Methoxy-isoporphyrins of water-soluble porphyrins: synthesis,characterization and electrochemical properties

Abstract

Methoxy-isoporphyrins of zinc [5,10,15,20-tetrakis(4-sulfonatophenyl)]porphyrin, ZnTSPP (1a) and zinc [5,10,15,20-tetrakis(4-carboxyphenyl)]porphyrin, ZnTCPP (1b) have been synthesized and characterized using standard spectroscopic techniques (Uv-visible, ¹H NMR) , ESI-mass spectrometry and powder X-ray diffraction studies. The isoporphyrins [5-(methoxy)-5,10,15,20-tetrakis(4-sulfonatophenyl)-5H,15H-porphinato]zinc(II) (2a) and [5-(methoxy)-5,10,15,20-tetrakis(4-carboxyphenyl)-5H,21H-porphinato]zinc(II) (2b) are formed due to nucleophilic attack of the methanol to the zinc porphyrin dication. Ceric ammonium nitrate (CAN) was used to oxidize zinc porphyrin and to form zinc porphyrin dication. The electronic spectra of the isoporphyrin complexes 2a and 2b exhibit an intense peak at near IR region . Electrochemical measurements of the synthesized isoporphyrins showed a typical irreversible reduction peak at lower potential. S-containing nucleophiles, which work as reducing agents, convert the zinc isoporphyrins to their parent porphyrins, which supports the electrochemical observations. Their structural properties have been studied using powder X-ray diffraction. The luminescence properties of isoporphyrins were compared with the parent zinc porphyrins.     

Cellular uptake and photocytotoxicity of glycoconjugated chlorins in HeLa cells

Eight 5,10,15,20-tetrakis[3- or 4-(beta-D-glycopyranosyloxy)phenyl]chlorins were synthesized by means of the Whitlock method with diimide reduction and purified by reversed-phase thin layer chromatography (RP-TLC). All compounds were characterized by (1)H NMR spectroscopy, electron-spray ionization time-of-flight mass spectrometry (ESI-TOF MS), and UV-Vis spectroscopy. ESI-TOF MS could detect the 2H difference in molecular weight between a glycoconjugated chlorin and its corresponding porphyrin (i.e., 5,10,15,20-tetrakis[3- or 4-(beta-D-glycopyranosyloxy)phenyl]porphyrin). The cellular uptake of the eight chlorins was evaluated in HeLa cells. All glycoconjugated chlorins showed higher cellular uptake than tetraphenylporphyrin tetrasulfonic acid (TPPS), and 5,10,15,20-tetrakis[3-(beta-D-xylopyranosyloxy)phenyl]chlorin showed 50-fold higher uptake than TPPS. The photocytotoxicity of 5,10,15,20-tetrakis[3-(beta-D-glucopyranosyloxy)phenyl]chlorin, 5,10,15,20-tetrakis[3-(beta-D-xylopyranosyloxy)phenyl]chlorin and TPPS towards HeLa cells was examined at the concentration of 2x10(-7) M (mol/dm(3)). These photosensitizers had no cytotoxicity in the dark, but their photocytotoxicity decreased in the order of 5,10,15,20-tetrakis[3-(beta-D-glucopyranosyloxy)phenyl]chlorin>5,10,15,20-tetrakis[3-(beta-D-xylopyranosyloxy)phenyl]chlorin>TPPS. The results indicate that the photocytotoxicity is not related simply to cellular uptake.     

Cellular uptake and photocytotoxicity of glycoconjugated porphyrins in HeLa cells

Thirty-two glycoconjugated porphyrins were synthesized by a modification of Lindsey method in the presence of Zn(OAc)(2).2H(2)O as a template. The Zn(2+) ion template strategy improved the yield about three-fold in the case of meta-substituted tetraphenylporphyrins. In addition, free-base porphyrins were obtained almost quantitatively by demetalation with 4 M HCl. Sixteen deacetylated glycoconjugated porphyrins were tested as candidate photodynamic therapy (PDT) drugs using HeLa cells. Most of the deacetylated glycoconjugated porphyrins showed higher cellular uptake than tetraphenylporphyrin tetrasulfonic acid (TPPS), and 5,10,15,20-tetrakis[4-(beta-D-arabinopyranosyloxy)phenyl]porphyrin (p-5d) in particular showed 18.5-fold higher uptake than TPPS. The photocytotoxicity of 5,10,15,20-tetrakis[4-(beta-D-glucopyranosyloxy)phenyl]porphyrin (p-5a), p-5d and TPPS was examined with HeLa cells, using a light dose of 16 J/cm(2). These photosensitizers had no cytotoxicity in the dark, but their photocytotoxicity increased in the order of TPPS < p-5a < p-5d. These results suggest p-5d is a good candidate for a PDT drug.     

