Soret Band Region Research Articles

The effect of π-extended indoleninyl-dibenzotetraaza[14]annulenes on their third-order nonlinear optical properties

Two new dibenzotetraaza[14]annulenes (DBTAAs) featuring indolenine moieties were synthesised via an acetic acid-catalysed condensation of 1,2-diaminobenzene with two diformyl indolenines in good yields. The structural identity of the compounds was confirmed by NMR (1H, 13C), FT-IR and elemental analysis. UV–visible analysis displays intense absorptions in the Soret band region with ɛ ranging from 0.32 to 1.02 × 105 M−1 cm−1 in various solvents of different polarities. The absorption spectra display a slight bathochromic shift upon replacing the indolenine (4a) with benz[e]indolenine (4b) groups on the DBTAA framework. Furthermore, the spectra exhibit solvatochromism shifts due to hydrogen-bonding interactions between the solutes and solvents. The effect of the extended π-conjugation on the electronic distributions at different energy levels was computed by Density Functional Theory (DFT) calculations. Compound 4b exhibits greater Nonlinear Optical (NLO) parameters compared to 4a due to the extended π-conjugation in the molecule. Hence, 4b could be utilised as a promising material for various NLO applications.

Femtosecond Dynamics of Excited States of Chlorophyll Tetramer in Water-Soluble Chlorophyll-Binding Protein BoWSCP.

The paper reports on the absorption dynamics of chlorophyll a in a symmetric tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP. It was measured by a broadband femtosecond laser pump-probe spectroscopy within the range from 400 to 750 nm and with a time resolution of 20 fs-200 ps. When BoWSCP was excited in the region of the Soret band at a wavelength of 430 nm, nonradiative intramolecular conversion S3→S1 was observed with a characteristic time of 83 ± 9 fs. When the complex was excited in the region of the Qy band at 670 nm, relaxation transition between two excitonic states of the chlorophyll dimer was observed in the range of 105 ± 10 fs. Absorption spectra of the excited singlet states S1 and S3 of chlorophyll a were obtained. The delocalization of the excited state between exciton-coupled Chl molecules in BoWSCP tetramer changed in time and depended on the excitation energy. When BoWSCP is excited in the Soret band region, an ultrafast photochemical reaction is observed. This could result from the reduction of tryptophan in the vicinity of chlorophyll.

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD● is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434nm and 446nm. This feature was absent when TyrD was reduced, but was present (i) when TyrD was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434nm and 446nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.

Modified porphyrin chromophores: Synthesis and spectroscopic properties of nitronaphtho[1,2-b]porphyrins, benzothieno[2,3-b]porphyrins and their metalated derivatives

A series of c-annulated pyrroles were prepared by reacting isocyanoacetate esters with dinitronaphthalenes in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene. The resulting nitronaphthopyrroles esters condensed with an acetoxymethylpyrrole to afford dipyrrylmethanes and following cleavage of the ester groups, acid-catalyzed condensation with a dipyrrylmethane dialdehyde afforded four isomeric nitronaphthoporphyrins. Three of these isomers showed only minor differences in their electronic spectra compared to a known naphtho[1,2-b]porphyrin, although in two cases the intensities of the Soret band absorptions were reduced. However, when the nitro group was placed at a position one removed from the porphyrin nucleus, the UV–vis spectrum showed two very broad peaks in the Soret band region as well as distorted Q bands. This effect was also noted for the corresponding nickel(II), copper(II) and zinc complexes. An example of a benzo[b]thiophene-fused porphyrin was also prepared in three steps from 3-nitrothiophene, but the fused heterocyclic unit had virtually no effect on the porphyrin chromophore. This study demonstrates that while fused naphthalene and benzo[b]thiophene units do not significantly affect the electronic structure of porphyrins, the introduction of a suitably placed electron-withdrawing group can induce profound changes.

