Soret Band Region Research Articles

Solvent Effects on the UV-Vis Absorption Properties and Tautomerism of N-Confused Tetraphenylporphyrin.

The two tautomeric forms of N-confused tetraphenylporphyrin (NCTPP) show distinctly different absorption spectra. The existence of each tautomer in solution has been shown to be strongly solvent-dependent. In the present work, we have studied the tautomerization using absorption spectroscopy in 15 different solvents. While changes in the two tautomers are not large in the Soret band region, the distinct spectral changes between the two tautomers in the Q-band region provide a convenient way to measure the concentration of each tautomer. The resulting data shows a strong correlation between the tautomer and the H-bond accepting ability of the solvent. The anomaly in this data is for the alcoholic solvents ethanol and methanol, for which we observe evidence for H-bonding, presumably between the exterior N2 nitrogen of the NCTPP and the O-H proton of the solvent.

Structural Studies on Porphyrin-PNA Conjugates in Parallel PNA:PNA Duplexes: Effect of Stacking Interactions on Helicity.

Parallel PNA:PNA duplexes were synthesized and conjugated with meso-tris(pyridyl)phenylporphyrin carboxylic acid at the N-terminus. The introduction of one porphyrin unit was shown to affect slightly the stability of the PNA:PNA parallel duplex, whereas the presence of two porphyrin units at the same end resulted in a dramatic increase of the melting temperature, accompanied by hysteresis between melting and cooling curves. The circular dichroism (CD) profile of the Soret band and fluorescence quenching strongly support the occurrence of a face-to-face interaction between the two porphyrin units. Introduction of a L-lysine residue at the C-terminal of one strand of the parallel duplex induced a left-handed helical structure in the PNA:PNA duplex if the latter contains only one or no porphyrin moiety. The left-handed helicity was revealed by nucleobase CD profile at 240-280 nm and by the induced-CD observed in the presence of the DiSC2 (5) cyanine dye at ~500-550 nm. Surprisingly, the presence of two porphyrin units led to the disappearance of the nucleobase CD signal and the absence of CD exciton coupling within the Soret band region. In addition, a dramatic decrease of induced CD of DiSC2 (5) was observed. These results are in agreement with a model where the porphyrin-porphyrin interactions cause partial loss of chirality of the PNA:PNA parallel duplex, forcing it to adopt a ladder-like conformation.

Optical properties of regiorandom polythiophene/Al2O3 nanocomposites and their application to ammonia gas sensing

Polythiophene (PT) and its composite with Al2O3 have been synthesized successfully using chemical oxidation method. The structural, morphological and chemical properties of the nanocomposites (NCs) are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The XRD spectra show the polycrystalline nature and improved crystallinity with Al2O3 doping concentrations. The SEM results reveal that all PT/Al2O3 samples form nano-dimensioned spherical structures spread uniformly throughout the surface. FTIR spectra show the shifting of some bands and appearance of some extra bands in PT/Al2O3 samples as compared to the pure PT indicates a complex formation between the dopant and polythiophene molecules. Optical properties are studied using UV–visible absorption and photoluminescence (PL) spectroscopy. PL spectra of pure PT and PT/Al2O3 samples exhibit mainly three visible emission peaks at around ~462, ~490 and ~522 nm. The two emission peaks centred at ~462 and ~490 nm in the Soret band region where as single peak at ~522 nm in the Q band emission. The NH3 gas sensing properties have been carried out at room temperature by varying the ammonia concentration over a range of 25–650 ppm. The response is found to increase with increasing Al2O3 doping concentration as well as increase of ammonia gas concentrations. The highest Al2O3 doped PT NCs sample shows maximum sensitivity at lower concentration of ammonia gas and rapid response and recovery time.

