C, in CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ (with CF₃COONa).     

Reaction of platinum(II) diamine and triamine complexes with selenomethionine

Four new coordination polymers namely {[Mn₂(BT)(DPS)₂(H₂O)₆]·10H₂O}_n (MnBTDPS), {[Co₂(BT)(DPS)₂(H₂O)₆]·10H₂O}_n (CoBTDPS), {[Cu₂(BT)(DPS)(H₂O)₄]·5H₂O}_n (CuBTDPS) and {[Zn₂(BT)(DPS)₂]·6H₂O}_n (ZnBTDPS), where BT = 1,2,4,5-benzenetetracarboxylate and DPS = di(4-pyridyl) sulfide, were synthesized and characterized by thermal analysis, vibrational spectroscopy (Raman and infrared) and single crystal X-ray diffraction analysis. In all compounds, the DPS ligands are coordinated to metal sites in a bridging mode and the carboxylate moiety of BT ligands adopts a monodentate coordination mode, as indicated by the Raman spectra data through the Δν (ν_asym(COO) − ν_sym(COO)) value. According to X-ray diffraction analysis, MnBTDPS and CoBTDPS are isostructural and in these cases, the metal centers exhibit a distorted octahedral geometry. In CuBTBPP, the Cu²⁺ centers geometries are best described as square-pyramids, according to the trigonality index τ = 0.14 for Cu1 and τ = 0.10 for Cu2. On the other hand, in ZnBTDPS, the Zn²⁺ sites adopt a tetrahedral geometry. Finally, the four compounds formed two-dimensional sheets that are connected to each other through hydrogen bonding giving rise to three-dimensional supramolecular arrays.     

Synthesis and structural characterization of two new polynuclear metal thiosaccharinates: Hexakis(thiosaccharinato)hexasilver(I) and tetrakis(thiosaccharinato)tetracopper(I)

The title compounds, for short Ag₆(tsac)₆ (1) and [Cu₄(tsac)₄(MeCN)₂] · 2MeCN (2), were prepared by the reaction of thiosaccharin with Ag(I) or Cu(II) salts in different solvents. The new complexes were characterized by FT-IR, Raman, UV-Vis and NMR spectroscopy. Their crystal and molecular structures were determined by X-ray diffraction methods. The structures were solved from 1621 (1) and 7080 (2) reflections with I > 2σ(I) and refined to agreement R1-factors of 0.0261 (1) and 0.0456 (2). Ag₆(tsac)₆ molecule derives from the clustering of six Ag(tsac) moieties related to each other through the crystallographic 3-bar (S₆) symmetry operations of the space group. This results in a highly regular molecular structure where the silver atoms are at the corners of an octahedral core slightly compressed along one of its three-fold axis [inter-metallic Ag?Ag contacts of 3.1723(4) and 3.1556(4) Å]. The six thiosaccharinate ligands bridge neighboring Ag atoms along the C₃-axis through Ag-N bonds [d(Ag-N) = 2.285(2) Å] at one end and bifurcated Ag-S(thione)-Ag bonds [Ag-S distances of 2.4861(7) and 2.5014(8) Å] at the other end. In contrast, the 2 compound is arranged in the lattice as an irregular tetrameric copper complex [Cu₄(tsac)₄(MeCN)₂] where the metals show different environments. Two copper ions are four-fold coordinated to three tsac ions through the N-atom of one tsac [Cu-N distances of 2.112(3) and 2.064(3) Å] and the thione sulfur atom of the other two [Cu-S distances in the range from 2.284(1) to 2.358(1) Å] and to a MeCN solvent molecule [Cu-N distances of 1.983(4) and 2.052(3) Å]. The other two copper ions are in three-fold environment, one trans-coordinated to two tsac ions [Cu-N distances of 1.912(3) and 1.920(3) Å] and to the thione S-atom of a third ligand [d(Cu-S) = 2.531(1) Å], the other one to the thione sulfur atom of three tsac ligands [Cu-S distances in the range from 2.229(1) to 2.334(1) Å]. The clustering renders the metals to short distances from each other, the shorter Cu?Cu distance being 2.6033(7) Å, as to presume the existence of weak inter-metallic interaction within the cluster.     

