Glauco Ponterini

Trans-cis photoisomerization mechanism of carbocyanines: experimental check of theoretical models

Abstract Two different theoretical models for the cyanine photoisomerization, emphasizing, respectively, the role of the solvent effects (model a) and that of the intramolecular forces (model b), have been experimentally tested by investigating the photophysical and photochemical behaviour of two carbocyanines (DOC and DTC). The decay kinetics of the excited-state (S1) stable isomer and of the ground-state (S0) unstable photoisomer have been measured in solvents of very different polarities, but similar and low viscosities (methanol, methylene chloride and chlorobenzene were extensively investigated, but some test measurements were also carried out in p-dioxane and in 1:1 toluene-chlorobenzene). The activation energy for the non radiative decay of the spectroscopic S1 minimum, corresponding to a twisting around one of the polymethine bonds, is independent of solvent polarity, whereas a small but significant dependence is displayed by the activation energy and preexponential factor of the S0 back isomerization. These results can be easily accounted for within model b, while they sharply contrast with the predictions of model a. We have also measured the quantum yields of photoisomer formation for DOC and DTC at various temperatures and have shown that, in both cases, a radiationless decay occurs from the perp S1 minimum to the ground state, with a branching ratio favouring reformation of the stable (trans) isomer. The parallel temperature dependence of the fluorescence decay and photoisomer formation has provided further support to model b. Finally, the temperature dependence of the quantum yield for the cis→trans photoisomerization of DOC has been analyzed. A very low activation energy has been found, suggesting that this process is probably intrinsically barrierless.

First- and second-order polarizabilities of simple merocyanines. An experimental and theoretical reassessment of the two-level model.

Taking four merocyanines [(CH3)2N-(CHCH)n-C(CH 3)O; n = 1-4] (Mc1-4) as test D-A systems, we performed a close experimental and theoretical examination of the two level model with reference to its ability to provide correct predictions of both absolute values and dependence on the conjugation path length of first- and second-order molecular polarizabilities. By (1)H NMR spectroscopy merocyanines Mc1-4 were found to be approximately 1:1 mixtures of two planar conformers with cis and trans arrangements of the C(CH 3)O electron-acceptor group and all trans structure of the polyene like fragment. The degree of bond length alternancy (BLA) in the -(CHCH)n- fragment, was quantified by extensive full geometry optimizations at both semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to be most reliable and were used for calculations of the excited-state properties. The applicability of the two level model, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g) and the lowest-lying (1)(pipi*) CT state (e), was checked by analysis of fluorescence and near UV absorption spectra. Measurements of the basic two-level model quantities ( Ege, microge and Deltamicro(eg)), by which the dominant components of alpha and beta tensors are expressed (alpha XX , beta XXX , X identical with long molecular axis), were designed to give approximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic method for the determination of Deltamicro(eg), based on accurate measurements of absorption spectral shifts in n-hexane/diethyl ether mixtures with small diethyl ether volume fractions. Such an approach led to Mc1-4 beta XXX s matching well in both value and n-dependence with EFISH data reported in the literature for similar merocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, which combines absorption and fluorescence solvent shifts. All the measured quantities were calculated for both trans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performances of different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In all cases, alpha XX and beta XXX were found to rise proportionally to about n (1.3) and n (2), respectively, in qualitatively good agreement with the experimental values. As to the absolute values, however, experimental alpha XX s and beta XXX s were best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including both single and double excitations. The validity of the two-level model was checked by comparison with converged SOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilities for all of the four dyes (Mc1-4).

Protein–protein interface-binding peptides inhibit the cancer therapy target human thymidylate synthase

Human thymidylate synthase is a homodimeric enzyme that plays a key role in DNA synthesis and is a target for several clinically important anticancer drugs that bind to its active site. We have designed peptides to specifically target its dimer interface. Here we show through X-ray diffraction, spectroscopic, kinetic, and calorimetric evidence that the peptides do indeed bind at the interface of the dimeric protein and stabilize its di-inactive form. The “LR” peptide binds at a previously unknown binding site and shows a previously undescribed mechanism for the allosteric inhibition of a homodimeric enzyme. It inhibits the intracellular enzyme in ovarian cancer cells and reduces cellular growth at low micromolar concentrations in both cisplatin-sensitive and -resistant cells without causing protein overexpression. This peptide demonstrates the potential of allosteric inhibition of hTS for overcoming platinum drug resistance in ovarian cancer.

Singlet-triplet intersystem crossing in 2,2':5',2-terthiophene and some of its derivatives

Abstract During the course of a study devoted to the investigation of the existence of a relationship between the phototoxic and photophysical properties of 2,2′:5′,2″-terthiophene ( 1a ) and related compounds, the singlet-to-triplet intersystem crossing quantum yields of compound 1a and six of its monosubstituted derivatives ( 1b – 1g ) were determined in 95% ethanol solution. The variations in the phototoxicity of these compounds against some insect and mite species are large in comparison with changes in the quantum yields of triplet state formation. The latter were very similar in the compounds studied (range, 0.88–0.99), with the exception of 5-formyl-2,2′:5′,2″-terthiophene. ( 1c ) (Φ T =0.50). Moreover, a complete characterization of the S 1 decays of compounds 1a – 1g was obtained by combining these data with the corresponding fluorescence quantum yields and lifetimes. Finally, the consequences of the nature and position of the substituents on the measured triplet—triplet absorption and rate constants of the two S 1 decay paths, i.e. fluorescence emission and intersystem crossing, are qualitatively discussed.