A negligible CT character of the lowest excited state of a novel complex of zinc tetraphenylporphyrin with axially bonded 2-(4-methoxy-trans-styryl)quinoline-1-oxide ligand: Experimental studies and TD DFT/CAM B3LYP [6-31G(d,p)] calculations

Abstract The crystal structure and photophysical behavior of the novel (1:1) ZnTPP–MSQNO complex composed of ZnTPP unit with axially bonded 2-(4-methoxy-trans-styryl)quinoline-1-oxide ligand (MSQNO) has been extensively investigated both in solution and in the solid state. The single-crystal X-ray measurements, stationary absorption and emission spectra and the time-resolved fluorescence spectroscopy combined with TD DFT/CAM B3LYP [6-31G(d,p)] calculations were employed. The crystal structure is characterized by the centrosymmetric triclinic unit cell, space group P 1 ¯ . Zinc atom in the studied complex is pulled out 0.365 A from the porphyrin macrocycle and the MSQNO ligand occurs in the anti-rotameric form of the trans-isomer. The complex formation in the solution is confirmed by the appropriate red shift of the Qx and Qy band in the absorption spectrum in toluene, which is in a good agreement with the results obtained by theoretical calculations. We have found that effective mixing of electronic configurations of the locally excited states with those of the charge transfer type leads to a negligible CT nature of the S1 state of ZnTPP–MSQNO complex. We also hypothesized, that due to an overlap of the absorption spectrum of MSQNO and ZnTPP in the Soret band region, the electronic coupling becomes possible and the nonradiative energy transfer process as an additional deactivation channel may take place. An unusually short lifetime (0.6 ns) was revealed for the solid complex, while in toluene (1.5 ns) and in n-propanol (2.7 ns), they shown to be several times longer.

The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions.

We have performed gas-phase absorption spectroscopy in the Soret-band region of chlorophyll (Chl) a and b tagged by quaternary ammonium ions together with time-dependent density functional theory (TD-DFT) calculations. This band is the strongest in the visible region of metalloporphyrins and an important reporter on the microenvironment. The cationic charge tags were tetramethylammonium, tetrabutylammonium, and acetylcholine, and the dominant dissociation channel in all cases was breakage of the complex to give neutral Chl and the charge tag as determined by photoinduced dissociation mass spectroscopy. Two photons were required to induce fragmentation on the time scale of the experiment (microseconds). Action spectra were recorded where the yield of the tag as a function of excitation wavelength was sampled. These spectra are taken to represent the corresponding absorption spectra. In the case of Chl a we find that the tag hardly influences the band maximum which for all three tags is at 403 ± 5 nm. A smaller band with maximum at 365 ± 10 nm was also measured for all three complexes. The spectral quality is worse in the case of Chl b due to lower ion beam currents; however, there is clear evidence for the absorption being to the red of that of Chl a (most intense peak at 409 ± 5 nm) and also a more split band. Our results demonstrate that the change in the Soret-band spectrum when one peripheral substituent (CH3) is replaced by another (CHO) is an intrinsic effect. First principles TD-DFT calculations agree with our experiments, supporting the intrinsic nature of the difference between Chl a and b and also displaying minimal spectral changes when different charge tags are employed. The deviations between theory and experiment have allowed us to estimate that the Soret-band absorption maxima in vacuo for the neutral Chl a and Chl b should occur at 405 nm and 413 nm, respectively. Importantly, the Soret bands of the isolated species are significantly blueshifted compared to those of solvated Chl and Chl-proteins. The protein microenvironment is certainly not innocent of perturbing the electronic structure of Chls.

Controlling the Orientation of Pendants in Two-Dimensional Comb-Like Polymers by Varying Stiffness of Polymeric Backbones

The electronic communications between chromophores are closely related to distances and orientation of these π-conjugated systems. Reported herein is a collection of well-defined two-dimensional comb-like polymers containing porphyrin pendants obtained by ring-opening metathesis polymerizations of norbornene and cyclobutene derivatives using ruthenium or molybdenum catalysts. The spacing separating the adjacent pendants are defined by ring sizes of cycloalkenes and the orientations are determined by the stiffness of the polymeric backbone, which is, in turn, discerned by the percentage of Z-double bonds. Both peak widths of the porphyrin radical cation of the EPR spectra and the absorption profiles in the Soret band region reflect the degree of the spin delocalization and exciton coupling between porphyrin pendants in these polymers and, hence, the stiffness of the polymeric backbone. Our approach offers a useful protocol to align an array of chromophores appended onto a rigid polymeric backbone so that t...

