Two new Fe(II) spin crossover complexes with tetrazol-1-yl-cycloalkane ligands

Two new 1-(tetrazol-1-yl)cycloalkanes [C_ntz] with n = 5 and 6 were synthesised as ligands for iron(II) spin crossover complexes. Just recently, the [Fe(C₃tz)₆](BF₄)₂ showed that the rigid cyclopropyl-substituent of the tetrazole yielded a rather abrupt and complete spin transition at T_½ ≈ 190 K [1]. Aiming for a deeper insight into the factors governing the spin transition behavior such as abruptness and spin transition temperature we synthesized the two new homologous complexes [Fe(C₅tz)₆](BF₄)₂ and [Fe(C₆tz)₆](BF₄)₂ which were characterized by XRPD, magnetic susceptibility measurements, DSC, ⁵⁷Fe-Mössbauer, UV-Vis-NIR and MIR spectroscopy. The magnetic and structural properties of both [Fe(C_ntz)₆](BF₄)₂ with n = 5 and 6 are also compared with the [Fe(C₃tz)₆](BF₄)₂ and its structural peculiarities are discussed.     

On the identity of the high spin heme components of cytochrome c oxidase

In oxidized, resting cytochrome c oxidase (EC 1.9.3.1) and under most conditions of partial reduction ? 50% of the heme components are detected by EPR spectroscopy. When the enzyme is fully reduced in the presence of equimolar quantities of cytochrome c, anaerobic reoxidation by an excess of a chemical oxidant (ferricyanide, porphyrexide) produces intense high and low spin heme signals simultaneously. The time range in which maximal high spin signals are observed is 0.1–2 s after mixing. Under these conditions 35–50% of the total heme a is accounted for by the low spin heme signal and 35–40% by the high spin signals, with the rhombic component accounting for 30–35% of the total heme. It is concluded that under these conditions, the major portion of both heme components must be EPR detectable. Thus, if the generally accepted assignment of the low spin signal to cytochrome a is adopted, it follows that in the experiments described, cytochrome a₃ is represented in the rhombic high spin signal. The quantities of heme represented in the axial high spin signal are too small for a definitive assignment; these signals could originate from either heme. When after formation of high spin signals as described, O₂ is admitted, the rhombic signal is eliminated within 4 ms. In the presence of the strongest rhombic high spin signals, the absorption band at 655 nm is only ? 25% developed. The implications of these findings are discussed in the context of present hypotheses concerning the state and interactions of cytochrome c oxidase components during oxidation-reduction.     

Conformational control of spin state in iron(II) tripodal imidazole complexes

Three new iron(II) N₆ tripodal complexes provide information on the role of ligand conformation on spin crossover behavior. The ligands (generated in situ) are the Schiff base condensate of tris(2-aminoethyl)amine (tren) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, H₃(1), and the condensates of tris(2-aminoethyl)methylammonium ion (N(Me)tren⁺) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, N(Me)H₃(1)⁺, or with 2-imidazole carboxaldehyde, N(Me)H₃(3)⁺. The structures of [FeH₃(1)](ClO₄)₂, [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃(1)](ClO₄)₃ are reported. The central tren nitrogen atom in these complexes exhibits three different geometries, pyramidal with the nitrogen pointed toward the iron (“N in”, Fe-N distance of 3.050 Å), planar (Fe-N distance of 3.527 Å), and pyramidal with the nitrogen pointed away from the iron atom (“N out”, Fe-N distance of 3.921 Å). With iron(II) the “N in” geometry is high spin while the planar and “N out” geometries are low spin. [FeH₃(1)](ClO₄)₂ exhibits spin crossover behavior between room temperature and 77 K as determined by Mössbauer spectroscopy and also exhibits a conformational change from “N in” to planar over this same temperature range. The structures of [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃ (3)](ClO₄)₃ are locked into the “N out” geometry due to the quaternary nitrogen atom and are low spin even at room temperature. The LS planar and “N out” conformations place a strain on the bond angles of the aliphatic arms of the ligand, which are more pronounced in the “N out” case. The HS “N in” geometry lacks this strain.     

