N-methylimidazole Complexes Research Articles

Evaluation of Heme Peripheral Groups Interactions in Low-Dielectric Constant Media

In this study, we measured the contributions of the ionization of the heme propionates to the reduction potentials of heme b and heme a (bis)N-methylimidazole complexes in various low-dielectric constant conditions. Additionally, we measured the effects of H-bond to the heme a formyl group on the reduction potential of the heme. The performed electrochemical measurements show that ionization of the heme propionates lead to the largest redox change in dichloromethane with no electrolyte. The measured reduction potential changes for heme b and heme a were - 55 and −47 mV (±10 mV) per ionized propionate, respectively. For heme a, the study demonstrates how the dielectric constant of the medium is important in the magnification of the αpKa upon redox-linked ionization of the heme propionates and their roles in the proton pump of cytochrome c oxidase. Additionally, we carried out a detailed study on the H-bonding properties of heme a model compounds (copper mono- and di-acetyl porphyrins) and the effects of the dielectric constant of the solvent on the measured dissociation constant (Kd) between various H-bond donors and the copper porphyrins. Our measurements show that H-bonds between the copper porphyrins and TFA or phenol are significant in benzene and dichloromethane, but extremely weak in the presence of acetonitrile. This study shows how the dielectric constant of the medium also plays a role in modulating the properties of heme a through H-bonding of its formyl group.

Evaluation of Heme Peripheral Group Interactions in Extremely Low-Dielectric Constant Media and Their Contributions to the Heme Reduction Potential.

In this study, we measured the contributions of the ionization of the heme propionates to the reduction potentials of heme b and heme a (bis)N-methylimidazole complexes in various low-dielectric constant conditions. Additionally, we measured the effects of H-bond to the heme a formyl group on the reduction potential of the heme. The performed electrochemical measurements show that ionization of the heme propionates lead to the largest redox change in dichloromethane with no electrolyte. The measured reduction potential changes for heme b and heme a were -55 and -47 mV (±10 mV) per ionized propionate, respectively. For heme a, the study demonstrates how the dielectric constant of the medium is important in the magnification of the ΔpKa upon redox-linked ionization of the heme propionates and their roles in the proton pump of cytochrome c oxidase.

NMR investigations of nitrophorin 2 belt side chain effects on heme orientation and seating of native N-terminus NP2 and NP2(D1A)

Nitrophorin 2 (NP2), one of the four NO-storing and NO-releasing proteins found in the saliva of the blood-sucking bug Rhodnius prolixus, has a more ruffled heme and a high preference for a particular heme orientation (B) compared with nitrophorin1 and nitrophorin4, which show not a preference (Ato Bratio ofapproximately1:1), suggesting that it fits more tightly in the β-barrel protein. In this work we have prepared a series of "belt" mutants of NP2(D1A) and (ΔM0)NP2 aimed at reducing the size of aromatic or other residues that surround the heme, and investigated them as the high-spin aqua and low-spin N-methylimidazole complexes. The belt mutants included Y38A, Y38F, F42A, F66A, Y85A, Y85F, Y104A, I120T, and a triple mutant of NP2(D1A), the F42L, L106F, I120T mutant. Although I120 has been mainly considered to be a distal pocket residue, CδH3 of I120 lies directly above the heme 3-methyl, at 2.67Å, of heme orientationB, or the 2-vinyl of A, and it thus plays a role as a belt mutant, a role that turns out to be extremely important in creating the strong favoring of the Bheme orientation [AtoB ratio of 1:14 for NP2(D1A) or 1:12 for (ΔM0)NP2]. The results show that the 1D (1)H NMR spectra of the high-spin forms are quite sensitive to changes in the shape of the heme binding cavity. The single mutation I120T eliminates the favorability of the Bheme orientation by producing a heme A to Borientation ratio of 1:1, whereas the single mutation F42A reverses the heme orientation from an A to Bratio of1:14 seen for NP2(D1A) to 10:1 for NP2(D1A,F42A). The most extreme ratio was found for the triple mutant of NP2(D1A), NP2(D1A,F42L,L105F,I120T), in which the Ato Bratio is approximately25:1, a ΔG change of about -3.5kcal/mol or -14.1kJ/mol with respect to NP2(D1A). The seating of the heme is modified as well in that mutant and in several others, by rotations of the heme by up to 4° from the seating observed in NP2(D1A), in order to relieve steric interactions between a vinyl β-carbon and a protein side chain, or to fill a cavity created by replacing a large protein side chain by a much smaller one; the latter was observed for all tyrosine to alanine mutants. These relatively small changes in seating have a measurable effect on the NMR spectra of the mutants, but are indeed minor in terms of overall seating and reactivity of the NP2(D1A) protein. The (1)H NMR resonances of the hemin substituents of the low-spin N-methylimidazole complexes of NP2(D1A,F42L,L105F,I120T) as well as NP2(D1A,I120T), NP2(D1A,Y104A), and NP2(D1A,F42A) have been assigned using natural abundance (1)H{(13)C} heteronuclear multiple quantum correlation and (1)H-(1)H nuclear Overhauser effect spectroscopy spectra.