Microsecond time-resolved absorption spectroscopy used to study CO compounds of cytochrome bd from Escherichia coli.

Cytochrome bd is a tri-heme (b 558, b 595, d) respiratory oxygen reductase that is found in many bacteria including pathogenic species. It couples the electron transfer from quinol to O2 with generation of an electrochemical proton gradient. We examined photolysis and subsequent recombination of CO with isolated cytochrome bd from Escherichia coli in one-electron reduced (MV) and fully reduced (R) states by microsecond time-resolved absorption spectroscopy at 532-nm excitation. Both Soret and visible band regions were examined. CO photodissociation from MV enzyme possibly causes fast (τ<1.5 µs) electron transfer from heme d to heme b 595 in a small fraction of the protein, not reported earlier. Then the electron migrates to heme b 558 (τ∼16 µs). It returns from the b-hemes to heme d with τ∼180 µs. Unlike cytochrome bd in the R state, in MV enzyme the apparent contribution of absorbance changes associated with CO dissociation from heme d is small, if any. Photodissociation of CO from heme d in MV enzyme is suggested to be accompanied by the binding of an internal ligand (L) at the opposite side of the heme. CO recombines with heme d (τ∼16 µs) yielding a transient hexacoordinate state (CO-Fe2+-L). Then the ligand slowly (τ∼30 ms) dissociates from heme d. Recombination of CO with a reduced heme b in a fraction of the MV sample may also contribute to the 30-ms phase. In R enzyme, CO recombines to heme d (τ∼20 µs), some heme b 558 (τ∼0.2–3 ms), and finally migrates from heme d to heme b 595 (τ∼24 ms) in ∼5% of the enzyme population. Data are consistent with the recent nanosecond study of Rappaport et al. conducted on the membranes at 640-nm excitation but limited to the Soret band. The additional phases were revealed due to differences in excitation and other experimental conditions.

Changes in the Energy Transfer Pathways within Photosystem II Antenna Induced by Xanthophyll Cycle Activity

Energy transfer pathways between photosystem II (PSII) antenna complexes in intact thylakoid membranes have been studied using low-temperature (77 K) excitation fluorescence spectroscopy. The focus of this study was to see whether de-epoxidation of violaxanthin into zeaxanthin causes any alterations in the energetic couplings between the core antenna complexes CP43 and CP47 and the peripheral light-harvesting antenna (LHCII). It was discovered that the appearance of zeaxanthin caused characteristic alterations in the PSII excitation fluorescence spectra in the Soret-band region. While in the dark violaxanthin was found to be largely uncoupled from any emitting chlorophylls, its intensive de-epoxidation resulted in the appearance of two additional bands at 509 and 492 nm. The former was attributed to weak coupling of zeaxanthin to emitters in the CP43 and LHCII complexes and the latter to efficient coupling of violaxanthin of the CP29 complex to emitters in the CP43, CP47, and LHCII complexes. The role of CP29-bound violaxanthin is discussed in light of both its efficient energetic coupling and strong physical binding to this complex. The finding that zeaxanthin is energetically coupled to chlorophyll a emitters of the PSII antenna is discussed with respect to its suggested role as a quencher involved in photoprotective energy dissipation, or non-photochemical quenching (NPQ), in the photosynthetic membrane.

