Possible Intermediates in Biological Metalloporphyrin Oxidative Degradation. Nickel, Copper, and Cobalt Complexes of Octaethylformybiliverdin and Their Conversion to a Verdoheme

Direct Oxidations of meso-Tetrakis(pentafluorophenyl)porphyrin: Porphotrilactones and Entry into a Nonbiological Porphyrin Degradation Pathway

The direct oxidations of meso-tetrakis(pentafluorophenyl)porphyrin using cetyltrimethylammonium permanganate (CTAP), RuCl3/Oxone/base or Ag+/oxalic acid each generate distinctive product mixtures that may contain, inter alia, porpho-mono-, di-, and trilactones. The CTAP and RuCl3/Oxone/base oxidations also generate a unique open chain tripyrrin derived from the degradation of a porpholactone oxazolone moiety. Thus, its formation and structure are distinctly different from all biological or nearly all other nonbiological biliverdin-like linear porphyrinoid degradation products that are derived from ring cleavages between the pyrrolic building blocks.

Making and Breaking Helical Open-Chain Oligopyrroles

Closed-chain oligopyrroles such as porphyrins or corroles have been well-established in literature and experience a steadily strong interest by several fields of science. However, their open-chain derivatives are comparatively underrepresented, despite their intriguing properties and promising applications. Here, we aim to review typical synthetic routes, as well as point towards several emergent properties, marking them as interesting candidates for various fields of study. The review focuses on two traditional methods (each starting from highly symmetric metalloporphyrins) and then expands its scope towards more recent variations before moving on to more exotic and recent highlights that have yet to be included into the canon. Key chemical reactivities (ring closure, substitution and fragmentation) are then followed by notable physicochemical properties, placing special emphasis on potential uses in molecular electronics and sensors.

Synthesis, Structure, and Properties of Helical Bis-Cu(II) Complex of Linear Hexapyrrolic Ligand

The chemistry of metal helical complexes has attracted wide interest not only because of their resemblance with DNA structure but also due to their unique photophysical and chiroptical properties. Linear hexapyrrolic ligand 1 has been designed and synthesized using 3-pyrrolyl BODIPY as a key precursor. The reactivity of the appended pyrrole group of 3-pyrrolyl BODIPY was taken as an advantage to synthesize bis(3-pyrrolyl BODIPY) by treating 3-pyrrolyl BODIPY with 10 equiv of acetone in CHCl₃ under acid-catalyzed conditions and afforded bis(3-pyrrolyl BODIPY) 2 in 20% yield. Bis(3-pyrrolyl BODIPY) 2, in which two 3-pyrrolyl BODIPY units were connected via sp³ meso carbon, was very stable, and its identity was confirmed by HR-MS, NMR, and X-ray crystallographic analysis. The X-ray structure revealed that the 3-pyrrolyl BODIPY moieties in bis(3-pyrrolyl BODIPY) 2 remained almost planar and arranged at an angle of 98.4° with each other, leading to a V-shaped conformation. In the next step, bis(3-pyrrolyl BODIPY) 2 was treated with AlCl₃ in acetonitrile/methanol at reflux to afford hexapyrrolic ligand 1. Hexapyrrolic ligand 1 was treated with CuCl₂ in acetonitrile at room temp for 1 h followed by crystallization to afford helical bis-Cu(II) complex 1-Cu. Bis-Cu(II) complex 1-Cu was characterized and studied by HR-MS, X-ray crystallography, ESR, absorption, and DFT/TD-DFT techniques. The X-ray structure revealed that the bis-Cu(II) complex was a double-stranded bimetallic helicate and each Cu(II) ion was coordinated to four nitrogen atoms of two dipyrrin units from two hexapyrrolic ligands in a distorted tetrahedral geometry. The crystal packing diagram showed that the bis-Cu(II) complex formed as a racemic mixture containing both M and P isomers which was unable to isolate. The ESR spectrum of bis-Cu(II) complex 1-Cu indicated the presence of two noninteracting Cu(II) ions in slightly different coordination environments. DFT and TD-DFT studies were in agreement with the experimental observations of bis(3-pyrrolyl BODIPY) 2 complex and bis-Cu(II) complex 1-Cu.