A porphyrin derivative with highly red shifted and intensified absorption bands immobilized into the mesopores of nanostructured amberlite: Synthesis, characterization and photocatalytic activity

The BF3 complex of meso-tetra(4-sulfonatophenyl)porphyrin (H2TPPS4), immobilized into the pores of a new nano-structured mesoporous anion exchange polymer, nanoAmbN(Me)3Cl (Amb = Amberlite IRA-410(Cl)) was used as the photosensitizer for aerobic oxidation of organic compounds. NanoAmbN(Me)3Cl, nanoAmbN(Me)3@H2TPPS4 and nanoAmbN(Me)3@H2TPPS4(BF3)2, were characterized by IR and UV–vis diffuse reflectance spectroscopy (UV–vis DRS) methods, TGA, FESEM, elemental mapping (SEM-EDX) and BET. The nanocomposite was used as photosensitizer in the aerobic oxidation of 1,5-dihydroxynaphthalene (DHN) and cyclohexene in acetonitrile. The results were compared with those obtained in the presence of the BF3 complex of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) immobilized on nanostructured amberlyst 15. The observed red shifts in the UV–vis DRS UV–vis spectrum upon the formation of nanoAmbN(Me)3@H2TPPS4(BF3)2 were much larger than those of nanoAmbSO3@H2TMPyP(BF3)2. The singlet quatum yield of nanoAmbN(Me)3@H2TPPS4 and nanoAmbN(Me)3@H2TPPS4(BF3)2 was determined by using 1,3-diphenylisobenzofuran (DPBF) and DHN as the quenchers of singlet oxygen, for the excitation of the photosensitizers in the Q and Soret band regions, respectively.

Globin X: A target for surfactant-like molecules.

Globin X (GbX) is recognized as a novel, intrinsically hexacoordinate, member of the vertebrate globin family. Previously we showed that GbX is significantly more stable than pentacoordinate vertebrate globins (Hb and Mb) at elevated temperatures, acidic pH as well as in the presence of denaturants. Such a unique feature of this protein led us to examine how the GbX interacts with surfactant-like molecules such as perfluorooctanoic acid (PFOA) and sodium dodecyl sulfate (SDS). In addition to GbXWT protein, the effect of intraprotein disulfide bridge and the heme iron pentacordination on GbX interactions with surfactant-like molecules was investigated by characterizing the GbXC65A and GXH90V constructs. The impact of surfactant on the structural properties of GbX was evaluated based on changes in the Soret band region and the emission properties of intrinsic Trp residues. The Kd values for SDS binding to the deoxy form of GbXWT, GbXC65A, and GbXH90V were determined to be 251 ± 4 μM, 188 ± 7 μM, and 358 ± 91 μM respectively, indicating that the intramolecular disulfide bond and hexacoordination do not have a significant effect on the surfactant-protein complex. However, the met form of GbXWT and GbXC65A binds SDS with Kd of 1426 ± 224 μM and 2106 ± 26 μM, pointing towards a lower affinity of the met form of the protein for SDS surfactant. Interestingly, PFOA produces a minimal change in the Soret band region, suggesting a distinct mechanism of binding. Based on the Trp emission data, the Kd values for PFOA binding to the met form of the protein were determined to be 40 ± 28 μM for GbX WT and 25 ± 1 μM for GbXC65A. The molecular mechanism of surfactant-like molecules binding to GbX was also evaluated using molecular dynamic simulations.

Thiourea Organocatalysts as Emerging Chiral Pollutants: En Route to Porphyrin-Based (Chir)Optical Sensing

Environmental pollution with chiral organic compounds is an emerging problem requiring innovative sensing methods. Amino-functionalized thioureas, such as 2-(dimethylamino)cyclohexyl-(3,5-bis(trifluoromethyl)phenyl)thiourea (Takemoto’s catalyst), are widely used organocatalysts with virtually unknown environmental safety data. Ecotoxicity studies based on the Vibrio fischeri luminescence inhibition test reveal significant toxicity of Takemoto’s catalyst (EC50 = 7.9 mg/L) and its NH2-substituted analog (EC50 = 7.2–7.4 mg/L). The observed toxic effect was pronounced by the influence of the trifluoromethyl moiety. En route to the porphyrin-based chemosensing of Takemoto-type thioureas, their supramolecular binding to a series of zinc porphyrins was studied with UV-Vis and circular dichroism (CD) spectroscopy, computational analysis and single crystal X-ray diffraction. The association constant values generally increased with the increasing electron-withdrawing properties of the porphyrins and electron-donating ability of the thioureas, a result of the predominant Zn⋯N cation–dipole (Lewis acid–base) interaction. The binding event induced a CD signal in the Soret band region of the porphyrin hosts—a crucial property for chirality sensing of Takemoto-type thioureas.