Selective Detection of Volatile Organic Compounds by Spectral Imaging of Porphyrin Derivatives Bound to TiO2 Porous Films

In this work, the carboxylic acid derivatives of a free-base porphyrin, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphyrin, and 10 of its metal derivatives (TCPPs) have been used for optical gas sensing. For this purpose, microstructured columnar TiO(2) thin films prepared by GAPVD (glancing angle physical vapor deposition) have been used as host materials for the porphyrins as they are non-dispersive and porous, allowing their use for UV-visible spectroscopy and gas sensing. The chemical binding between the dye molecules and the TiO(2) has been studied through infrared spectroscopy, and the obtained spectral changes have been found to be compatible with chelating and/or bidentate binding modes of the carboxylate groups on the TiO(2) surface. When hosted in the film, the UV-visible spectra of the porphyrins featured a blue shift and broadening of the Soret band with respect to the solution, which has been attributed to the formation of π-π aggregates between porphyrin molecules. The composite porphyrin/TiO(2) films obtained from each of the 11 porphyrins have been exposed to 12 different volatile organic compounds (VOCs), and their respective gas-sensitive properties have been analyzed as a function of the spectral changes in their Soret band region in the presence of the analytes. The set of composite films has shown high selectivity to the analyzed volatile compounds. For each analyte, an innovative way of showing the different responses has been developed. By means of this procedure, an imagelike recognition pattern has been obtained, which allows an easy identification of every compound. The kinetics of the exposure to several analytes showed a fast, reversible and reproducible response, with response times of a few seconds, which has been attributed to both the sensitivity of the porphyrins and the high porosity of the TiO(2) films. Also, increasing concentrations of the analytes resulted in an increase in the magnitude of the response, indicating that the sensor behavior is also concentration-dependent.

A Resonance Raman Enhancement Mechanism for Axial Vibrational Modes in the Pyridine Adduct of Myoglobin Proximal Cavity Mutant (H93G)

The proximal cavity mutant of myoglobin consists of a mutation of the proximal histidine to glycine (H93G), which permits exogenous ligands to bind to the heme iron. A non-native pyridine ligand can ligate to the heme to yield a five-coordinate adduct, H93G(Pyr), that cannot be formed freely in solution since the six-coordinate bis-pyridine adduct is more stable than the five-coordinate adduct. We have used resonance Raman spectroscopy in the Soret band region of the heme to study the enhancement of axial vibrations of bound pyridine in the H93G(Pyr) adduct. The observation that the pyridine ring breathing mode (ν(1)) and the symmetric ring stretching (ν(3)) modes are enhanced under these conditions is explained by a computational approach that shows that coupling of the π-system of the heme with the p-orbitals of the pyridine is analogous to π-backbonding in diatomic ligand adducts of heme proteins. The result has the broader significance that it suggests that the resonance enhancement of pyridine modes could be an important aspect of Raman scattering of pyridine on conducting surfaces such as those studied in surface enhanced Raman scattering experiments.

Aluminium(iii) porphyrin based axial-bonding type dyads containing thiaporphyrins and expanded thiaporphyrins as axial ligands

Four axial-bonding type Al(III) porphyrin based dyads, containing thiaporphyrins with N3S 1 and N2S22 cores and expanded thaiporphyrins with N2S33 and N2S44 cores, were synthesised by treating [(TPP)AlIIIOH] with the corresponding mono-functionalised meso-4-hydroxyphenyl thiaporphyrin or expanded thiaporphyrin building blocks in benzene at refluxing temperature. The stable dyads 1–4 are freely soluble in common organic solvents and characterized by mass, 1D and 2D NMR, absorption, electrochemical and fluorescence techniques. 1D and 2D NMR studies unambiguously confirmed the dyads. The absorption and electrochemical studies show weak interaction between the two macrocycles in dyads and the constituted macrocycles in dyads mostly retain their characteristic individual features. The steady state fluorescence studies indicate a decrease in the quantum yield of Al(III) porphyrin unit in dyads 1–4 and invoked the possibility of singlet–singlet energy transfer from the Al(III) porphyrin unit to the N3S porphyrin unit in dyad 1 and N2S2 porphyrin unit in dyad 2. Structural information of these dyads in their ground (S0) state are generated by applying density functional theory (DFT) based quantum chemical calculations. Excited state calculations are carried out following time dependent density functional theory (TD-DFT) and UV-Vis spectra of these dyads are simulated in the Soret band region.