Experimental spin density in the high spin ground state of the Fe8pcl cluster

The induced spin density was determined by polarised neutron diffraction in the S = 10 ground state of [(tacn)₆Fe₈O₂(OH)₁₂]Br_4.3(ClO₄)_3.7 · 6H₂O, Fe₈pcl, which differently from the bromide analogous, Fe₈Br_8, presents a centre of symmetry. The relative spin arrangement is in agreement with the model proposed from classical magnetic measurements and with the previous spin density study in the noncentrosymmetrical Fe₈Br₈ compound. The experimental spin populations on the iron atoms are in good quantitative agreement with calculations using exact diagonalisation of the exchange Hamiltonian with experimental J values obtained from magnetic susceptibility measurements on Fe₈Br₈. This determination confirms that the spin density determination provides a valuable evaluation of the relative strengths (and sign) of the intracluster magnetic interactions and that the dissymmetry observed on the spin populations in the Fe₈Br₈ compound with respect to quasi-D₂ symmetry of the molecular frame was an artefact due to the data refinement method for non centrosymmetrical structures.     

Conformational effect of a spin crossover iron(II) complex: Bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamineiron(II) perchlorate

A iron(II) complex of the linear hexadentate N₆ ligand H₂L^2-3-2, [Fe(H₂L^2-3-2)](ClO₄)₂, was synthesized and the spin crossover properties were investigated, where H₂L^2-3-2 denotes bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamine. The complex showed a gradual and reversible one-step spin crossover (SCO) between the high-spin (S = 2) and low-spin (S = 0) states at T_1/2 = 208 K without hysteresis. The crystal structures were determined at 296 and 250 K (HS state), 230, 210, and 200 K (intermediate between the HS and LS states) and 150 and 110 K (LS state). The spin transition from 296 to 150 K accompanies with the conformation change of the chelate rings at the triamine moiety and the formation of the hydrogen bond network in the same space group of orthorhombic Pbcn (no. 60). However, in the LS state at 110 K, the space group changed from orthorhombic Pbcn at 150 K (Pcan when the same axial setting to 110 K was used) to monoclinic P2₁/a (no. 14) at 110 K, although no spin transition and no change of assembly structure between 150 and 110 K were observed. It give us an idea that the space group transformation is mainly related to the conformational thermodynamic stability of the chelate rings at the triamine moiety and is not directly correlated with the spin transition.     

Interaction of transmembrane helices in ATP synthase subunit a in solution as revealed by spin label difference NMR

Subunit a in the membrane traversing F₀ sector of Escherichia coli ATP synthase is known to fold with five transmembrane helices (TMHs) with residue 218 in TMH IV packing close to residue 248 in TMH V. In this study, we have introduced a spin label probe at Cys residues substituted at positions 222 or 223 and measured the effects on the Trp ?NH indole NMR signals of the seven Trp residues in the protein. The protein was purified and NMR experiments were carried out in a chloroform-methanol-H₂O (4:4:1) solvent mixture. The spin label at positions 222 or 223 proved to broaden the signals of W231, W232, W235 and W241 located at the periplasmic ends of TMH IV and TMH V and the connecting loop between these helices. The broadening of W241 would require that the loop residues fold back on themselves in a hairpin-like structure much like it is predicted to fold in the native membrane. Placement of the spin label probe at several other positions also proved to have broadening effects on some of these Trp residues and provided additional constraints on folding of TMH IV and TMH V. The effects of the 223 probes on backbone amide resonances of subunit a were also measured by an HNCO experiment and the results are consistent with the two helices folding back on themselves in this solvent mixture. When Cys and Trp were substituted at residues 206 and 254 at the cytoplasmic ends of TMHs IV and V respectively, the W254 resonance was not broadened by the spin label at position 206. We conclude that the helices fold back on themselves in this solvent system and then pack at an angle such that the cytoplasmic ends of the polypeptide backbone are significantly displaced from each other.     