Assignment of the ferriheme resonances of high- and low-spin forms of the symmetrical hemin-reconstituted nitrophorins 1–4 by 1H and 13C NMR spectroscopy: the dynamics of heme ruffling deformations

The four major nitrophorins (NPs) of the adult blood-sucking insect Rhodnius prolixus have been reconstituted with the "symmetrical hemin" 2,4-dimethyldeuterohemin, and their NMR spectra have been investigated as the high-spin (S=5/2) aqua and low-spin (S=1/2) N-methylimidazole (NMeIm) and cyanide complexes. The NMeIm complexes allow assignment of the high-spin hemin resonances by saturation transfer difference spectroscopy. The cyanide complexes were investigated as paramagnetic analogues of the NO complexes. It is shown that the hemin ring is highly distorted from planarity, much more so for NP2 than for NP1 and NP4 (with ruffling being the major distortion mode), for both high- and low-spin forms. For the cyanide complexes, the conformation of the distorted ring changes on the NMR timescale to yield chemical exchange (exchange spectroscopy, EXSY) cross peaks for NP1sym(CN), NP3sym(CN) and NP4sym(CN) but not for NP2sym(CN). These changes in nonplanar conformation are visualized as a "rolling" of the ruffled macrocycle ridges through some number of degrees, the lowest-energy ruffling mode. This probably occurs in response to slow protein dynamics that cause the I120 and L132 side chains in the distal heme pocket to move in opposite directions (up and away vs. down and toward the hemin ring). This in turn changes the out-of-plane displacements of the 2M and 3M of the symmetrical hemin on the NMR timescale. Two other types of dynamics, i.e., changes in heme seating and NMeIm rotation, are also observed. The highly distorted heme and the dynamics it causes are unique to the NPs and a few other heme proteins with highly distorted macrocycles.

Synthesis of poly(dihalophenylene oxide)s by atom transfer radical rearrangement polymerization via Ce(IV)/Ce(III) complexes in solid state

Poly(dihalophenylene oxide)s by solid-state thermal decomposition of (trihalophenolato) cerium complexes with either N-methyl imidazole or pyridine as neutral ligands were synthesized at different temperatures (80–250°C) and different time intervals (3–48 h) via atom transfer rearrangement radical polymerization. The complexes were characterized by FT-IR, DSC, SEM, mass spectroscopy, magnetic susceptibility and elemental analyses. Structural analyses of polymers were performed by 1H- and 13C-NMR, SEM and FT-IR spectroscopic analyses, T g by DSC and the molecular weights by viscometry. 2,4,6-Trichlorophenolate displayed a slightly higher selectivity in favor of 1,2-addition, whereas 1,2- and 1,4-additions took place at equal rates in the case of 2,4,6-tribromophenolate. Pyridine complexes decomposed at lower temperatures and had lower polymer yield than the N-methyl imidazole complexes. Among the synthesized polymers in solid state, the highest intrinsic viscosity value of 0.058 dl/g (M w = 7.15 × 104) was achieved only with Ce(TCP)3(Py)2 complexes. All the polymers having high T g values (178–189°C) were rigid.