Hydrophobic quercetin encapsulated hemoglobin nanoparticles: formulation and spectroscopic characterization

Various natural proteins are finding application in drug delivery for their high biodegradability and biocompatibility. Albumins are well explored and now focus is shifting to other proteins like hemoglobin (Hb) with unique structural properties. In the present study Hb is allowed to denature at pH 5.0 and model hydrophobic drug quercetin (Q) is encapsulated via self-assembly and hydrophobic interactions. Fluorimetric titrations record highest binding between Hb and Q at pH 5.0, rendering significant structural changes in Hb as captured in CD spectra. A decrease in fluorescence life time of tryptophan residues from 3.31 ns in Hb to 2.89 ns in presence of Q at pH 5.0; surmises efficient binding of Q at the hydrophobic core housing tryptophan. Peak shifts in Fourier transform infrared spectroscopy spectra of Hb-Q compared to Hb evidence significant interactions between them at pH 5.0. Significant spectral changes in soret band region of Hb on addition of Q at pH 5.0 envisages unfolding of porphyrin ring and binding influence of Q. Efficient formation of Hb-Q nanoparticles (NPs) at pH 5.0 is established by DLS, SEM and TEM. Communicated by Ramaswamy H. Sarma

Bandgap Energy of TiO₂/M-Chlorophyll Material (M=Cu²⁺, Fe³⁺)

The bandgap energy (Egap) of TiO2 material modified with metal-chlorophyll complex compounds (M = Cu2+, Fe3+) was observed. Chlorophyll (Chl) was isolated from cassava leaves, and its UV-Vis spectra showed absorption peaks in the Soret band region (410 nm) and in the Q band region (665 nm), which is the typical peak of chlorophyll. Copper(II)-chlorophyll complex was prepared from the reaction between chlorophyll and CuSO4.5H2O, while the iron(III)-chlorophyll was synthesized from chlorophyll and FeCl3.6H2O in methanol solvent under reflux at 65°C. The presence of copperand iron metals in the chlorophyll metal complexes was identified using Atomic Absorption Spectroscopy in methanol solution. The absorption of copper measured in Cu2+-Chl was 0.0488 (0.4805 mg/L), while the iron atom in Fe3+-Chl was 0.0050 (0.0195 mg/L). The UV-vis spectra demonstrate the hypsochromic shift of the Soret band to 405 nm (Cu2+-Chl) and 402 nm (Fe3+-Chl). The Infrared spectra of chlorophyll after being complexed with copper(II) shows the increase of vibrational absorption wavenumber of the C=N group from 1225.06 cm-1 to 1241.94 cm-1 indicates the coordination of the metal ion on the N atom in the pyrrole ring. The shift in the absorption band on the Fe3+-Chl spectrum was seen for the C=O ester group from 1720.49 cm-1 to 1721.10 cm-1 indicating the metal ion bonding in the C=O group of esters. The DR-UVis analysis of TiO2/metal-chlorophyll shows a bathochromic shift towards the visible light region. By using the Tauc plot method, it was observed that the Egap of TiO2 reduces from 3.08 eV to 2.89 eV and 2.93 eV in the compound of TiO2/Cu2+-Chl and TiO2/Fe3+-Chl, respectively.

Supramolecular Chirogenesis in Bis-Porphyrin: Crystallographic Structure and CD Spectra for a Complex with a Chiral Guanidine Derivative

The complexation of (3aR,7aR)-N-(3,5-bis(trifluoromethyl)phenyl)octahydro-2H-benzo[d]imidazol-2-imine (BTI), as a guest, to ethane-bridged bis(zinc octaethylporphyrin), bis(ZnOEP), as a host, has been studied by means of ultraviolet-visible (UV-Vis) and circular dichroism (CD) absorption spectroscopies, single crystal X-ray diffraction, and computational simulation. The formation of 1:2 host-guest complex was established by X-ray diffraction and UV-Vis titration studies. Two guest BTI molecules are located at the opposite sides of two porphyrin subunits of bis(ZnOEP) host, which is resting in the anti-conformation. The complexation of BTI molecules proceed via coordination of the imine nitrogens to the zinc ions of each porphyrin subunit of the host. Such supramolecular organization of the complex results in a screw arrangement of the two porphyrin subunits, inducing a strong CD signal in the Soret (B) band region. The corresponding DFT computational studies are in a good agreement with the experimental results and prove the presence of 1:2 host-guest complex as the major component in the solution (97.7%), but its optimized geometry differs from that observed in the solid-state. The UV-Vis and CD spectra simulated by using the solution-state geometry and the TD-DFT/ωB97X-D/cc-pVDZ + SMD (CH2Cl2) level of theory reproduced the experimentally obtained UV-Vis and CD spectra and confirmed the difference between the solid-state and solution structures. Moreover, it was shown that CD spectrum is very sensitive to the spatial arrangement of porphyrin subunits.