Porphyrin–DNA conjugates: porphyrin induced adenine–guanine homoduplex stabilization and interduplex assemblies

DNA has found widespread uses as a nanosized scaffold for assembly of patterned multichomophoric nanostructures. Herein we report the synthesis, self-assembly, stability, and spectroscopic studies of short alternating non-self-complementary DNA sequences 5'-(dGdA)(4) and 5'-(dAdG)(4) with non-charged tetraarylporphyrins covalently linked to the 5' position of deoxyadenosine or deoxyguanosine via a phosphate or amide linker. The linker, the metal in the porphyrin coordination center, and the neighboring nucleobase have very distinct effects on the duplex formation of porphyrin-deoxyguanosine-deoxyadenosine oligodeoxynucleotides. At ionic strength between 5 mM and 40 mM, free base trispyridylphenylporphyrin appended to the 5' termini of 5'-(dAdG)(4) oligonucleotide via short non-polar amide linker served as a hydrophobic molecular cap inducing deoxyadenosine-deoxyguanosine antiparallel homoduplex. At ionic strength of ≥60 mM, the free base porphyrin functioned as a molecular 'glue' and induced the formation of porphyrin-DNA inter-homoduplex assemblies with characteristic tetrasignate CD Cotton effects in the porphyrin Soret band region. When the porphyrin cap was covalently attached to 5' position of deoxyguanosine or deoxyadenosine via charged phosphate linker, no significant deoxyadenosine-deoxyguanosine hybridization was observed even at elevated ionic strengths.

Chiral 1,2-Subnaphthalocyanines

Following the first suggestion of inherent molecular chirality in asymmetrically substituted subphthalocyanines by Torres and co-workers in 2000, elucidation of the relationship between structure and chirality has become an important issue. However, separation of the enantiomers has been prevented by the low solubility of the molecules synthesized to date, and it has not been possible to link the CD signs and intensities to their absolute structures. Recently, we observed that 1,2-subnaphthalocyanines possess two diastereomers with respect to the arrangement of the naphthalene moieties and that these novel chiral molecules exhibit moderate solubility in common organic solvents. This has enabled us to separate all of the diastereomers and enantiomers. The two diastereomers have been completely characterized by NMR spectroscopy and X-ray diffraction analysis. The absorption and magnetic circular dichroism spectra, together with theoretical calculation, reveal a small variation in the frontier molecular orbitals of the 1,2-subnaphthalocyanines compared with conventional subphthalocyanines, except for destabilization of the HOMO-3, which results in a characteristic absorption in the Soret band region. The chirality of 1,2-subnaphthalcyanines, including the CD signs and intensities, is discussed in detail for the first time with enantiomerically pure molecules whose absolute structures have been elucidated by single-crystal X-ray diffraction analysis.

One-pot synthesis of 5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin (ENTPP) and spontaneous resolution upon crystallization

5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin (ENTPP) has been synthesized in a one-pot reaction, and the corresponding chiral crystalline samples have been obtained by spontaneous resolution. 1 H NMR spectrum suggests it is mononaphthyl substituted species and an ethyl group is over the porphyrin plane. The structure has been further confirmed by X-ray crystallography. ENTPP·C6H14 (C57H50N4O2 ): monoclinic, P21, a = 10.707(2) Å, b = 12.203(2) Å, c = 17.858(4) Å, β = 103.06(3)°, V = 2272.8(8) Å3, Z = 2. The 8-position substituent, ester group, lies above the porphyrin plane and leads to the conformational chirality. The entire structure is built up with homochiral molecules, which leads to a chiral crystal through packing in P21 space group. Circular dichroism (CD) spectra have exhibited remarkable absorptions in the Soret band region, which further confirms the homochirality of the crystalline samples.