Comparative investigations on a series of [hexakis(1-(tetrazol-1-yl)alkane-N4)iron(II)] bis(tetrafluoroborate) spin crossover complexes: Methyl- to butyl-substituted species

A series of 1-(tetrazol-1-yl)alkanes [ntz] with n = 1-4 were synthesised as ligands for iron(II) spin crossover complexes. Within this series 1-(tetrazol-1-yl)butane [4tz] was prepared for the first time, whereas 1-(tetrazol-1-yl)methane [1tz], 1-(tetrazol-1-yl)ethane [2tz], 1-(tetrazol-1-yl)propane [3tz] and the [hexakis(ntz)iron(II)]bis(tetrafluoroborate) complexes were prepared according to the literature. Aiming for a comparative study we characterized all four compounds by XRPD, magnetic susceptibility measurements, ⁵⁷Fe-Moessbauer spectroscopy and IR spectroscopy. [Fe(4tz)₆](BF₄)₂ yielded appropriate single crystals and an X-ray structure of the new compound [Fe(4tz)₆](BF₄)₂ is presented. The magnetic and structural properties of all [Fe(ntz)₆](BF₄)₂ are compared and discussed.     

Pulse ENDOR and density functional theory on the peridinin triplet state involved in the photo-protective mechanism in the peridinin-chlorophyll a-protein from Amphidinium carterae

The photoexcited triplet state of the carotenoid peridinin in the Peridinin-chlorophyll a-protein of the dinoflagellate Amphidinium carterae has been investigated by pulse EPR and pulse ENDOR spectroscopies at variable temperatures. This is the first time that the ENDOR spectra of a carotenoid triplet in a naturally occurring light-harvesting complex, populated by energy transfer from the chlorophyll a triplet state, have been reported. From the electron spin echo experiments we have obtained the information on the electron spin polarization dynamics and from Mims ENDOR experiments we have derived the triplet state hyperfine couplings of the α- and β-protons of the peridinin conjugated chain. Assignments of β-protons belonging to two different methyl groups, with a_iso = 7.0 MHz and a_iso = 10.6 MHz respectively, have been made by comparison with the values predicted from density functional theory. Calculations provide a complete picture of the triplet spin density on the peridinin molecule, showing that the triplet spins are delocalized over the whole π-conjugated system with an alternate pattern, which is lost in the central region of the polyene chain. The ENDOR investigation strongly supports the hypothesis of localization of the triplet state on one peridinin in each subcluster of the PCP complex, as proposed in [Di Valentin et al. Biochim. Biophys. Acta 1777 (2008) 186-195]. High spin density has been found specifically at the carbon atom at position 12 (see Fig. 1B), which for the peridinin involved in the photo-protective mechanism is in close contact with the water ligand to the chlorophyll a pigment. We suggest that this ligated water molecule, placed at the interface between the chlorophyll-peridinin pair, is functioning as a bridge in the triplet-triplet energy transfer between the two pigments.     

Photothermal studies of the room temperature photo-induced spin state change in the Fe(III)(salten)(mepepy) complex