Controlling conformations and physical properties of meso-tetrakis(phenylethynyl)porphyrins by ring fusion: synthesis, properties and structural characterizations

The boron trifluoride-catalyzed Rothemund condensations of phenylpropargylaldehyde with 4,7-dihydro-4,7-ethano-2H-isoindole or 3,4-diethylpyrrole in dichloromethane at low temperature give 5,10,15,20-tetrakis(phenylethynyl)porphyrins bearing bicyclo[2.2.2]octadiene and octaethyl substituents, respectively. The former undergoes a retro Diels-Alder reaction to afford 5,10,15,20-tetrakis(phenylethynyl)benzoporphyrin quantitatively. The different conformations of the porphyrin periphery were determined by X-ray diffraction and their redox and spectroscopic properties have been investigated.     

Indirect spectrophotometric determination of trace cyanide with cationic porphyrins

Three highly sensitive methods for the determination of cyanide have been developed, based on the fact that the complexation of silver ions with three cationic porphyrins, 5,10,15,20-tetrakis-(1-methyl-2-pyridinio)porphine [T(2-MPy)P], 5,10,15,20-tetrakis(1-methyl-3-pyridinio)porphine [T(3-MPy)P] and 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphine [T(4-MPy)P], in alkaline media is inhibited by cyanide and the decrease in absorbance of the silver(II) complex is proportional to the cyanide concentration. Sensitivities of the procedures developed are 0.133, 0.126 and 0.234 ng/cm(2), respectively for an absorbance of 0.001. Cadmium(II), copper(II), mercury(II), zinc(II), iodide and sulfide interfere with the cyanide determination. One of the proposed methods was applied to the determination of cyanide in waste-water samples, with satisfactory results.     

Molecular imprinting inside dendrimers

Synthetic hosts capable of binding porphyrins have been produced by a mixed-covalent-noncovalent imprinting process wherein a single binding site is created within cross-linked dendrimers. Two synthetic hosts were prepared, using as templates 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin and 5,10,15,20-tetrakis(3,5-dihydroxyphenyl)porphyrin. These two templates were esterified with, respectively, fourth- and third-generation Fréchet-type dendrons containing homoallyl end-groups. The resulting tetra- and octadendron macromolecules underwent the ring-closing metathesis reaction using Grubbs' Type I catalyst, RuCl(2)(P(C(6)H(5))(3))(2)(CHCH(2)C(6)H(5)), to give extensive interdendron cross-linking. Hydrolytic removal of the porphyrin cores afforded imprinted hosts whose ability to bind porphyrins with various peripheral substituents was investigated by UV-visible spectrophotometric titrations and size exclusion chromatography. The results indicate a high yield of imprinted sites that show high selectivity for binding of porphyrins capable of making at least four hydrogen bonds, but only a moderate degree of shape selectivity.     

A general method for constructing optically active supramolecular assemblies from intrinsically achiral water-insoluble free-base porphyrins

We have developed a general method to construct optically active porphyrin supramolecular assemblies by using a simple air-water interfacial assembly process. The method involved the in situ diprotonation of the free-base porphyrins at the air-water interface and subsequent assembly under compression. We showed that two intrinsically achiral water-insoluble free-base porphyrin derivatives, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (H(2)OEP) and 5,10,15,20-tetra-p-tolyl-21H,23H-porphine (H(2)TPPMe), could be diprotonated when spread onto a 2.4 M hydrochloric acid solution surface, and the Langmuir-Schaefer (LS) films fabricated from the subphase exhibited strong circular dichroism (CD) absorption, whereas those fabricated from pure Milli-Q water subphase did not. The experimental data suggested that the helical stacking of the achiral porphyrin building blocks was responsible for the supramolecular chirality of the assemblies. Interestingly, such a method was successfully applied to a series of other intrinsically achiral free-base porphyrins such as 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (H(2)TPPOMe), 5,10,15,20-tetraphenyl-21H,23H-porphine (H(2)TPP), 5,10,15,20-tetrakis(4-(allyloxy)phenyl)-21H,23H-porphine (H(2)TPPOA), and 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine (H(2)TPPDOMe). A possible mechanism has been proposed. The method provides a facile way to obtain optically active porphyrin supramolecular assemblies by using intrinsically achiral water-insoluble free-base porphyrin derivatives.     