Stereochemistry and chiroptical properties of bimetallic single helicates of hexapyrrole-α, ω-dicarbaldimines with high diastereoselectivity

Bimetallic complexes of hexapyrrole-[Formula: see text],[Formula: see text]-dicarbaldimines consisting of a pair of four-coordinate metal sites adopt a helical closed [Formula: see text]-symmetric form or sigmoidal open forms depending on whether the 2,2[Formula: see text]-bipyrrole subunit at the center of the hexapyrrole chain takes cis- or trans-conformation. X-ray crystallography of a bisNi complex having N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units at both ends of the hexapyrrole chain revealed a non-symmetric heterohelical open form where the metal coordination sites of opposite helical sense sit on opposite sides of the central 2,2[Formula: see text]-bipyrrole subunit. BisPd complexes preferred a closed [Formula: see text] form and a steric bulk at the 3,3[Formula: see text]-position of the 2,2[Formula: see text]-bipyrrole subunit improved the helical sense bias. A bisPd complex with N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units adopts a helical closed [Formula: see text] form exclusively with full bias for a [Formula: see text]-helical sense. These bimetallic single stranded helicates were reversibly oxidized to [Formula: see text]-cation radicals at 0.1[Formula: see text]0.3 V vs. a ferrocene/ferrocenium couple and spectroelectrochemistry revealed remarkable absorption and CD spectral changes in the Vis-NIR region.

Biliverdin-copper complex at physiological pH

Biliverdin (BV), a product of heme catabolism, is known to interact with transition metals, but the details of such interactions under physiological conditions are scarce. Herein, we examined coordinate/redox interactions of BV with Cu2+ in phosphate buffer at pH 7.4, using spectrophotometry, HESI-MS, Raman spectroscopy, 1H NMR, EPR, fluorimetry, and electrochemical methods. BV formed a stable coordination complex with copper in 1 : 1 stoichiometry. The structure of BV was more planar and energetically stable in the complex. The complex showed strong paramagnetic effects that were attributed to an unpaired delocalized e-. The delocalized electron may come from BV or Cu2+, so the complex is formally composed either of BV radical cation and Cu1+ or of BV radical anion and Cu3+. The complex underwent oxidation only in the presence of both O2 and an excess of Cu2+, or a strong oxidizing agent, and it was resistant to reducing agents. The biological effects of the stable BV metallocomplex containing a delocalized unpaired electron should be further examined, and may provide an answer to the long-standing question of high energy investment in the catabolism of BV, which represents a relatively harmless molecule per se.

Oxygenation of Phenanthriporphyrin and Copper(III) Phenanthriporphyrin: An Efficient Route to Phenanthribilinones

Photooxidation of copper(III) 5,6-dimethoxyphenathriporphyrin and copper(III) 5,6-dioxophenanthriporphyrin, which contain phenanthrene or dioxophenathrene moieties built into the macrocyclic frameworks, resulted in the regioselective cleavage that afforded organometallic copper(III) complexes of open-chain phenanthribilinone-type acyclic ligands terminated by carbonyl groups. The copper(III) coordinates two carbon atoms of phenantherene (dioxophenanthrene) and two nitrogen atoms of pyrrole and pyrrolone units, preserving the donor sets of the paternal complexes. The primary dioxygen attack is located at the meso carbon atom adjacent to the phenanthrene moiety. Demetalation of copper(III) 21-benzoyl-phenanthribilin-1-one and copper(III) 21-benzoyl-dioxophenanthribilin-1-one yielded mainly two diastereomers [15Z, 20E] and [15Z, 20Z], which differ in the configurations at two C_α-C_meso double bonds. The regioselectivity of the cleavage, detected in the course of experimental studies, has been substantiated by DFT investigations. The regioselective cleavage of 5,6-dimethoxyphenanthriporphyrin in reaction with basic iron(III) acetate was detected, providing the synthetically efficient methodology to produce 21-benzoyl-dioxophenanthribilin-1-one.

Oxidative ring-openings of octaethyl-2-oxochlorin: Regioisomeric β-oxobiliverdins

We describe the oxidative ring opening of octaethyl-2-oxochlorin using two different oxidation methods, both providing a mixture of all possible regioisomeric products (8-[Formula: see text] through 8-[Formula: see text]. While isomers 8-[Formula: see text], 8-[Formula: see text], and 8-[Formula: see text] formed in isolable yields and relative ratios that varied with the oxidation method used, isomer 8-[Formula: see text] was invariably formed in trace amounts only. The three major products were spectroscopically characterized (IR, MS, 1D- and 2D NMR spectroscopy) and their configurations were deduced by NMR spectroscopy. The spectroscopic findings correlated well with the single crystal X-ray structure of the novel cleavage product 8-[Formula: see text] and the known compound of 8-[Formula: see text]. The work broadens the number of octaethylporphyrin-derived biliverdin derivatives available and presents a methodology of controlling the biliverdin backbone configuration by introduction of a [Formula: see text]-ketone functionality into select positions.