The complex [Fe(III)(salten)(mepepy)]BPh₄ (salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate; mepepy = 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)-ethene; BPh₄ = tetraphenyl borate) has been investigated to determine the volume and enthalpy changes associated with the room temperature photo-induced spin crossover. Here we report the photophysical properties of the trans to cis isomerization of the mepepy ligand as well as the spin crossover of the Fe(III)(salten)(mepepy) complex in acetonitrile:water mixtures using photoacoustic calorimetry (PAC). The PAC studies indicate that the trans to cis transition of the mepepy ligand occurs faster than the ∼20 ns response time of the acoustic detector and gives rise to a negligible volume change (0.7 ± 0.3 mL mol⁻¹) and an enthalpy change of 33 ± 10 kcal mol⁻¹. These results are consistent with the loss of a charge assisted hydrogen bond between a water molecule and the pyridyl ring of the mepepy upon photoisomerization. In the case of Fe(III)(salten)(mepepy) photoexcitation, PAC results indicate that the high-spin to low-spin transition, also occurring in ?20 ns, gives rise to small volume and enthalpy changes (0.9 mL mol⁻¹ and 4 kcal mol⁻¹). Analysis of the results indicate that the observed thermodynamics are related to a distortion of the Fe(II)(salten)(mepepy) complex associated with the cleavage of an Fe?N bond upon spin conversion.     

Inductive influence of 4′-terpyridyl substituents on redox and spin state properties of iron(II) and cobalt(II) bis-terpyridyl complexes

A series of iron and cobalt bis-terpyridine (terpy) complexes were prepared with the general formula [M(R-terpy)₂](PF₆)₂, where M represents Co(II) and Fe(II), and R is the following terpyridine substituents in order of increasing electron-withdrawing behavior [(C₄H₈)N, (C₄H₉)NH, HO, CH₃O, CH₃-phenyl, H, Cl, CH₃SO, CH₃SO₂]. The complexes were prepared to investigate the extent of redox and spin state control that is attainable by simply varying the electron donating/withdrawing influence using a single substituent site on the terpyridine ligand. Cyclic voltammetry was used to assess the substituents influence on the M(III/II) redox couple. A plot of the M(III/II) redox potential (E_1/2) versus the electron donating/withdrawing nature of the substituents (Hammett constants), shows a strong linear trend for both metals; however, the substituents were observed to have a stronger influence on the Fe(III/II) couple. Solution magnetic susceptibility measurements at room temperature were carried out using standard NMR methodology (modified Evans method) where all of the Fe(II) complexes exhibited a diamagnetic, low spin (S = 0) behavior. In the cobalt series where R = H for [Co(R-terpy)₂]²⁺, the complex is known to be near the spin cross-over where the room temperature effective magnetic moment (μ_eff) in solution is ≈3.1 Bohr magnetons; however, in this study the μ_eff is observed to vary between 2.7 and 4.1 Bohr magnetons depending on the R-substituent.     

New insight in coordination of vic-dioximes: Bis- and tris(E,E-dioximato)Ni(II) complexes

N,N′-Bis[allylamino]glyoxime, N,N′-bis[anilino]glyoxime, and N,N′-bis[1,2,3,4-tetrahydro-5-naphthalenamino]glyoxime have been prepared from corresponding amines and (E,E)-dichloroglyoxime. These ligands gave orange-red compound with NiCl₂ in less acidic medium (pH ∼ 5) that are bis(E,E-dioximato)nickel(II) complexes {[(E,E)-Ni(HL)₂]} (1a-3a) and green compounds in acidic medium (pH ∼ 2) that are tris(E,E-dioximato)nickel(II) dichloride complexes {[(E,E)-Ni(LH₂)₃]Cl₂} (1b-3b). The crystal structures of all complexes have been determined by X-ray diffraction on a single crystal. The study of absorption spectra of these two types of complexes shows that they may be converted to each other by addition of acids (1a-3a) or bases (1b-3b) and there is no way for the amphi form.     

Characterization of some low spin complexes of ferric hemeoctapeptide from cytochrome-c

Complexes of hemeoctapeptide, derived from bovine cytochrome c, show similar magnetic properties to those of low spin complexes of cytochrome c and hemoglobin. The electronic properties of hemeoctapeptide and cytochrome complexes are also similar while the Soret and beta bands of these analogs are generally blue shifted from those of corresponding complexes of hemoglobin due largely to the differences in the type of heme. Electron spin resonance calculations were carried out using Taylor's method to elucidate d orbital splittings and structural differences in hemeoctapeptide, hemoglobin, and cytochrome c. A correlation between V, the rhombicity, and the position of the beta band was found to exist and was dependent on protein type. However, neither the electronic not magnetic data was largely dependent on protein bulk. A large rhombic splitting caused shifts to the blue, and showed a dependence of the porphyrin pi orbitals on the placement of the metal relative to the porphyrin plane. A structural basis for the degree of rhombic splitting and thermodynamic parameters for ligand binding is proposed.     