Crystallographic and computational studies of a tartaric acid amide linked zinc bisporphyrinate

A chiral zinc bisporphyrinate ([Zn2(D-BTABis)]) showed a bisignate CD signal in the Soret band region. In order to understand the origin of this CD spectrum, we have done crystallographic and computational studies. The crystal structure shows this bisporphyrin is composed of two zinc porphyrin subunits with a tartaric acid amide linker. There are π-π interactions between the phenyl substituents in the linker and porphyrin planes. Bisporphyrin adopts syn-conformation in the crystal structure, but that cannot rationalize the solution CD. Density function theory (DFT) calculations reveal that the conformation of the optimized structure is different from that of the crystal structure. Based on the optimized structure, TDDFT calculations provide the simulated CD spectrum consistent with the observed CD. Our studies suggest the comprehensive understanding of the CD origin should rely on the combination of crystal structure and theoretical calculations.

Spectroscopic Investigation of Interaction between the 4f Electronic System and the Photoexcited Cyclic π System in Terbium(III) Monoporphyrinato Complex.

Electronic interaction between the total angular momentum (J) of the 4f electronic system and an angular momentum (L) of the photoexcited cyclic π system in a terbium(III) monoporphyrinato complex with a capping ligand cyclen, [Tb(TPP)(cyclen)]Cl (TPP = 5,10,15,20-tetraphenylporphyrinato; cyclen = 1,4,7,10-tetraazacyclododecane), have been investigated by varied-temperature and varied-magnetic-field magnetic circular dichroism (MCD) spectroscopy. Three MCD A-term patterns which correspond to B(0,0), Q(0,0), and Q(1,0) absorption bands were observed for the complex which was incorporated into a thin film of poly(methyl methacrylate), PMMA, as well as that in solution phase. The A-term patterns in the Soret band region and Q-band region were found to show opposite behavior with changes in temperature and magnetic field. This finding clearly indicated not only the presence of the J-L interaction but also the different directions of the J-L interaction; i.e., J and L interact ferromagnetically in the Q(0,0) and Q(1,0) states, while antiferromagnetically in the B(0,0) state. The magnitude of the interaction was determined by simulation-based fitting to experimental ratios, where refers to the A-term intensity and to the oscillator strength of the band. The first ab initio study was also carried out for further insight into the interaction. From the computational study, the relationship between the orbital angular momentum of π electronic system, L(π), and the orbital angular momentum of the 4f system, L(f), was clearly revealed. A computational approach to determine the magnitude and the direction of J-L interaction was established.

C2 symmetric borneol-porphyrin hybrids: Synthesis, characterization, electronic structure and their anti-cancer behaviors

A series of five C2 Symmetric A2B2 type borneol-porphyrin hybrids have been synthesized and characterized. Moreover, a detailed analysis of their optical and redox properties was carried out by comparing the results obtained via optical spectroscopy and electrochemistry. The current results demonstrated that the meso borneol-substitutions excitation coupled strongly with porphyrin core that leads to significant CD signals in the Soret band region, and molecular anti-cancer behaviors were also investigated.

Spectroscopic and theoretical optical properties of indoleninyl‐substituted dibenzotetraaza[14]annulenes

AbstractTwo new macrocyclic dibenzotetraaza[14]annulene (DBTAA) compounds with indolenine (5) and pyridoindolenine (6) moieties were synthesized and characterized by spectroscopy. Both DBTAAs exhibit strong UV‐Vis absorption properties in the Soret band region. The theoretical second‐order nonlinear optical property, electric dipole moment (μ), dispersion‐free dipole polarizability (α) and first hyper‐polarizability values were calculated by density functional theory and time dependent density functional theory. The ab‐initio quantum mechanical calculation by time‐dependent Hartree‐Fock method was utilized to investigate the dynamic dipole polarizabilities, dynamic second‐order, static, and dynamic third‐order (γ) hyper‐polarizabilities of the DBTAAs. The configuration interaction technique of all doubly occupied molecular orbitals possesses theoretically defined single‐photon absorption (OPA) specifications for the examined structures. The computed maximum OPA wavelengths on both macrocyclic compounds coincide with the preceding measurement outcomes.