Effect of high external pressures on the vibrational spectra of crystalline dichloro(1,5-cycloctadiene)platinum(II), Pt(η-C8H12)Cl2

The effects of high external pressures on the principal IR and Raman bands of crystalline dichloro(1,5-cycloctadiene)platinum(II), Pt(COD)Cl₂ (COD = η⁴-C₈H₁₂), have been investigated for pressures up to ∼30 kbar by diamond-anvil cell microspectroscopy. This square-planar Pt(II) complex does not undergo any pressure-induced structural change throughout the pressure range investigated and the pressure dependences (dν/dP) for the vibrational modes range from −0.18 to 0.79 cm⁻¹ kbar⁻¹. The negative dν/dP value observed for the IR band at 1426 cm⁻¹ (−0.18 cm⁻¹ kbar⁻¹) suggests that this band is chiefly associated with a CC stretching mode of the Pt-COD group. This observation provides yet another indication that high-pressure vibrational spectroscopy may indeed be a general method for establishing the presence of π-backbonding in organometallic complexes, such as metal carbonyls and alkenes.     

Sequences of tryptic peptides containing the five cysteinyl residues of ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum

Tryptic peptides which account for all five cysteinyl residues in ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum have been purified and sequenced. Collectively, these peptides contain 94 of the approximately 500 amino acid residues per molecule of subunit. Due to one incomplete cleavage at a site for trypsin and two incomplete chymotryptic-like cleavages, eight major radioactive peptides (rather than five as predicted) were recovered from tryptic digests of the enzyme that had been carboxymethylated with [³H]iodoacetate. The established sequences are: GlyTyrThrAlaPheValHisCys¹Lys TyrValAspLeuAlaLeuLysGluGluAspLeuIleAla GlyGlyGluHisValLeuCys¹AlaTyr AlaGlyTyrGlyTyrValAlaThrAlaAlaHisPheAla AlaGluSerSerThrGlyThrAspValGluValCys¹ ThrThrAsxAsxPheThrArg AlaCys¹ThrProIleIleSerGlyGlyMetAsnAla LeuArg ProPheAlaGluAlaCys¹HisAlaPheTrpLeuGly GlyAsnPheIleLys In these peptides, radioactive carboxymethylcysteinyl residues are denoted with asterisks and the sites of incomplete cleavage with vertical wavy lines. None of the peptides appear homologous with either of two cysteinyl-containing, active-site peptides previously isolated from spinach ribulosebisphosphate carboxylase/oxygenase.     

A high-pressure, Raman spectroscopic study of bis(pyrophosphato)tetrazinc(II) and -tetramanganese(II) decahydrate salts

The Raman spectra of crystalline bis(pyrophosphato)tetrazinc(II) and -manganese(II) decahydrates, M₄(P₂O₇)₂ · 10H₂O (M = Zn and Mn), have been investigated at room temperature. The Mn(II) salt readily underwent photodecomposition upon extended exposure to 785-nm laser irradiation and so could not be examined under high pressures in a diamond-anvil cell. The Zn(II) salt was much more stable and was studied successfully at pressures up to ∼35 kbar. The pressure dependences of the δ_sPO₂, ν_sPOP, ν_asPOP, ν_sPO₂, and ν_asPO₂ internal modes of the pyrophosphate group and of an external H₂O mode indicated the occurrence of a pressure-induced phase transition in the 15-20 kbar region. Further confirmation of this structural change came from the appearance of four initially forbidden modes in the 19-26 kbar range. This phase transition proved to be reversible upon release of the applied pressure. The pressure dependences before and after the phase transition were −0.22 to 0.66 and 0.06 to 0.33 cm⁻¹/kbar, respectively.     