Organic Porphyrin Nanoparticles with Induced Optical Activity: Ion-Based Synthesis from Achiral Chromophore and Chiral Counterions

Organic porphyrin nanoparticles with induced optical activity are successfully synthesized via ion-association between the achiral meso-tetrakis(1-methylpyridinium-4-yl) porphine (TMPyP) cation and the chiral hexacoordinated phosphate (D3-symmetric TRISPHAT or C2-symmetric BINPHAT with Δ or Λ chirality, respectively) anion in the presence of poly(vinylpyrrolidone). The particle size ranges from 30 to 50 nm in diameter, which is almost independent of the counteranion used. The Soret band of the porphyrin nanoparticles exhibits a large bathochromic shift in comparison with that of the porphyrin monomer in solution. Simple quantum chemical calculations suggest that the meso-substituent flattening of the TMPyP molecule as well as ruffling/saddling deformation can be an origin of this bathochromic shift. The induced circular dichroism (CD) of the porphyrin nanoparticles exhibits split Cotton effects at the Soret band region, although the split modes are different between the nanoparticles prepared from TMPyP/T...

Doubly β‐Functionalized Zinc(II) Porphyrin‐sensitized TiO2 Solar Cells

AbstractFunctionalized porphyrin at β‐positions with carboxylic acid groups was prepared to investigate the photovoltaic function as dye‐sensitized nanocrystalline‐TiO2 solar cells. The electronic structures of the porphyrin macrocyclic core are strongly coupled with olefinic side chains so that the absorption spectrum exhibits broad and red‐shifted Soret and Q‐bands. 2b‐Zn, which is a metallated porphyrin with 2‐propenoic acid groups at two β‐positions, exhibits the power conversion efficiency of 5.18%. From such photovoltaic performance, we can suggest that multiple pathways through olefinic side chains at two β‐positions enhance the electron injection efficiency. For improvement of the solar cell efficiency, we have employed empirical surface modification which is a post‐treatment of the TiO2 film by aqueous TiCl4. As a result, post‐TiCl4 treated cell exhibits the power conversion efficiency of 5.37% and the maximum incident photon to current efficiency of 67.2% in the Soret band region.

ChemInform Abstract: Synthesis, Optical Properties, Structures, and Molecular Orbital Calculations of Subazaporphyrins, Subphthalocyanines, Subnaphthalocyanines, and Related Compounds

The synthesis of and spectroscopic data on subazaporphyrins, subphthalocyanines, subnaphthalocyanines and their related compounds reported to date are summarized, together with the results of molecular orbital (MO) calculations from our laboratory. Thus the problems and key points in the synthesis of these subazamacrocycles and their electronic absorption and magnetic circular dichroism (MCD) spectra are discussed, together with those of the monosubstituted-type low-symmetrical phthalocyanines obtained by their ring expansion reaction. The Q bands of these subazamacrocycles are orbitally doubly degenerate, and in the Soret band region, three transitions to the orbitally degenerate state are found. The MO calculations suggest that the HOMO levels destabilize considerably while the LUMO levels change slightly with increasing size of the π system. In addition, the electronic adsorption data are reproduced by the calculation only when the two pyrrole hydrogens in the monosubstituted-type phthalocyanines are a...

Spectral-luminescence properties of a norbornene-substituted tetraazachlorin and its metal complexes

The spectral-luminescence properties of a tetraazachlorin derivative with a norbornene fragment annelated to a reduced pyrrole ring and its complexes with zinc and palladium have been studied at 293 and 77 K. For the norbornene-substituted free base, differences in fluorescence from unsubstituted tetraazachlroin and its dibenzobarrelene-substituted analog are found. The fluorescence lifetime is observed to rise by ∼7 times for the free base and by ∼1.6 ties for the Zn complex on going from 293 to 77 K. An essential dependence of the photophysical parameters on the nature of the solvent is noted. The fluorescence polarization spectrum of the norbornene-substituted tetraazachlorin reveals in the Soret band region at least four electronic transitions. For the Pd complex, weak phosphorescence in the near IR region has been detected; the 0-0 band maximum is at 990 nm and the singlet-triplet interval amounts to 5800 cm -1 , which is larger by 400 cm -1 than for Pd tetraazaporphine. The quantum yields of the photosensitized formation of singlet oxygen have been determined using a relative luminescence method.

