Study of tetraphenylporphyrins as modifiers of insulin amyloid aggregation.

Amyloid fibrils are rigid ß-pleated protein aggregates that are connected with series of harmful diseases and at the same time are promising as base for novel nanomaterials. Thus, design of compounds able to inhibit or redirect those aggregates formation is important both for the biomedical aims and for nanotechnology applications. Here, we studied the effect of tetraphenylporphyrins (metal free, their Cu and Pd complexes, and those functionalized by carboxy and amino groups on periphery) on insulin amyloid self-assembling. The strongest impact on insulin aggregation was demonstrated by a metal-free porphyrin bearing four carboxy groups. This compound strongly suppresses insulin aggregation (about 88% reduction in amyloid-sensitive probe emission) inducing formation of fibrils with the length close to this of free insulin (1.7 ± 0.6 µm as compared with 1.4 ± 0.4 µm, respectively) with an essentially reduced tendency to lateral aggregation. Contrarily, the presence of tetraphenylporphyrin containing four amino groups only slightly affects fibrils' morphology and makes weaker impact on insulin aggregation yield (about 44% reduction). This is explained by the ability of aromatic carboxy groups of 5,10,15,20-(tetra-4-carboxyphenyl)porphyrin to interact with complementary protein-binding groups and thus stabilize the supramolecular complex. For 5,10,15,20-(tetra-4-aminophenyl)porphyrin, full protonation takes place in acidic medium of protein aggregation reaction; this results in the high positive charge of TPPN4 (equal or close to +6) and hence higher contribution of coulombic repulsion to interaction of TPPN4 with insulin. One more possible mechanism of the lower inhibition effect of TPPN4 as compared with TPPC4 could be the more restricted possibility of the former as compared with the latter to form H bonds with insulin groups. It was also shown that metal-free, Pd-containing, and Cu-containing tetraphenylporphyrins without peripheral substituents make almost the same impact on the protein self-assembling. We suppose this to be due to coordination saturation of these metal atoms.

Combined Picosecond Time-Resolved UV-Vis and NMR Techniques Used for Investigation of the Excited State Intramolecular Triplet-Triplet Energy Transfer.

The phenomenon of the intramolecular triplet-triplet (T-T) energy transfer observed for spiro[9,10-dihydro-9-oxoanthracene-10,2'-5',6'-benzindan] (AN) molecule was investigated using stationary and time-resolved techniques in the UV/vis spectral region. The rate constant for energy transfer from anthrone chromophore to the triplet state localized on the naphthalene subunit of AN molecule is 2.8 × 1010 s-1. NMR spectroscopy is rarely used for investigation of molecules in the electronically excited states. Here, we propose 1H NMR combined with UV laser irradiation as a useful method for the recognition of an electron spin densities distribution in the excited triplet state that exists for tens of microseconds in the liquid phase. The direct registration of the 1H NMR signals from molecules in the excited triplet state was not possible due to its short lifetime. However, even the short interaction between unpaired electrons and nuclear spins leads to the changes in the NMR spectrum. The analysis of difference NMR spectra delivers information about the electron spin densities distribution over the skeleton of the molecule in the excited triplet state. In order to understand the nature of the excited states involved in the triplet-triplet energy transfer process, quantum chemical calculations were performed.

Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability.

The photophysics and photostability of 12,13-dihydro-5H-indolo[3,2-c]acridine (IA), a rigid bifunctional indole derivative with proton donor/acceptor functionalities, can be drastically changed by the environment. The formation of hydrogen bonds with alcohols leads to a significant decrease of the triplet formation efficiency and an increase of photostability. The photodegradation yield was found to be about two hundred times lower in methanol and 1-propanol than in n-hexane or acetonitrile. A similar effect has been reported for two indole-naphthyridines, molecules that can exist in syn and anti rotameric forms. We demonstrate that IA, which can exist only in the syn form, is more photostable in alcohols than similar, but non-rigid molecules. This additional photostability enhancement is due to the elimination of a slower channel of excited state deactivation in alcohol complexes, S0 ← S1 internal conversion. The dominant, faster channel of S1 depopulation is the excited state double proton transfer, manifested by the presence of low energy tautomeric fluorescence.

Near Infrared Phosphorescent, Non-oxidizable Palladium and Platinum Perfluoro-phthalocyanines.

New Pd(II) and Pt(II) complexes with a highly electron-deficient ligand (H2 PcF64 ) were conveniently prepared in a three-step synthesis. This is the first time that the phosphorescence of phthalocyanines with a H2 PcF64 framework has been measured. Based on these measurements, the triplet-state energies (ET ) were directly determined. Transient absorption experiments revealed broad T1 →Tn absorption spanning from ca. 350 to ca. 1000 nm and allowed determination of the triplet-state lifetimes. Removal of the Pd or Pt from the perfluoro-phthalocyanine resulted in a significant increase of the triplet lifetime for H2 PcF64 . The very efficient intersystem crossing observed for both PdPcF64 and PtPcF64 leads to residual fluorescence and suppresses the fluorescence lifetimes to less than 50 ps. The absence of Pd and Pt in the perfluoro-phthalocyanine ligand, viz. H2 PcF64 , led to a recovery of fluorescence. Cyclic voltamperometry studies pointed to complete resistance of PdPcF64 and PtPcF64 to oxidation and very strong electron affinity, which rendered these materials very good electron acceptors (n-type materials). The presence of d-orbital metals such as Pd(II) and Pt(II) in the phthalocyanine ring stabilizes their reduced forms, as indicated by the spectroelectrochemical experiments. PdPcF64 and PtPcF64 easily sensitize singlet oxygen production with very high quantum yields. Both phthalocyanines presented resistance to photodegradation in the solid state under aerobic conditions and under intense irradiation.