In situ redox reactions facilitate the assembly of a mixed-valence metal-organic nanocapsule

C-alkylpyrogallol[4]arenes (PgCs) have been studied for their ability to form metal-organic nanocapsules (MONCs) through coordination to appropriate metal ions. Here we present the synthesis and characterization of an Mn^II/Mn^III-seamed MONC in addition to its electrochemical and magnetic behavior. This MONC assembles from 24 manganese ions and 6 PgCs, while an additional metal ion is located on the capsule interior, anchored through the introduction of bridging nitrite ions. The latter originate from an in situ redox reaction that occurs during the self-assembly process, thus representing a new route to otherwise unobtainable nanocapsules.

New approaches are required to access metal-organic assemblies with unusual structural properties. Here, the authors use an in situ redox reaction to obtain a mixed-valence, Mn(II)/Mn(III)-containing metal-organic nanocapsule with an odd number of metal ions.

Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting Pd^II or Pt^II centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single Pd^II and Pt^II center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV-visible region, especially in the 350-600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ∼500-700 nm and phosphorescence from ∼700-875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier Pt^II center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of ¹O₂ (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize ¹O₂ may make them attractive platforms for development of new agents for photodynamic therapy.

Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn²⁺ and Cu²⁺ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of ¹O₂ at wavelengths longer than 500 nm was probed. Both the free base and Zn²⁺ 10,10-dimethylbiladiene architectures show modest efficiencies for ¹O₂ sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O₂ and other small molecules.

Electrochemical investigations of tripyrrin complexes

Electrochemical investigations on divalent transition metal complexes with a conjugated linear tripyrrole ligand, namely 3,4,8,9,13,14-hexaethyl-2,15-dimethyltripyrrin (HTrpy) are reported. This tripyrrin ligand behaves as a tridentate monoanionic ligand and forms a series of neutral metal complexes of the type TrpyMX, where M = Zn ( II ), Cu ( II ), Ni ( II ) Co ( II ) or Pd ( II ) and X is a chloride anion ( Cl -). The studied nickel, cobalt and zinc complexes undergo respectively three and two ligand-centered reversible one-electron reductions and a reversible ligand-centered one-electron oxidation. Only irreversible electron transfers are observed for TrpyPdCl , due to the instability of the reduced complex. TrpyCoCl does not undergo any metal-centered electron transfer which is unexpected compared to related complexes. The copper(II) complex shows an additional reduction step occurring on the metal, generating a stable copper(I) complex. A comparison of the redox behavior of tripyrrin complexes with porphyrins and linear tetrapyrrolic complexes, namely bidipyrrins, clearly indicates that the observed redox behavior of tripyrrin complexes is mainly metal dependent. The observed HOMO-LUMO gap in the studied series of tripyrrins is rather small (about 1.70 V) compared to porphyrins (2.25 V), but it is close to bidipyrrins (1.60 V).

Copper(II) and Copper(III) Complexes of Pyrrole-Appended Oxacarbaporphyrin

The reaction of an O-confused porphyrin with a pendant pyrrole 4 and copper(II) acetate yields an organocopper(III) diamagnetic complex 4-Cu(III) substituted at the C(3) position by the pyrrole and H. The transformation of 4-Cu(III), performed in aerobic conditions, gave a rare copper(II) organometallic compound 6-Cu(II). In the course of this process, the tetrahedral-trigonal rearrangement originated at the C(3) atom but effects the whole structure. The electron paramagnetic resonance spectroscopic features correspond to a copper(II) oxidation state. A crystallographic analysis of 6-Cu(II) confirmed the formation of a direct metal-C bond [Cu(II)-C 1.939(4) A]. It was found that the Cu(II) complex of O-confused oxaporphyrin is sensitive to oxidative conditions. The degradation of 6-Cu(II) to yield copper(II) tripyrrinone complexes has been observed, which was considered as a peculiar case of dioxygen activation in a porphyrin-like environment. This process is accompanied by regioselective oxygenation at the inner C to form the 2-oxa-3-(2'-pyrrolyl)-21-hydroxycarbaporphyrinatocopper(II) complex ((pyrr)OCPO)CuII (8). The reaction of 6-Cu(II) with hydrogen peroxide, performed under heterophasic conditions, resulted in quantitative regioselective hydroxylation centered at the internal C(21) atom, also producing 8. Treatment of 8 with acid results in demetalation to form the nonaromatic 21-hydroxy O-confused porphyrin derivative ((pyrr)OCPOH)H (9).