Porphyrazines with Annulated Chalco¬gen-Containing Heterocycle: Study of Spectral-Luminescent Properties and Quantum-Chemical Calculations of the Excited Electronic States

Joint experimental and theoretical study of spectral-luminescent properties and electronic structure of the molecules of new phthalocyanine analogues – derivatives of octaphenylporphyrazine and tetrakis(tert-butyl)phthalocyanine carrying an annulated 1,2,5-thiadiazole or 1,2,5-selenadiazole cycle instead of two phenyl groups or a benzene ring – has been performed. The annulation of a five-membered heterocycle results in bathochromic shifts of the Q x band and Soret band; the Q y – Q x (S 2 –S 1 ) interval increases and the fluorescence spectra become mirror-symmetrical to the absorption spectra in the region of the G→Q x transition. Replacement of a benzene ring in the tetrakis(tert-butyl)phthalocyanine molecule by the 1,2,5-selenadiazole cycle lowers the fluorescence quantum yield from 0.77 to 0.14 which may be accounted for by the internal heavy-atom effect as well as by intramolecular charge transfer. The fluorescence quantum yield of octaphenylporphyrazine is very low, but for the derivatives it is comparatively high, ϕ F =0.06, i.e. the fluorescence is not quenched but enhanced when the heterocycle is annulated. The quantum-chemical calculations of the ground and excited states of the original structures of phthalocyanine, octaphenylporphyrazine, and their derivatives with the annulated thiadiazole cycle have been performed. It is shown that the chalcogen substituted compounds exist as isomers with the NH – NH axis passing through the pyrrole rings devoid of the heterocycle. The calculations based on the INDO/Sm method give reasonable interpretation of the experimental absorption spectra. The description of the properties of the lowest excited Q states is sufficiently good for the phenyl substituted compounds. At the same time, for the Soret band region it is rather difficult to achieve agreement with experiment. Specifically, the energies of the G→B and G→N transitions and the relation between their intensities strongly depends on the values of the dihedral angle made by phenyl ring and the macrocycle.

Unraveling the Mystery of Ferricytochrome C: An Investigation into Induced Non-Native Conformational Changes

Cytochrome_c, in its oxidized state, adopts a multitude of conformations depending on solution conditions. Some of these conformations are relevant for the protein's functions in the electron transport process and in apoptosis. As frequently as cytochrome_c has been investigated limited experiments have been carried out under low ionic conditions, which is of significant biological importance since it's required for Apaf-1 complex formation and anionic lipids. This investigation explores the energy landscape of cytochrome_c under well-defined thermodynamic conditions. A comparison of CD and absorption of the Soret band region of both the native and non-native states of ferricytochrome c adopted between pH1-13, and temperatures between 278-353K at ionic strengths below 0.1mM was performed. Avoiding the binding of anions to positive patches on the proteins surface. State-I shows the protein unfolded with the iron in a high spin state, as the protein environment was acidified a Cotton band emerges in the CD spectra, the intensity of the bands decreased, starting around pH4. Approaching state-III, the iron enters a low spin state, a stronger couplet emerges reflecting band-splitting, predominantly caused by a combination of electronic and vibronic perturbations, maintained below 343K. Suggesting a conformational transition from the native state, into a thermally activated intermediate state, affecting the internal electric field causing moderate rearrangements of the heme, until it enters a thermally unfolded state. This state of the protein consistently becomes populated at higher temperatures across the pH range. This couplet remains into pH9 possible reflecting an intermediate transition of state III-IV, moving more alkaline this couplet disappears. Using Kuhn anisotropy, Δɛ/e vs temperature, the population of intermediates is indicated as temperature increased. Characterization of ferricytochrome_c transitions at low ionic strength showed significant heterogeneity of the protein throughout the pH range.