Single molecule Raman spectra of porphycene isotopologues.

Single molecule surface-enhanced resonance Raman scattering (SERRS) spectra have been obtained for the parent porphycene (Pc-d0) and its deuterated isotopologue (Pc-d12), located on gold and silver nanoparticles. Equal populations of "hot spots" by the two isotopologues are observed for 1 : 1 mixtures in a higher concentration range of the single molecule regime (5 × 10(-9) M). For decreasing concentrations, hot spots are preferentially populated by undeuterated molecules. This is interpreted as an indication of a lower surface diffusion coefficient of Pc-d12. The photostability of single Pc molecules placed on nanoparticles is strongly increased in comparison with polymer environments. Trans tautomeric species dominate the spectra, but the analysis of time traces reveals transient intermediates, possibly due to rare cis tautomeric forms.

Solvent-Induced Changes in Photophysics and Photostability of Indole-Naphthyridines.

Molecules that can simultaneously act as hydrogen bond donors and acceptors often exhibit completely different photophysical behavior in protic and aprotic solvents. Formation of multiple hydrogen bonds with, for example, water or alcohols, may lead to enhanced internal conversion; as a result, triplet formation efficiency can be reduced. These changes in photophysical characteristics may influence the photostability. In order to check this hypothesis, we have investigated spectroscopy, photophysics, and changes in photostability caused by interaction with aprotic and protic solvents for 2-(1'H-indol-2'-yl)-[1,5]naphthyridine and 2-(1'H-indol-2'-yl)-[1,8]naphthyridine, molecules with hydrogen bond accepting and donating functionalities. The photostability of these compounds in n-hexane, acetonitrile, and alcohols was studied in the regime of 365 nm irradiation. The photodegradation yield was found to be significantly lower in alcohols. In polar and protic solvents, the presence of two species was detected and attributed to syn and anti rotameric forms; the former are dominant in all environments.

Arresting consecutive steps of a photochromic reaction: studies of ß-thioxoketones combining laser photolysis with NMR detection.

Photochromism of monothiodibenzoylmethane has been studied in a number of environments at different temperatures. Direct laser irradiation of a sample located in the NMR magnet allowed in situ monitoring of the phototransformation products, determining their structure, and measuring the kinetics of the back reaction. These observations, along with the data obtained using electronic and vibrational spectroscopies for rare gas matrix-isolated samples, glasses, polymers, and solutions, as well as the results of quantum-chemical calculations, provide insight into the stepwise mechanism of the photochromism in ß-thioxoketones. At low temperature in rigid matrices the electronic excitation leads to the formation of the -SH exorotamer of the (Z)-enethiol tautomer. In solutions, further steps are possible, producing a mixture of two other non-chelated enethiol forms. Photoconversion efficiency strongly depends on the excitation wavelength. Analysis of the mechanisms of the photochromic processes indicates a state-specific precursor: chelated thione-enol form in the excited S2(ππ*) electronic state. The results show the potential of using laser photolysis coupled with NMR detection for the identification of phototransformation products.

1H-pyrrolo[3,2-h]quinoline: a benchmark molecule for reliable calculations of vibrational frequencies, IR intensities, and Raman activities.

Reliable assignment of 55 out of 57 vibrational modes has been achieved for 1H-pyrrolo[3,2-h]quinoline using IR, Raman, and fluorescence spectroscopy combined with quantum chemical calculations. The experiments provided a data set for assessing the performance of different models/basis sets for predicting the vibrational frequencies, as well as IR and Raman intensities for a molecule with 13 heavy atoms. Among six different tested DFT functionals, the hybrid B3LYP used with Pople's split-valence basis sets is suggested as the best choice for accurate and cost-effective IR/Raman spectral simulations. Neither HF nor MP2 methods can satisfactorily describe the vibrational structure. Increasing the basis set size from double to triple-ζ and by adding polarization and diffuse functions does not necessarily improve the results, especially regarding the predictions of vibrational frequencies. With respect to the intensities, extending the basis set helps, with the accuracy increasing systematically for the Raman spectra, and in a less regular fashion for the IR. A large difference in accuracy is observed while comparing the spectral parameters predicted for in-plane and out-of-plane normal modes. The former are reliably computed with modest basis sets, whereas for the out-of-plane vibrations, larger basis sets are necessary, but even in this case, the out-of-plane vibrations are reproduced with much less accuracy than in-plane modes. This effect is general, as it has been observed using different functionals and basis sets.