Diphenic acid as a general conformational lock in the design of bihelical structures

The bihelical (figure of "infinity") topology was examined from vantages of design, crystal structures, chirality, circular dichroism (CD) studies and molecular-orbital calculations. The minimalistic design envisaged the sequential linking of cystine to the anchor diphenic acid, which proved to be a general conformational lock. The bihelical compound 4 was obtained in two steps from diphenic anhydride 1 and cystine di-OMe. The chirality of 4 arises largely from the L-cystine. The bihelical compound 5 obtained from D-cystine di-OMe was found, by X-ray crystallography, CD studies, and optical rotation, to be the perfect mirror image of 4 prepared from L-cystine. The crystal structure of prototype 8, prepared by protocols used for 4 from the achiral cystine analogue cystamine, had a "U"-shaped conformation held together by intramolecular hydrogen bonds. Analysis of 4 and 5 show that the pairs of nine-membered beta-turn-like constructs made compact through hydrogen bonding with DMSO hold the key for the bihelical conformation. Another factor is the need for the presence of a ligand at the Calpha position. The absence of this, as in 8, allows major flexibility in the torsional angles around this critical region, promoting flexible alternatives. The CD analysis of 4, confirmed to be bihelical by X-ray crystallography, showed a typical negative band at about 210 A attributed to the beta-turn-like motif, and in the positive-band region a peak at about 227 A, generally related to the twist of the biphenyl unit. The cystamine analogue 8, which showed a "U"-type structure, presented a CD spectrum with no typical features. The total energy, derived from theoretical calculations by using the X-ray structure data, support the bihelical structure for 4 and a "U"-shaped one for 8. The limited utility of such calculations was tested with composite 9. Composite 9, in which the anchor diphenic acid is linked to cystamine on the one hand and to cystine on the other, showed a CD spectrum similar to that of 4, and this coupled with molecular-orbital calculations, using data from 4 and 8, predict a bihelical structure for this compound.

A Cu-Zn-Cu-Zn heterometallomacrocycle shows significant antiferromagnetic coupling between paramagnetic centres mediated by diamagnetic metal

An unprecedented antiferromagnetic exchange mediated by two -O-Zn-O- bridges, with singlet-triplet splitting J= 35.0 cm-1, was observed between two copper centers separated by 5.7062(9)A in the heterometallomacrocyclic diethanolamine (H2L) complex [Cu2Zn2(NH3)2Br2(HL)4]Br2.CH3OH.

Metal Complexes of meso-Amino-octaethylporphyrin and the Oxidation of NiII(meso-amino-octaethylporphyrin)

The crystal structures of meso-NH2-OEPH2, NiII(meso-NH2-OEP), and CuII(meso-NH2-OEP) (where OEP is the dianion of meso-amino-octaethylporphyrin) have been determined to examine the effects of the meso-substituent on the geometry of the ligand. CuII(meso-NH2-OEP) has a nearly planar geometry while the free ligand itself and NiII(meso-NH2-OEP) have ruf conformations. NiII(meso-NH2-OEP) is much less reactive toward oxidation than are (py)2FeII(meso-NH2-OEP), ClFeIII(meso-NH2-OEP), or NiII(meso-HO-OEP), which all undergo oxidation in pyridine solution when exposed to dioxygen. Treatment of NiII(meso-NH2-OEP) with iron(III) chloride in chloroform solution does result in oxidation of the ligand in two separate processes. One involves oxygenation at the trans-meso position, while the other results in ring cleavage and removal of the amino function. The open-chain tetrapyrrole complex, NiII(OEB-CO2Et), has been characterized by single-crystal X-ray diffraction and shown to contain a helical ligand with a four-coordinate nickel ion.