Synthesis and Crystal Structure of 15,16-Bis（（4-nitrophenoxy）methyl）-2,3,5,6,8,9,11,12- octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecine

Double-armed crown ether 15,16-bis((4-nitrophenoxy)methyl)-2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13] pentaoxacyclopentadecine F.W. 570.54 was synthesized by dibromomethyl benzo-15-crown-5 with 4-nitrophenol in the presence of anhydrous K2CO3, characterized by spectroscopic techniques and confirmed by X-ray crystal structure analysis. The title compound crystallizes in monoclinic, space group P21/c with a = 12.840(4), b = 9.425(3), c = 22.447(7), β = 98.128(7)° and Z = 4. The structure exhibits intermolecular hydrogen bonding of C-H···O type.     

Dendritic ruthenium porphyrins: a new class of highly selective catalysts for alkene epoxidation and cyclopropanation

Attachment of Fréchet-type poly(benzyl ether) dendrons [G-n] to carbonylruthenium(II) meso-tetraphenylporphyrin (5) using covalent etheric bonds forms a series of dendritic ruthenium(II) porphyrins 5-[G-n](m) (m=4, n=1, 2; m=8, n=0-2). The attachment was realized by treating the carbonylruthenium(II) complex of 5,10,15,20- tetrakis(4'-hydroxyphenyl)porphyrin or 5,10,15,20-tetrakis(3',5'-dihydroxyphenyl)porphyrin with [G-n]OSO(2)Me in refluxing dry acetone in the presence of potassium carbonate and [18]crown-6. Complexes 5-[G-n](m) were characterized by UV/Vis, IR, and NMR spectroscopy and mass spectrometry. All of the dendritic ruthenium porphyrins are highly selective catalysts for epoxidation of alkenes with 2,6-dichloropyridine N-oxide (Cl(2)pyNO). The chemo- or diastereoselectivity increases with the generation number of the dendron and the number of dendrons attached to 5, and complex 5-[G-2](8) exhibits remarkable selectivity or turnover number in catalyzing the Cl(2)pyNO epoxidation of a variety of alkene substrates including styrene, trans-/cis-stilbene, 2,2-dimethylchromene, cyclooctene, and unsaturated steroids such as cholesteryl esters and estratetraene derivative. The cyclopropanation of styrene and its para-substituted derivatives with ethyl diazoacetate catalyzed by 5-[G-2](8) is highly trans selective.     

Photocatalytic hydrogen evolution based on efficient energy and electron transfers in donor-bridge-acceptor multibranched-porphyrin-functionalized platinum nanocomposites

Two donor-bridge-acceptor conjugates (5,10,15,20-tetrakis[4-(N,N-diphenylaminobenzoate)phenyl] porphyrin (TPPZ) and 5,10,15,20-tetrakis[4-(N,N-diphenylaminostyryl)phenyl] porphyrin (TPPX)) were covalently linked to triphenylamine (TPA) at the meso-position of porphyrin ring. The triphenylamine entities were expected to act as energy donors and the porphyrins to act as an energy acceptor. In this paper, we report on the synthesis of these multibranched-porphyrin-functionalized Pt nanocomposites. The conjugates used here not only served as a stabilizer to prevent agglomeration of Pt nanoparticles, but also as a light-harvesting photosensitizer. The occurrence of photoinduced electron-transfer processes was confirmed by time-resolved fluorescence and photoelectrochemical spectral measurements. The different efficiencies for energy and electron transfer in the two multibranched porphyrins and the functionalized Pt nanocomposites were attributed to diverse covalent linkages. Moreover, in the reduction of water to produce H(2), the photocatalytic activity of the Pt nanocomposite functionalized by TPPX, in which the triphenylamine and porphyrin moieties are bonded through an ethylene bridge, was much higher than that of the platinum nanocomposite functionalized by TPPZ, in which the two moieties are bonded through an ester. This investigation demonstrates the fundamental advantages of constructing donor-bridge-acceptor conjugates as highly efficient photosensitizers based on efficient energy and electron transfer.     

Differential substituent effects of beta-halogens in water-soluble porphyrins

The first water-soluble beta-octafluorinated porphyrins, 5,10,15,20-tetrakis(4-sulfonatophenyl)-2,3,7,8,12,13,17,18- octafluoroporphyrin, 1, and 5,10,15,20-tetrakis(2,6-difluoro-3-sulfonatophenyl)-2,3,7,8,12,13,17,18- octafluoroporphyrin, 2, have been prepared and their aqueous aggregation, acid-base, and optical properties have been characterized. The porphyrins are tetraanionic at neutral pH (at pH = 3-11 for 1 and pH = 0-9 for 2). Semiempirical (AM1) calculations provide evidence that somewhat unusual acidity characteristics of the fluorinated compounds (with respect to similar brominated porphyrins) can be rationalized solely on the basis of chemical hardness and electronegativity arguments. These results indicate that the large conformational differences seen in the structures of brominated and fluorinated water-soluble porphyrins have little impact upon N-H acidity. Metalation of 1 and 2 with ZnCl2 yielded the zinc complexes, which were characterized by optical spectroscopy and electrochemistry.     