Large third-order nonlinear optical response of porphyrin J-aggregates oriented in self-assembled thin films

The preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl)porphyrin diacid (H4TPPS2−) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H4TPPS2− is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient βTPA of 50 cm GW−1. The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H4TPPS2− from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayers.

Homodimeric enzymes as drug targets.

Many enzymes and proteins are regulated by their quaternary structure and/or by their association in homo- and/or hetero-oligomer complexes. Thus, these protein-protein interactions can be good targets for blocking or modulating protein function therapeutically. The large number of oligomeric structures in the Protein Data Bank (http://www.rcsb.org/) reflects growing interest in proteins that function as multimeric complexes. In this review, we consider the particular case of homodimeric enzymes as drug targets. There is intense interest in drugs that inhibit dimerization of a functionally obligate homodimeric enzyme. Because amino acid conservation within enzyme interfaces is often low compared to conservation in active sites, it may be easier to achieve drugs that target protein interfaces selectively and specifically. Two main types of dimerization inhibitors have been developed: peptides or peptidomimetics based on sequences involved in protein-protein interactions, and small molecules that act at hot spots in protein-protein interfaces. Examples include inhibitors of HIV protease and HIV integrase. Studying the mechanisms of action and locating the binding sites of such inhibitors requires different techniques for different proteins. For some enzymes, ligand binding is only detectable in vivo or after unfolding of the complexes. Here, we review the structural features of dimeric enzymes and give examples of inhibition through interference in dimer stability. Several techniques for studying these complex phenomena will be presented.

Electronic spectra and trans—cis photoisomerism of carbocyanines. A theoretical (CS INDO CI) and experimental study

Abstract Two carbocyanine dyes, DOC(3,3′-diethyloxacarbocyanine) and DTC (3,3′-diethylthiacarbocyanine), were studied in detail by a combined experimental and theoretical treatment previously adjusted on simple streptocyanines. The BPPC (bisphenylaminopentamethine cyanine) dye was also studied to better correlate the properties of DOC and DTC with those of their parent streptocyanine BMPC (bisdimethylaminopentamethine cyanine). In the experimental part of the work, carried out on methanol solutions of BPPC + , Cl − , DOC + , I − and DTC + , I − , we determined the separate spectra of the stable forms and the photoisomers produced by irradiation into the visible absorption region. The theoretical part, based on CS INDO S + D + T - CI calculations, was especially devised to provide as thorough an interpretation as possible of the electronic spectra (both S 0  S n and S 0  T n ) of BPPC, DOC and DTC in terms of molecular subunits and to try to identify each observed species as a specific geometrical isomer (all- trans or mono- cis ). In all cases the properties of the stable forms were consistent with the molecules assuming the all- trans structure. The lowest singlet excited state ( S 1 ), responsible for the colour band, retained nearly pure cyanine character ( 1 B in Platts notation), while the states falling in the second (medium UV) absorption region had prevailing aromatic ( 1 H ) or charge transfer ( 1 G ) character. This provided an explanation for the absence of a definite “ cis peak” effect in these compounds. BPPC was found to give rise to the same photoisomer as BMPC (3–4 cis ) and an additional photoisomer clearly identified as the 1–2 cis form. On the other hand, in DOC and DTC, where the spectral changes caused by the irradiation were far weaker, the single observed photoisomer could be better assigned as 2–8 cis , i.e. corresponding to the 2–3 cis form of BMPC.

Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines

The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S1 state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying 1(nπ*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the 1(πHπL*) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.

Dimerization of green sensitizing cyanines in solution. A spectroscopic and theoretical study of the bonding nature

Abstract The bonding nature in cyanine-dye aggregates has been investigated by studying dimerization in solution of 3,3′-disulfopropyl-4,5,4′,5′-dibenzo-9-ethyloxacarbocyanine (D) and three other oxacarbocyanine analogues (E, F, G) used as photographic sensitizers in the green spectral region. Quantitative information on the monomer–dimer equilibrium of dye D in different solvents and of its analogues (E, F, G) in water was obtained by measurement of the absorption spectrum as a function of dye concentration and of temperature. Dimerization was found to be generally driven by enthalpic factors traceable to strong attractive van der Waals interactions between the two large and highly polarizable dye molecules. Entropic contributions to ΔG0 usually favour dissociation but are smaller than the enthalpic ones. The visible absorption spectrum of the dimer consists in a classic two-branched exciton band with a marked splitting ( 1600 cm −1 in water). The experimental observations were the subject of a theoretical study including classical molecular dynamics (MD) and Monte Carlo (MC) calculations of the dimer structure and comparative analysis of monomer and dimer spectra by the CS INDO CI method. Computer simulations led to three similar H-type structures, the most stable of which is characterized by a distance of 4.7 A between the planes of the chromophores and an endo–endo configuration of the sulfopropyl substituents. The calculated dimer spectrum was clearly interpreted in terms of exciton model but a quantitative agreement with the two-maximum exciton band could be obtained only by assuming substantial deviation of the long molecular axes from parallelism. On the basis of normal coordinate calculations it is suggested that such configurations may occur with a high probability in virtue of twisting vibrational motions of extremely low frequency.

Electronic spectrum of porphyrins. CS INDO CI study

Abstract The electronic spectra of porphin, chlorin and their magnesium derivatives have been studied quantum-mechanically with the CS INDO CI method. Both the S 0 -S n and T 1 -T n one-photon absorption spectra have been analyzed together with the correlation effects. The main result regards the T-T spectrum of porphin. To obtain a correct description of the complete spectrum it is necessary to take account of the correlation effects. Goutermans model of four orbitals is too simple to describe the region of the Soret band.