Metallobiliverdin radicals--DFT studies

Several aspects of the molecular and electronic structure of biliverdin derivatives have been studied using density functional theory (DFT). The calculations have been performed for complexes of trianion (BvO2)3- and dianion [BvO(OH)]2-, derived from two tautomeric forms of biliverdin, BvO2H3 and [BvO(OH)]H2, with redox innocent metal ions: lithium(I), zinc(II), and gallium(III). One-electron-oxidized and reduced forms of each complex (cation and anion radicals) have been also considered. The molecular structures of all species investigated are characterized by a helical arrangement of tetrapyrrolic ligands with the metal ion lying in the plane formed by the two central pyrrole rings. The spin density distribution in four types of metallobiliverdin radicals--[(BvO2.)Mn+]n-2,[[BvO(OH).]Mn+]n-1 (cation radicals),[(BvO2.)Mn+]n-4,[[BvO(OH).]Mn+]n-3 (anion radicals)--has been investigated. In general, the absolute values of spin density on meso carbon atoms were larger than for the beta-carbon atoms. Sign alteration of spin density has been found for meso positions, and also for the beta-carbon atoms of at least two pyrrole rings. The calculated spin density maps accounted for the essential NMR spectroscopic features of iron biliverdin derivatives, including the considerable isotropic shifts detected for the meso resonances and shift alteration at the meso and beta-positions.

A remarkable skeletal rearrangement of a coordinated tetrapyrrole: chemical consequences of palladium pi-coordination to a bilindione

Pd4(OEB)2, in which a [Pd2]2+ unit is bound in pi-fashion to olefinic sites that are exocyclic to pyrrole rings of the octaethylbilindione ligand, undergoes an unprecedented sequence of reactions that results in the rearrangement of the framework of the bilindione ligand and the formation of trans-Pd(py)2I2. This process of bilindione rearrangement and oxidation occurs as a direct consequence of the pi-coordination of the palladium. The reaction results in the migration of a nitrogen atom from a pyrrole carbon atom to what was formerly a meso carbon atom to transform a former pyrrole ring into a six-membered ring. This process also involves cleavage of the Pd-Pd and Pd-C bonds, oxidation of palladium, and introduction of an oxygen atom (from water) not necessarily in this particular sequence.

Ring-opening and meso substitution from the reaction of cyanide ion with zinc verdohemes

The reactivity of zinc verdoheme, [Zn(II)(OEOP)](O(2)CCH(3)) where OEOP is the monoanion of octaethyl-5-oxaporphyrin, with cyanide ion has been shown to be a complex process that involves not only the expected ring-opening of the macrocycle, as occurs with other nucleophiles (methoxide, methanethiolate, dimethylamide), but also substitution at one or two of the meso positions. The ring-opened products have been subjected to crystallographic study. The structures of mu-H(2)O-[Zn(II)(OEB-10,19-(CN)(2))](2) and mu-H(2)O-[(Zn(II)(OEB-10,15,19-(CN)(3))](2) both consist of two helical tetrapyrrole subunits that are coordinated to a zinc ion through four Zn-N bonds. The two zinc ions are coordinated to a bridging water molecule that is also hydrogen bonded to a lactam oxygen atom at one end of each tetrapyrrole subunit. Thus the chiral sense of one helical Zn(II)(OEB-10,19-(CN)(2)) portion is transmitted to the other Zn(II)(OEB-10,19-(CN)(2)) unit and the resulting binuclear unit is chiral. In contrast Co(II)(OEB-15,19-(CN)(2)), which was obtained by the insertion of Co(II) into the free ligand, is monomeric with a four-coordinate cobalt ion. A series of DFT geometry optimization calculations were performed on zinc complexes of 5-oxaporphyrins (verdoheme), verdins (bilindione), 4-cyano-5-oxaporphyrins, and 19-cyanoverdins in an effort to gain insights to the features of these complexes and the reactions that lead to meso-cyano-substituted cyanoverdins.

Biladienones from the photooxidation of a meso-gem-disubstituted phlorin: crystal and molecular structures of the 3N + O coordinated nickel(II) and copper(II) complexes

The photooxidation of a meso-gem-disubstituted phlorin gave two isomeric biladienones in an equilibrium involving a Z-E double bond photoisomerism. The structures of these bile pigments were elucidated using NMR techniques and show terminal benzoyl and pyrrolone moieties. Complexation with divalent cations (nickel(II), copper (II)) gave stable compounds whose crystal and molecular structure could be determined by X-ray diffraction. The metal is coordinated with three pyrrole nitrogen atoms and one oxygen atom of a terminal benzoyl group. In the crystal, the molecules are arranged in pairs through hydrogen bonds between the free terminal pyrrolones.