Association of chlorophyll a/c2 complexes to photosystem I and photosystem II in the cryptophyte Rhodomonas CS24

Photosynthetic supercomplexes from the cryptophyte Rhodomonas CS24 were isolated by a short detergent treatment of membranes from the cryptophyte Rhodomonas CS24 and studied by electron microscopy and low-temperature absorption and fluorescence spectroscopy. At least three different types of supercomplexes of photosystem I (PSI) monomers and peripheral Chl a/c₂ proteins were found. The most common complexes have Chl a/c₂ complexes at both sides of the PSI core monomer and have dimensions of about 17 × 24 nm. The peripheral antenna in these supercomplexes shows no obvious similarities in size and/or shape with that of the PSI-LHCI supercomplexes from the green plant Arabidopsis thaliana and the green alga Chlamydomonas reinhardtii, and may be comprised of about 6-8 monomers of Chl a/c₂ light-harvesting complexes. In addition, two different types of supercomplexes of photosystem II (PSII) dimers and peripheral Chl a/c₂ proteins were found. The detected complexes consist of a PSII core dimer and three or four monomeric Chl a/c₂ proteins on one side of the PSII core at positions that in the largest complex are similar to those of Lhcb5, a monomer of the S-trimer of LHCII, Lhcb4 and Lhcb6 in green plants.     

The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme

In Photosystem II (PSII), the Mn₄CaO₅-cluster of the active site advances through five sequential oxidation states (S₀ to S₄) before water is oxidized and O₂ is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S₂ using EPR spectroscopy, quantum chemical calculations using Density Functional Theory (DFT), and time-resolved UV-visible absorption spectroscopy. The EPR experiments show that the equilibrium between S₂^LS and S₂^HS is pH dependent, with a pK_a?≈?8.3 (n?≈?4) for the native Mn₄CaO₅ and pK_a?≈?7.5 (n?≈?1) for Mn₄SrO₅. The DFT results suggest that exchanging Ca with Sr modifies the electronic structure of several titratable groups within the active site, including groups that are not direct ligands to Ca/Sr, e.g., W1/W2, Asp61, His332 and His337. This is consistent with the complex modification of the pK_a upon the Ca/Sr exchange. EPR also showed that NH₃ addition reversed the effect of high pH, NH₃-S₂^LS being present at all pH values studied. Absorption spectroscopy indicates that NH₃ is no longer bound in the S₃Tyr_Z state, consistent with EPR data showing minor or no NH₃-induced modification of S₃ and S₀. In both Ca-PSII and Sr-PSII, S₂^HS was capable of advancing to S₃ at low temperature (198?K). This is an experimental demonstration that the S₂^LS is formed first and advances to S₃via the S₂^HS state without detectable intermediates. We discuss the nature of the changes occurring in the S₂^LS to S₂^HS transition which allow the S₂^HS to S₃ transition to occur below 200?K. This work also provides a protocol for generating S₃ in concentrated samples without the need for saturating flashes.     