The Electrochemical Evaluation of the Antioxidant Activity of Substituted Tetraphenylporphyrins

Oxidative–reductive and antioxidant properties of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin, and 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin in their reaction with the 2,2-diphenyl-1-picrylhydrazile free radical are studied. Two of the three abovelisted compounds, namely, 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin, were found to possess antioxidant activity, the former’s antioxidant activity being higher, while 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin showed no antioxidant properties. A probable mechanism of antioxidant activity of the studied porphyrins involves hydrogen homolytic detachment from functional substituent in phenyl ring and the hydrogen radical interaction with 2,2-diphenyl-1-picrylhydrazile.     

Structure and photophysics of beta-Octafluoro-meso-tetraarylporphyrins

The structure of THF-coordinated [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]zinc, Zn(F(8)TPP).THF, and photophysical studies of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, F(8)TPP, Zn(F(8)TPP), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, F(28)TPP, and [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphinato]zinc, Zn(F(28)TPP), in benzonitrile, are reported. A key point from these studies is that the octafluorinated F(8)TPP and perfluorinated F(28)TPP porphyrins possess similar absorption spectra, but dissimilar X-ray crystal structures and disparate photophysical characteristics. These data cannot be easily accommodated within currently accepted theories which relate macrocycle distortion and optoelectronic properties.     

Oxidation of caffeine with hydrogen peroxide catalyzed by metalloporphyrins

The biomimetic oxidation of caffeine with hydrogen peroxide using metalloporphyrins as catalysts, which are known to be good biomimetic models of cytochrome P450 enzymes, is investigated. The two manganese porphyrins tested, chloro[5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinato]manganese(III), [Mn(TDCPP)Cl], and chloro [5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]manganese(III), [Mn(TPFPP)Cl], afford high conversions of caffeine for all substrate/catalyst molar ratios. Selectivity was found to be dependent on the catalyst employed, and a new spiro racemic compound is described and fully characterized in the solid state from X-ray diffraction studies.     

Photobleaching of compounds of the 5,10,15,20-Tetrakis(m-hydroxyphenyl)porphyrin Series (m-THPP,m-THPC,and m-THPBC)

[reaction: see text] 5,10,15,20-Tetrakis(m-hydroxyphenyl)porphyrin (m-THPP) yielded novel quinonoid porphyrins upon irradiation in aqueous methanol. True photobleaching was observed for 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorin (m-THPC) and 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (m-THPBC) under the same conditions; several fragmentation products (imides, methyl p-hydroxybenzoate, dipyrrin derivatives) were recognized.     

Sterically encumbered systems for two low-coordinate phosphorus centers

Tetraarylphenyls of the form 2,3,5,6-Ar4C6 (Ar = p-tert-butylphenyl) are investigated as sterically demanding ligands for the syntheses of compounds having two p-phenylene-bridged phosphorus centers. The precursor to such materials, 1,4-diiodo-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (1), is readily obtained via a one-pot procedure in 68% yield. Compound 1 is then used to provide the bis(dichlorophosphine) 1,4-bis(dichlorophosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (2) and the derived bis(phosphine) 1,4-bis(phosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (3) in yields of 56 and 94% respectively. These materials provide access to novel materials containing two low-coordinate phosphorus centers bridged by a sterically encumbered phenylene unit. Compound 2 reacts with benzaldehyde and 2,6-dichlorobenzaldehyde in the presence of excess trimethylphosphine and zinc to produce the new pale yellow crystalline bis(phosphaalkenes) (E,E)-PhC(H)=PAr4C6P=C(H)Ph (4a; 42%) and (E,E)-Ar'C(H)=PAr4C6P=C(H)Ar' (4b; 46%; Ar' = 2,6-dichlorophenyl). The crystal structure of 4a shows a P=C bond length of 1.676(5) A. Compound 2 is also used to provide the unusual red-orange bis(diphosphene) DmpP=PAr4C6P=PDmp (5; 55%; Dmp = 2,6-Mes2C6H3). Compound 5 is structurally characterized, and a P=P bond length of 2.008(2) A is ascertained.     