Effect of degree of β-chlorination on photocatalytic activity of meso-tetraphenylporphyrin under homogeneous and nanoscale heterogeneous conditions: Chlorination vs. bromination

β-Chlorination of meso-tetraphenylporphyrin, H2TPP leads to the formation of H2TPPClx (X = 2, 4, 6 and 8) with absorption bands in the range of 421–451 and 650–715 nm, for the Soret and Q(0,0) bands, respectively. H2TPPClx were used as photosensitizers for the aerobic photooxidation of cyclooctene under mild conditions and compared with that of the non-chlorinated porphyrin. The lowest degree of catalyst degradation was observed in the case of H2TPPCl2. H2TPPCl2 and H2TPPCl4 showed the highest photocatalytic activity among the series. In spite of the electron-withdrawing effects of the chlorine atoms, with the exception of H2TPPCl2, the chlorinated porphyrins showed a comparable oxidative degradation to that of H2TPP towards oxidative degradation. The preference of blue LED lamp over the red one shows that the absorption at the Soret band region of the chlorinated porphyrins is mainly responsible for the photosensitizer ability of the catalyst. In this regard, H2TPPCl2 and H2TPPCl4 which have the highest overlap with the emission bands of the blue LED lamp showed the highest photocatalytic activity than the other H2TPPClx and H2TPP. Also, control reactions performed in the presence of 1,3-diphenylisobenzofuran and 1,4-benzoquinone provided evidence for the involvement of singlet oxygen as the main reactive oxygen species in this reaction. Singlet oxygen quantum yields (φF) of H2TPPClx were smaller than that of the non-chlorinated porphyrin and decreased as H2TPPCl8 > H2TPPCl4 > H2TPPCl6 ≥ H2TPPCl2. H2TPPClx showed much higher photocatalytic activities than their brominated counterparts. Also, the φF values and oxidative stability of the chlorinated porphyrins were usually higher than those of the brominated porphyrins, with the same degree of β-halogenation. Protonation of H2TPPClx with weak and strong acids (HCOOH, CF3COOH and HCl), also their immobilization on a nano-structured acidic polymer (Amberlyst 15) were used to study the influence of core protonation on photocatalytic activity and oxidative stability of the chlorinated porphyrins.

Crystallographic and DFT studies on host-guest complexes consisting of zinc bisporphyrinates and 1-phenylethylamine

We have investigated the chirality transfer from 1-phenylethylamine to a 5-amino-1,3-phthalic acid diamide-linked zinc bisporphyrinate through crystallographic and DFT studies. When the hosts were mixed with optically pure 1-phenylethylamine, CD showed moderate signals in the Soret band region. Single crystals of the corresponding 1:1 and 1:2 host-guest complexes were obtained. We present the first crystallographic structure of a 1:2 host-guest complex consisting of a bisporphyrin host and chiral monoamine guests. The structure reveals that the host-guest interactions are different for two guest molecules. The first guest is involved in a hydrogen bond and π-π interactions, while the second one is only involved in π-π interactions, which has not been observed in previous studies. The corresponding chirality transfer mechanism was also rationalized by DFT calculations.

Spectroscopic, thermodynamic and molecular docking studies on the interaction of two water-soluble asymmetric cationic porphyrins with calf thymus DNA

The water-soluble cationic porphyrin, 5-(1-dodecyl pyridinium-4-yl)-10,15,20-tris(1-methylpyridinium-4-yl)-21H,23H-porphyrin tetrachloride (MDTMPyP), and its Cu(II) complex (CuMDTMPyP) have been prepared and characterized by 1H NMR, UV–visible spectroscopy and elemental analyses. The binding properties of the porphyrins with calf thymus DNA (CT-DNA) have been investigated by electrochemical, spectroscopic and molecular docking methods. These observations were further confirmed by monitoring DNA viscosity and also circular dichroism studies in the Soret region. The absorption spectrum changes of both porphyrins in Soret band region showed that they could intercalate in DNA structure. The obtained results showed that MDTMPyP has a higher affinity to CT-DNA than CuMDTMPyP. The porphyrin solutions were titrated with DNA and the UV–visible titration spectra were analyzed with multivariate curve resolution-alternating least squares (MCR-ALS) technique to obtain spectrum of porphyrin–DNA complex, and concentration profiles of free and DNA-bonded porphyrin. Then the binding constant was calculated by the combination of MCR-ALS and McGhee–von Hippel equation. Also, the thermodynamic parameters of the DNA binding process were acquired from the van’t Hoff equation at various temperatures. The positive and large values of the entropy, and enthalpy revealed that the interaction is endothermic, and the entropically driven process. Two water soluble cationic asymmetric porphyrins have been synthesized and structurally characterized with various techniques. Their binding properties with CT-DNA have been explored by electrochemical and spectroscopic methods. The DNA binding studies offered that these porphyrins can intercalate into CT-DNA.