N-Alkylation of tripodal iron(III) imidazolate complexes: Reactivity and structure

The N-alkylation of iron(III) complexes of the tripodal imidazolate complexes derived from the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 2-imidazolecarboxaldehyde (2ImH), 4-imidazolecarboxaldehyde (4ImH) or 4-methyl-5-imidazolecarboxaldehyde (5-Me4ImH) was investigated. While each complex possesses three nucleophilic imidazolate nitrogen atoms, only the complex derived from 2-imidazolecarboxaldehyde, Fetren(2Im)₃, was completely alkylated under the ambient conditions used in this work. Using methyl iodide as the alkylating agent, a correlation between spin state of the product and degree of methylation was observed. Low spin iron complexes were more nucleophilic than high spin systems. The structure reactivity relationship was exploited in the reaction of Fetren(2Im)₃ with methyl iodide and allyl iodide to give [Fetren(N-Me2Im)₃]²⁺ and [Fetren(N-allyl2Im)₃]²⁺. The products are iron(II) due to reduction of the iron(III) by iodide ion which builds up in the reaction mixture as the alkylation reaction proceeds. These complexes were characterized by a number of methods including EA, IR, ES-MS, Mössbauer spectroscopy, magnetic susceptibility and X-ray diffraction.     

New amino-dithiaphospholanes and phosphoramidodithioites and their rhodium and iridium complexes

New sulfur derivatives of phosphoramidite ligands were synthesized and the impact of the sulfur unit on the spectroscopic properties of their rhodium and iridium complexes was investigated. The new ligands Bn₂NPSCH₂CH₂S^a(P-S^a) (Bn = benzyl, 4), Bn₂NPSCHCHS^a(CH₂)₃C^aH₂(P-S^a)(C^a-S^a) (6) and Bn₂NP(4-XC₆H₄OMe)₂ (X = S, 7a; X = O, 7b) were converted to the rhodium and iridium complexes trans-[Rh(CO)Cl(L)₂] (L = 4, 6, 7), [RhCl(COD)(L)] (L = 4, 6, 7), [IrCl(COD)(7a)] and [IrCl₂Cp∗(6)]. For comparison, some phosphoramidite complexes of these formulations also were synthesized. The new metal complexes were spectroscopically analyzed. For the carbonyl complexes, the ν_CO IR stretching frequencies were lower than for the corresponding phosphite and phosphoramidite ligands. The ¹J_PRh coupling constants for the rhodium complexes with the new ligands were also smaller than for the respective phosphoramidite and phosphite complexes. Finally, the ¹J_PSe coupling constants of the selenides of the new ligands were lower than those of the phosphoramidite ligands but higher than for PPh₃. The spectroscopic data reveal that the new thio ligands 4, 6 and 7a are more electron donating than phosphites and phosphoramidites but less electron donating than PPh₃.     

Capturing the spin state diversity of iron(III)-aryl porphyrins: OLYP is better than TPSSh

DFT calculations with a variety of exchange-correlation functionals, including PW91, OLYP, TPSSh, B3LYP and B3LYP*, have been carried out on the low-energy spin states of chloroiron(III) porphyrin and four aryliron(III) porphyrins, viz. Fe^III(P)Ph (S = 1/2), Fe^III(P)C₆F₅ (S = 5/2), Fe^III(P)(3,4,5-C₆F₃H₂) (S = 1/2), Fe^III(P)(2,4,6-C₆F₃H₂) (S = 5/2), where the expected spin states have been indicated within parentheses. Qualitatively, OLYP reproduces all the expected ground spin states. B3LYP appears to have some difficulty yielding the observed sextet ground states. B3LYP*, TPSSh and PW91 all fail to reproduce the sextet ground states, the latter two by rather large margins of energy. As far as this study is concerned, the overall performance of the functionals appears to be OLYP/OPBE > B3LYP > B3LYP* >> TPSSh > PW91/BLYP/BP86/TPSS.     