Oxidation of copper(II) hydroxyporphyrin (oxophlorin); oxidative ring opening and formation of an ester-linked, dinuclear copper complex

Addition of copper(II) acetate to octaethyl(meso-hydroxy)porphyrin (H2OEPOH) in THF under a dinitrogen atmosphere produces CuII(OEPOH), which has been isolated as red crystals and shown to have a meso-hydroxyporphyrin structure by UV-vis and EPR spectroscopy. CuII(OEPOH) undergoes oxidation by dioxygen to form a novel dinuclear copper complex {CuII(OEPOC(O)OEB)CuII}. This dinuclear complex is composed of a copper(II) meso-substituted porphyrin portion (with a planar CuN4 unit), which is attached through an ester linkage to a helical copper(II) tetrapyrrole that has been ring-opened through oxidative cleavage of a second molecule of CuII(OEPOH). The oxidative ring-opening reaction resembles that of natural heme catabolism but is arrested at a stage where the oxidized meso-carbon is still appended to the helical open tetrapyrrole. A mechanism is suggested for this process.Key words: copper porphyrin, crystal structure, porphyrin oxidation, linear tetrapyrrole, hydroxy-porphyrin, oxophlorin.

Double-helical cyclic peptides: design, synthesis, and crystal structure of figure-eight mirror-image conformers of adamantane-constrained cystine-containing cyclic peptide cyclo (Adm-Cyst)(3)

A large number of macrocycles containing alternating repeats of cystine diOMe(-NH-CH(CO(2)Me)-CH(2)-S-)(2) and either a conformationally rigid aromatic/alicyclic moiety or a flexible polymethylene unit (X) in the cyclic backbone with ring size varying from 13- to 78-membered have been examined by spectral ((1)H NMR, FT-IR, CD) and X-ray crystallography studies for unusual conformational preferences. While (1)H NMR measurements indicated a turnlike conformation for all macrocycles, stabilized by intramolecular NH.CO hydrogen bonding, as also supported by FT-IR spectra in chloroform, convincing proof for beta-turn structures was provided by circular dichroism studies. Single-crystal X-ray studies on 39-membered cyclo (Adm-L-Cyst)(3) revealed a double-helical fold (figure-eight motif) for the macrocycle. Only a right-handed double helix was seen in the macrocycle constructed from L-cystine. The mirror-image macrocycle made up of D-cystine units exhibited a double helix with exactly the opposite screw sense, as expected. The enantiomeric figure-eights were stabilized by two intramolecular NH. CO hydrogen bonds and exhibited identical (1) H NMR and FT-IR spectra. The CD spectra of both isomers had a mirror-image relationship. The present results have clearly brought out the importance of cystine residues in inducing turn conformation that may be an important deciding factor for the adoption of topologically important structures by macrocycles containing multiple S-S linkages.

Reactivity of the Verdoheme Analogues, 5-Oxaporphyrin Complexes of Cobalt(II) and Zinc(II), with Nucleophiles: Opening of the Planar Macrocycle by Alkoxide Addition To Form Helical Complexes

The 5-oxaporphyrin macrocycle is a modified porphyrin (with an oxygen atom replacing a meso-methine unit) that is produced during heme degradation in biological and chemical systems. We have undertaken an investigation into the chemical reactivity of metal complexes of this macrocycle. Here we report that the diamagnetic zinc(II) and paramagnetic cobalt(II) verdoheme analogues, [Zn(II)(OEOP)](O(2)CCH(3)) and [Co(II)(OEOP)](PF(6)) (OEOP is the monoanion of octaethyl-5-oxaporphyrin), undergo ring opening when treated with alkoxide ions. The zinc(II) complex Zn(II)(OEBOMe), where OEBOMe is the dianion of octaethylmethoxybiliverdin, is sufficiently stable to be isolated in crystalline form, but the cobalt(II) analogue, Co(II)(OEBOMe), is less stable and has been characterized primarily by (1)H NMR spectroscopy in solution. The reactions are accompanied by a color change from blue-green for the verdoheme complexes to yellow-green for the ring-opened compounds. The ring-opened product Zn(II)(OEBOMe) has been subjected to a crystallographic study which reveals that the complex contains a four-coordinate zinc(II) ion whose geometry is significantly controlled by the helical tetrapyrrole ligand so that tetrahedral coordination is not achieved. The structure of this complex is compared to that of a closely related free ligand and those of copper and cobalt complexes of tetrapyrrole ligands that also form helical complexes.