Synthesis of Glucoside Bonded Metal Porphyrins

Enzyme catalyzed reaction often has high selectivity and efficiency under mild conditions. However, disadvantage of enzyme catalysts is the difficulty of recovery. Metalloporphyrin plays an important role in biological system such as redox reaction, electron transfer,oxygen transportation and charge separation etc.1,2 Metalloporphyrins as superoxide dismutase (SOD) mimics have showed the ability of catalyzing the redox reaction of some harmful radicals , such as O2·―, ·OH. Grove and co-…     

Allosteric Binding of an Ag+ Ion to Cerium(IV) Bis-porphyrinates Enhances the Rotational Activity of Porphyrin Ligands

A series of cerium(IV) bisporphyrinate double-deckers [Ce(bbpp)2] (BBPP = 5,15-bis(4-butoxyphenyl) porphyrin dianion), [Ce(tmpp)2] (TMPP = 5,10,15,20-tetrakis(4-methoxyphenyl)-porphyrin dianion), [Ce(tfpp)2] (TFPP = 5,10,15,20-tetrakis(4-fluorophenyl)porphyrin dianion), [Ce(tmcpp)2] (TMCPP = 5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin dianion), and [Ce(tmpp)(tmcpp)] was prepared. They bind three Ag+ ions to their concave porphyrin pi subunits (pi-clefts) according to a positive homotropic allosteric mechanism with Hill coefficients (nH) of 1.7-2.7. The rotation rates of the porphyrin ligands in [Ce(bbpp)2] were evaluated to be 200 s-1 at 20 degrees C (delta G++293 = 14.1 kcal mol-1) and 220 s-1 at -40 degrees C (delta G++233 = 11.0 kcal mol-1) without and with Ag+ ions, respectively. These results consistently support our unexpected finding that Ag+ binding can accelerate rotation of the porphyrin ligand. On the basis of UV-visible, 1H NMR, and resonance Raman spectral measurements, the rate enhancement of the rotational speed of the porphyrin ligands is attributed to conformational changes of the porphyrin in cerium(IV) bis-porphyrinate induced by binding of Ag+ guest ions in the clefts. This novel concept of positive homotropic allosterism is applicable to the molecular design of various supramolecular and switch-functionalized systems.     

New class of potent catalysts of O2.-dismutation. Mn(III) ortho-methoxyethylpyridyl- and di-ortho-methoxyethylimidazolylporphyrins

Three new Mn(III) porphyrin catalysts of O2.-dismutation (superoxide dismutase mimics), bearing ether oxygen atoms within their side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP(5+)), Mn(III)5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP(5+)) and Mn(III) 5,10,15,20-tetrakis[N,N'-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP(5+)). Their catalytic rate constants for O2.-dismutation (disproportionation) and the related metal-centered redox potentials vs. NHE are: log k(cat)= 8.04 (E(1/2)=+251 mV) for MnTMOE-2-PyP(5+), log k(cat)= 7.98 (E(1/2)=+356 mV) for MnTM,MOE-2-ImP(5+) and log k(cat)= 7.59 (E(1/2)=+365 mV) for MnTDMOE-2-ImP(5+). The new porphyrins were compared to the previously described SOD mimics Mn(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)), Mn(III) 5,10,15,20-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP(5+)) and Mn(III) 5,10,15,20-tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP(5+)). MnTMOE-2-PyP(5+) has side chains of the same length and the same E(1/2), as MnTnBu-2-PyP(5+)(k(cat)= 7.25, E(1/2)=+ 254 mV), yet it is 6-fold more potent a catalyst of O2.-dismutation , presumably due to the presence of the ether oxygen. The log k(cat)vs. E(1/2) relationship for all Mn porphyrin-based SOD mimics thus far studied is discussed. None of the new compounds were toxic to Escherichia coli in the concentration range studied (up to 30 microM), and protected SOD-deficient E. coli in a concentration-dependent manner. At 3 microM levels, the MnTDMOE-2-ImP(5+), bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP(5+), while MnTDE-2-ImP(5+) was of very low efficacy.     

Basic and Coordination Properties of Tetraphenylporphine Derivatives

Basic and coordination properties of 5,10,15,20-tetrakis(3,5-dibromophenyl)porphine and 5,10,15,20-tetrakis(4-methoxy-3-bromophenyl)porphine in acetonitrile have been investigated by means of spectrophotometry. The optimization of the geometry parameters of polysubstituted porphyrins has been performed using quantum-chemical method (РМ3). The relationship between geometric structure of the studied porphyrins and their coordination, basic, and spectral properties is discussed.