Electron spin resonance spectra of penta- and hexacoordinated nitrosyl iron protoporphyrin IX complexes

Electron spin resonance (ESR) spectra of frozen aqueous solutions of NO · haem · base complexes and NO · haem intercalated into dodecyl sulfate micelles have been measured at 77 K and analyzed for the hyperfine components of ¹⁵NO,¹⁴N-base, ¹⁴N-pyrroles and ⁵⁷Fe which coincide with the principal directions of the g tensor. The influence of the basicity of the nitrogen base on the spin distribution and geometry of the Fe-N-O grouping has been demonstrated by replacing imidazole for pyridine and by comparing the ESR spectra with those obtained for the monomeric insect haemoglobin CTT IV.The comparison of the hyperfine parameters described for the so-called pentacoordinated nitrosyl complex of CTT IV with those of the NO · haem intercalated into detergent micelles has furnished evidence that the ESR spectrum of this conformation state of haemoglobin has to be definitely assigned to a pentacoordinated nitrosyl complex.The a_zz values increase with the following orders: CTT IV (2.98 mT) < imidazole complex (3.04mT) < pyridine complex (3.15mT) for ¹⁵NO, and pyridine complex (0.59 mT) < imidazole complex (0.67 mT) < CTT IV (0.70 mT) for the ¹⁴N-base. This result is in conformity with an increase of the donor and the acceptor strengths of the nitrogen base in trans-position to ¹⁵NO. The a_yy and a_xx components of ¹⁵NO and the ¹⁴N-base are strongly nonequivalent in the nitrosyl haemoglobin CTT IV, and less nonequivalent in the NO · haem · pyridine complex, indicating bending of the Fe-N-O grouping. The hyperfine components of the axial ligands coinciding with the x and y component of the g tensor are nearly equal for the NO · haem · imidazole complex.     

The physical chemistry of Hemes II. Electron paramagnetic resonance study of the interaction of some iron(III)-porphyrins with fluoride ion in dimethylformamide

The interaction of F⁻ with high and low spin ferric deuteroporphyrin IX dimethyl ester and a low spin model compound, bis(histidine methyl ester) deuterohemin IX has been studied in dimethylformamide solution by low-temperature EPR. The reaction of F⁻ with these complexes leads to high spin compounds. The structure of the EPR line at g = 2 is due to superhyperfine interactions with axial fluoride ligands. It allows their identification as mono- or difluoride complexes. Their optical absorption spectra are reported. In the particular cases of bis(imidazole) deuterohemin IX dimethyl ester and of the model compound, the variations of the EPR spectra as functions of concentration of ionic ligand are reported. Three new low spin complexes are thus obtained. They are characterized by a specific interaction of F⁻ with the NH group of the imidazole ring. This is proved following a second independent study in which we report the changes in g tensor principal values of low spin ferric porphyrins with the basicity (pK_a) of various nitrogenous bases.     

Electron spin resonance melting of chemically spin-labeled nucleic acids.

Conformational transitions of nitroxide labeled and unlabeled nucleic acids were analyzed by esr and uv spectroscopy to evaluate potential perturbation effects caused by chemical modifications of nucleic acids with spin labels. The melting temperature (T_m) determined by uv or esr melting profiles of 2 → 1 or 3 → 1 transitions is similar for labeled and unlabeled polyadenylic acid [(A)_n] and polyuridylic acid [(U)_n] complexes provided spin-labeled (A)_n with a nitroxide to nucleotide ratio of 0.002 is used. Complexes formed with spin-labeled (A)_n of greater spin-labeling extent display a noticeable perturbation of their thermal melting profiles. The studies reconfirm the existence of a low temperature esr transition at about 20 °C with calf thymus and T4 DNA duplexes spin-labeled with a nitroxide to nucleotide ratio of about 0.006. The uv melting profiles of the spin-labeled duplexes reveal no low-temperature discontinuity, but the T_m values reflecting the 2 → 1 transitions were reduced by several degrees versus those of the unlabeled duplexes. Thus, these studies suggest that with homopolymers, chemically modified to a low extent with nitroxides, the monitoring of local conformational transitions of duplexes or triplexes reflect the overall 2 → 1 or 3 → 1 transitions. In the case of the heteropolymers the possibility that the chemical modification is responsible for the low-temperature phenomenon cannot be ruled out.