A new series of iodocarbonyl ruthenium (II) complexes with unsymmetrical phosphine-phosphine sulfide ligands of the type Ph2P(CH2)nP(S)Ph2, n = 1-4: Isolation and structural investigation

Hexa-coordinated chelate complex cis-[Ru(CO)₂I₂(P∩S)] (1a) {P∩S = η²-(P,S)-coordinated} and penta-coordinated non-chelate complexes cis-[Ru(CO)₂I₂(P∼S)] (1b-d) {P∼S = η¹-(P)-coordinated} are produced by the reaction of polymeric [Ru(CO)₂I₂]_n with equimolar quantity of the ligands Ph₂P(CH₂)_nP(S)Ph₂ {n = 1(a), 2(b), 3(c), 4(d)} in dichloromethane at room temperature. The bidentate nature of the ligand a in the complex 1a leads to the formation of five-membered chelate ring which confers extra stability to the complex. On the other hand, 1:2 (Ru:L) molar ratio reaction affords the hexa-coordinated non-chelate complexes cis,cis,trans-[Ru(CO)₂I₂(P∼S)₂] (2a-d) irrespective of the ligands. All the complexes show two equally intense terminal ν(CO) bands in the range 2028-2103 cm⁻¹. The ν(PS) band of complex 1a occurs 23 cm⁻¹ lower region compared to the corresponding free ligand suggesting chelation via metal-sulfur bond formation. X-ray crystallography reveals that the Ru(II) atom occupies the center of a slightly distorted octahedral geometry. The complexes have also been characterized by elemental analysis, ¹H, ¹³C and ³¹P NMR spectroscopy.     

Thiodiacetato-copper(II) chelates with or without N-heterocyclic donor ligands: molecular and/or crystal structures of [Cu(tda)]n, [Cu(tda)(Him)2(H2O)] and [Cu(tda)(5Mphen)] · 2H2O (Him = imidazole, 5Mphen = 5-methyl-1,10-phenanthroline)

Three new thiodiacetato-Cu(II) chelates have been synthesized and studied by X-ray crystallography and by thermal, spectral and magnetic methods. [Cu(tda)]_n (1) is a 3D-polymer with a pentadentate tda, which acts with a fac-O₂ + S(apical)-tridentate chelating conformation and as a twofold anti, syn-μ-η¹:η¹ carboxylate bridge. In its square pyramidal Cu(II) coordination (type 4 + 1) four O(carboxylate) donors define a close regular square base, but the Cu-S(apical) bond deviates 27.4° from the perpendicular to the mean basal plane. Each anti,syn-bridging carboxylate group exhibits two C-O (average 1.26(1) Å) and two Cu-O bonds (average 1.958(7) Å), which are very similar in length to each other. In contrast, the mixed-ligand complexes of [Cu(tda)(Him)₂(H₂O)] (compound 2, distorted octahedral, type 4 + 1 + 1) and [Cu(tda)(5Mphen)] · 2H₂O (compound 3, distorted square pyramidal, type 4 + 1) have molecular structures and the tda ligand displays only a fac-O₂ + S(apical)-tridentate conformation. The Cu-S(apical) bond lengths (2.570(1), 2.623(1) or 2.573(1) Å for 1, 2 or 3, respectively) are shorter than those previously reported for closely related Cu(II)-tda derivatives. The different tda ligand roles in their Cu(II) derivatives are rationalized on the basis of crystal packing forces driving in the absence or presence of auxiliary ligands (with two or three N-donor atoms).     

Nickel(II)-substituted azurin I from Alcaligenes xylosoxidans as characterized by resonance Raman spectroscopy at cryogenic temperature