Synthesis, crystal structure and characterization of new transition metal compounds of bromophenols: Bis(2,4,6-tribromophenolato) di(N-methylimidazole)M(II) (M=Co, Cu)

Hacettepe University, Department of Engineering Physics, Beytepe 06532, Ankara, Turkey Received 26 February 2004, revised 13 May 2004, accepted 24 May 2004 Published online 1 February 2005 Key words crystal structure, tri bromophenolate complexes, n-methyl imidazole complexes, cobalt(II) complex, copper(II) complex. PACS 61.66.Hq Bis(2,4,6-tribromophenolato)di(N-methyl imidazole)M(II), where M stands for cobalt and copper metals, was synthesized via reaction of the corresponding metal sulphate and 2,4,6-tribromophenolate in aqueous media in the presence of N-methyl imidazole and sodium hydroxide. Although various crystallization procedures were applied only cobalt complex was obtained as single crystals. The Co(II) ion has a distorted octahedral enviroment involving two O atoms and two N atoms of the Bis(2,4,6-tribromophenolato)di(N-methyl imidazole) ligand. Powder x-ray diffraction pattern of copper compound was used for cooper complex. For characterizations of complexes carbon, hydrogen and nitrogen elemental analysis, FTIR and UV spectroscopy, DSC thermal analysis and magnetic susceptibility measurements at room temperature were performed.

The structural mobility of zinc complexes with tetradentate tripodal ligands derived from the amino acids glycine and phenylalanineElectronic supplementary information (ESI) available: Fig. S1: 1H NMR spectra (CDCl3 solutions) of the bpaAc–Phe–OMe complexes 1b/2b, 3bo, 5b and 8b at room temperature. See http://www.rsc.org/suppdata/dt/b2/b212049f/

Crystallographic studies indicate that aromatic interactions favor the formation of trigonal-bipyramidal zinc(II) complexes with the tetradentate tripodal ligand bpaAc–Phe–OMe (N,N-bis(2-picolyl)aminoacyl-(S)-phenylalanine-methylester). The benzyl side chain in [(bpaAc–Phe–OMe)Zn(OTf)]+ (1b, OTf = triflate), [(bpaAc–Phe–OMe)Zn(H2O)]2+ (2b) and [(bpaAc–Phe–OMe)Zn(pz)]2+ (3btt, pz = pyrazole; t = trigonal-bipyramidal) is oriented towards the axial active coordination site with the phenyl ring ca. 4.5 A away from the metal center. Here we show that this conformation is retained in solutions of 1b, 2b and [(bpaAc–Phe–OMe)Zn(Cl)]+ (8b). Temperature-dependent 1H NMR spectra reveal that the phenylalanine side chain is locked in its position as long as the coordination sphere stays trigonal-bipyramidal. Based on our crystallographic results we expected the pyrazole and N-methylimidazole complexes 3btt and [(bpaAc–Phe–OMe)Zn(N-Meim)]2+ (4bttt, N-Meim = N-methylimidazole) to be labile with respect to addition of a sixth ligand such as triflate, water, pyrazole or N-methylimidazole. 1H NMR studies confirm that the six-coordinate species 3boo and 4booo (o: octahedral) are formed in solution. This is presumably due to solvation effects and mobilizes the benzyl side chain which is flexible in octahedral complexes.

Characterization and catecholase-mimetic behavior of imidazole adducts of copper(II) valproate. Crystal structure of the 2-methylimidazole adduct

Four mononuclear imidazole adducts of copper(II) valproate have been prepared and characterized with IR, UV—Vis, EPR spectroscopic techniques and fast atom bombardment mass spectrometry. The catecholase-mimetic catalytic activities of the complexes have been determined by monitoring the formation of o-quinone from catechol. The catalytic activities of the mononuclear complexes are lower than that of the dinuclear copper(II) valproate. The complexes with 2-methylimidazole and 1,2-dimethylimidazole show slightly lower activities that the imidazole and N-methylimidazole complexes. The structure of Cu(Val) 2(2mIm) 2 (Val=valproate; 2mIm=2-methylimidazole) has been determined. The complex crystallizes in the P2 1/ c space group with unit cell dimensions of a=9.309(1), b=12.456(2), c=12.560(2) Å, β=99.84(1)°, V=1435.0(3) Å 3, Z=2.

Thermal behaviour of biologically interesting coordination compounds obtained by reaction of some divalent metal ions with imidazole and N-methyl imidazole

The imidazole ring appears in a number of biological ligands and many different parameters, such as basicity of the ligands, π-electron acceptor ability, steric factors, hydrogen bonding, etc., play an essential role either on the structural types of coordination or on the thermal characteristics of the coordination compounds obtained by reaction of imidazole and its derivatives with transition metal ions. The influence of some of these parameters on the thermal properties of coordination compounds obtained by reaction of imidazole (Im) and N-methyl imidazole (NmIm) with Co(NO 3) 2, NO(NO 3) 2 and Cu(NO 3) 2 is studied here by means of thermogravimetric and differential scanning calorimetric methods supported by infrared spectroscopy. The compounds obtained were Me(L) n (NO 3) 2 (Me=Co(II), Ni(II), Cu(II); n = 4, 6). The imidazole complexes are always more stable than the corresponding N-methyl imidazole complexes and the ΔH value corresponding to metal-ligand bond breaking is higher for the imidazole compounds. The decomposition mechanisms are also discussed.