Carbon‐bridged Oligo(phenylenevinylene)s as Light‐harvesting Antenna for Porphyrins

The efficacy of carbon-bridged oligo(phenylenevinylenes)s (COPVs) as light-harvesting antenna for porphyrins is demonstrated using a series of 5,15-di-COPVn-substituted free-base and zinc porphyrins, COPVn-MP-COPVn (n=1-3, M=H2 , Zn). These molecules were synthesized by Suzuki-Miyaura cross-coupling reactions of COPVn-Bpin and Br-H2 P-Br. The absorption spectra of these compounds in solution show a significant expansion of the Soret band region together with a bathochromic shift of the Q band, suggesting a significant interaction between these chromophores in the ground state. The photoluminescence quantum yield of the porphyrin-COPV conjugates is enhanced up to four times relative to the parent porphyrins. Theoretical calculations also indicated interactions between these chromophores in the HOMO, which suggests that the light-harvesting ability stems from the expansion of the π-electron-conjugation system.

Structures and electronic properties of halogenated Au(III) phthalocyanine AuPcX (X = Cl, Br): A Density Functional Theoretical Study

The structure and electronic property of halogenated Au(III) phthalocyanine AuPcX (X = Cl, Br) have been investigated theoretically with density functional theory (DFT). The calculation results show that the Au cation is coordinated with one halogen atom and four nitrogen atoms of phthalocyanine ligand. In addition, the electro-absorption spectrum of AuPcX (X = Cl, Br) have been calculated by time-dependent density functional theory. The calculated results show that both of the two compounds have strong absorption in the Q-band and Soret-band region, the solvent has a great influence on the absorption spectrum as the two compounds have different transition mechanism in vacuum and solvent environment. In addition, with the increase of solvent polarity, the absorption spectrum especially the Q-band of the two compounds is red-shifted evidently.

Determination of the Absolute Configuration of Side Chains of Basic Amino Acid Residues Using the Water-Soluble Porphyrin-Based Exciton Chirality Method.

We demonstrated that the circular dichroism (CD) exciton chirality method, based on the supramolecular interactions of meso-tetra(4-sulfonatophenyl)porphyrin (MTPPS4, M = Zn or H2), was applicable for the determination of the absolute configuration between the side chains of two basic amino acid residues of stable monomeric β-hairpin peptides (tryptophan zipper: Trpzip). When MTPPS4 was added to an aqueous solution containing Trpzip, a bisignate CD signal was detected in the Soret band region in addition to a decrease in absorbance. These spectral changes indicated the formation of a supramolecule consisting of Trpzip and MTPPS4 via electrostatic interactions between the positively charged lysine residue of Trpzip and the negatively charged sulfonate group of MTPPS4. On the basis of the Job plots, the supramolecular structure of Trpzip-ZnTPPS4 is ZnP-Tz-ZnP or ZnP-Tz-ZnP-Tz-ZnP, whereas that of Trpzip-H2TPPS4 is -(-H2P-Tz-)n- (MP and Tz denote MTPPS4 and Trpzip, respectively). To explain the bisignate CD spectra of the supramolecules, a plausible model, that is, ZnP-Tz-ZnP, was carefully analyzed by the CD exciton chirality method: two orthogonalized electric transition dipole moments of each MTPPS4 and the effects of free rotation of MTPPS4 around the electrostatic bonding axis were considered. The exciton-coupled CD spectral pattern based on ZnTPPS4 reflected the absolute configuration between the side chains of two lysine residues. Our approach can be used to determine the absolute configuration of side chains of other peptides with two basic amino acid residues in the amino acid sequences.