Metal-substituted blue copper proteins (cupredoxins) have been successfully used to study the effect of metal-ion identity on their active-site properties, specifically the coordination geometry and metal–ligand bond strengths. In this work, low-temperature (77 K) resonance Raman (RR) spectra of the blue copper protein Alcaligenes xylosoxidans azurin I and its Ni(II) derivative are reported. A detailed analysis of all observed bands is presented and responsiveness to metal substitution is discussed in terms of structural and bonding changes. The native cupric site exhibits a RR spectrum characteristic of a primarily trigonal planar (type 1) coordination geometry, identified by the ν(Cu–S)_Cys markers at 373, 399, 409, and 430 cm⁻¹. Replacement of Cu(II) with Ni(II) results in optical and RR spectra that reveal (1) a large hypsochromic shift in the main (Cys)S → M(II) charge-transfer absorption from 622 to 440 nm, (2) greatly reduced metal–thiolate bonding interaction, indicated by substantially lower ν(Ni–S)_Cys stretching frequencies, (3) elevation of the cysteine ν(C _β –S) stretching, amide III, and ρ _s(C _β H₂) scissors vibrational modes, and (4) primarily four-coordinated, trigonally distorted tetrahedral geometry of the Ni(II) site that is marked by characteristic ν(Ni–S)_Cys stretching RR bands at 347, 364, and 391 cm⁻¹. Comparisons of the electronic and vibrational properties between A. xylosoxidans azurin I and its closely structurally related azurin from Pseudomonas aeruginosa are made and discussed. For cupric azurins, the intensity-weighted average M(II)–S(Cys) stretching frequencies are calculated to be ν(Cu–S)_iwa = 406.3 and 407.6 cm⁻¹, respectively. These values decreased to ν(Ni–S)_iwa = 359.3 and 365.5 cm⁻¹, respectively, after Ni(II) → Cu(II) exchange, suggesting that the metal–thiolate interactions are similar in the two native proteins but are much less alike in their Ni(II)-substituted forms.     

Investigation of the electron density of iridium(I) Vaska-type complexes using DFT calculations and structural results: Structure of trans-carbonyl-chloro-bis(tricyclohexylphosphine)-iridium(I)

[Ir(CO)Cl(PCy₃)₂] was obtained by the slow attack of 1,2-dichloromethane on (Bu₄N)[Ir₂(μ-Dcbp)(CO)₂(PCy₃)₂] (Dcbp = 3,5-dicarboxylatepyrazole). The Ir-CO, Ir-Cl and Ir-P bond distances are 1.778(10), 2.374(3) and 2.3486(8) Å, respectively. The Ir-P bond distances for a number of different Vaska complexes indicate the shortest bond distances for phoshines containing electron withdrawing groups. An excellent correlation between DFT (OLYP/ZORA/TZP) and experimental structures is obtained as reflected by the RMSD values (H excluded) of between 0.083 and 0.268 Å for the different complexes studied. The calculated Ir-P bond distances and ν(CO) stretching frequencies closely follow the trends obtained from the experimental results.     

Balance between ultrafast parallel reactions in the green fluorescent protein has a structural origin

The fluorescence photocycle of the green fluorescent protein is functionally dependent on the specific structural protein environment. A direct relationship between equilibrium protein side-chain conformation of glutamate 222 and reactivity is established, particularly the rate of ultrafast proton transfer reactions in the fluorescence photocycle. We show that parallel transformations in the photocycle have a structural origin, and we report on the vibrational properties of responsive amino acids on an ultrafast timescale. Blue excitation of GFP drives two parallel, excited-state deuteron transfer reactions with 10 ps and 75 ps time constants to the buried carboxylic acid side chain of glutamate 222 via a hydrogen-bonding network. Assignment of 1456 cm⁻¹ and 1441 cm⁻¹ modes to ν_sym and assignment of 1564 cm⁻¹ and 1570 cm⁻¹ features to ν_asym of E222 in the 10 ps and 75 ps components, respectively, was possible from the analysis of the transient absorption data of an E222D mutant and was consistent with photoselection measurements. In contrast to the wild-type, measurements of E222D can be described with only one difference spectrum, with the ν_sym mode at 1435 cm⁻¹ and the ν_asym mode at 1567 cm⁻¹, also correlating a large Δν_asym-sym with slow excited-state proton transfer kinetics. Density Functional Theory calculations and published model compound and theoretical studies relate differences in Δν_asym-sym to the strength and number of hydrogen-bonding interactions that are detected via equilibrium geometry and COO⁻ stretching frequency differences of the carboxylate. The correlation of photocycle kinetics with side-chain conformation of the acceptor suggests that proton transfer from S205 to E222 controls the rate of the overall excited-state proton transfer process, which is consistent with recent theoretical predictions. Photoselection measurements show agreement for localized CO vibrations of chromophore, Q69, and E222 with Density Functional Theory and ab initio calculations placed in the x-ray geometry and provide their vibrational response in the intermediates in the photocycle.