Studies of Zn(II) and Co(II) complexes of imidazole and N-methylimidazole with regard to the activity related ionization in carbonic anhydrase

Mixed aquo-N-methylimidazole complexes of Co(II) have been studied as a function of pH to gain a fuller understanding of the metal-binding site in Co(II)-carbonic anhydrase. The inherent affinity of N-methylimidazole for Co(II) has been calculated along with a species distribution for the stepwise addition of ligand to the metal ion. From these studies, it is apparent that the occurrence of Zn(II) rather than Co(II) in native carbonic anhydrase can be explained by the stronger affinity of Zn(II) for imidazole and the preference of Zn(II) for a tetrahedral geometry as offered by the enzyme. Octahedral Co(II) fails to ionize metal bound water. However, at high pH, Co(II)-N-methylimidazole complexes interact directly with the hydroxide ion, generating species with visible spectra very similar to that of Co(II)-carbonic anhydrase. Tentative structures have been proposed for these species.

NMR studies of hemoproteins VI. Acid-base transitions of ferric myoglobin and its imidazole complex

220 MHz proton NMR was applied to the acid-base transition of ferric myoglobin and its imidazole complex. In horse and sperm whale ferric myoglobins: 1. (1) pH-dependent shift of heme-ring methyl signals above p 2H 10 was analyzed on the basis of rapid exchange between alkaline and acidic forms by the use of p K value 9.1 of acid-base transition in 2H 2O solution; 2. (2) limiting shifts of three methyl signals were reasonably determined for purely alkaline form. For the imidazole complex: 3. (3) a drastic high field shift of each signal was observed above p 2H 9.0, whereas N-methyl imidazole complex did not exhibit such a shift, which suggests the 2H + dissociation from liganded imidazole > N 2 H. It is concluded that one 2H + dissociation and association is involved in the p 2H-dependent transitions.

Coboglobins

Abstract It has been previously found possible to reconstitute apomyoglobin with cobalt protoporphyrin IX to form oxygen-carrying cobaltoproteins which are presumably isomorphous with the natural hemoproteins. We have here observed the influence of an apoprotein upon the reactivity of a prosthetic group by comparing oxygen binding to the dimethyl ester of cobalt protoporphyrin IX with that to the metalloporphyrin incorporated in apomyoglobin. The protein environment causes no major alteration on the electronic structure of cobalt porphyrin or its oxygen adduct, yet produces a large effect on the stabilities of the cobaltoxygen linkage: at 25°, both whale and horse skeletal muscle cobaltomyoglobin have the same free energy changes upon oxygenation (+2.4 Cal per mole), and a binding constant ∼300 times greater than that for the N-methyl imidazole complex of porphyrin in solution. Furthermore, the two cobaltomyoglobins have substantially different enthalpies and entropies of oxygen binding: for horse the values are ΔH = -11.3 Cal per mole, ΔS = -46 e.u.; for whale ΔH = -13.3, ΔS = -53 e.u. A formal thermochemical cycle is used to obtain the contributions from apoprotein-prosthetic group interactions to the enthalpy and entropy of oxygen binding. These in turn are used to partition the binding enhancement (300-fold) for the two proteins into enthalpic and entropic contributions. The enhancement of oxygen binding in horse cobaltomyoglobin is entirely entropic, whereas that in whale cobaltomyoglobin is roughly balanced between enthalpic (20-fold) and entropic (15-fold) components. We have thus, for the first time, directly observed how two proteins of similar structure and function achieve similar modifications in prosthetic group reactivity through a different balance of enthalpic and entropic factors.

Spectroscopic studies of substituted imidazole complexes. II. N-methylimidazole complexes of divalent cobalt, nickel, copper and zinc

The preparations are reported of various 6:1, 4:1, 2:1 and 1:1 complexes of N-methylimidazole with divalent cobalt through zinc. The results of electronic and low frequency vibrational spectral and magnetic susceptibility measurements are presented and are used to assign coordination geometries. E.p.r. data are also given for the copper complexes.