Mario J. Politi

Photophysical characterization of a 1,4,5,8-naphthalenediimide derivative

Abstract The photophysical properties of N , N -dibutyl-1,4,5,8-naphthalenediimide ( 1 , Scheme 1) were studied using absorption and fluorescence spectroscopy. The absorption and emission spectra of 1 are mirror images and show resolved vibrational structure indicating that 1 is a rigid molecule which does not relax substantially from the initially formed Franck-Condon state. The fluorescence of 1 is rather weak (quantum yield, approximately 0.002), supposedly due to fast intersystem crossing to a close-lying triplet level. Diimide 1 aggregates in acetonitrile and in aqueous medium, but aggregation is prevented by the presence of α-cyclodextrin (α-CD) due to complex formation between α-CD and the butyl substituents of 1 . Diimide 1 displays unusual spectral characteristics in aromatic hydrocarbon solvents, which can be attributed to ground state complex formation between 1 and the aromatic compound. The addition of benzene, toluene and p -xylene to an acetonitrile solution of 1 was followed by fluorescence spectroscopy, and the data obtained were treated as simple 1 : 1 equilibria. Association constants were calculated for the complexes between 1 and the aromatic compounds. The magnitude of the constants suggests that these ground state complexes are basically of the π-stacking type, with no charge transfer character. In the excited state, however, the complexes show polar character, suggesting that electron transfer occurs following excitation.

Intramolecular excimer formation from 1,8-N-alkyldinaphthalimides

Abstract Conjugation of π electrons between imide groups vicinal to arene moieties can drive the photophysical and photochemical properties to a mixed transition of close lying states having n , π ∗ and π,π ∗ characters (Almeida et al., J. Photochem. Photobiol. A: Chem., 58 (1991) 289). Furthermore, the extent of mixing between these states is a function of the molecular geometry (Barros et al., J. Photochem. Photobiol. A: Chem., 76 (1993) 55). New imides were prepared (1,8-N-propyldinaphthalimide (C3) and 1,8-N-butyldinaphthalimide (C4) to investigate the n , π ∗ and π,π ∗ conjugative effects through space. The steady state and time-resolved fluorescence emission of C3 and C4 in solvents of high polarity shows the presence of a new emitting species at longer wavelength. This emission arises from a rotation in the alkyl chain, resulting in an intramolecular energy transfer between the two chromophores during the excited state lifetime of the extended form.

Photodynamic therapy for American cutaneous leishmaniasis: The efficacy of methylene blue in hamsters experimentally infected with Leishmania (Leishmania) amazonensis

The aim of this study was to investigate the effectiveness of Photodynamic Therapy (PDT) using Methylene Blue (MB) as the photosensitizing compound and a Light-Emitting Diode (LED) in American cutaneous leishmaniasis (ACL). Hamsters were experimentally infected with Leishmania (Leishmania) amazonensis. After the development of the lesions in the footpad, the animals were treated with MB three times a week for 3 months. Ten minutes after each application of MB, the lesions were irradiated with LED for 1 h. The lesions were evaluated weekly by the measurement of the hamster footpad thickness. At the end of the treatment the parasitic load was quantified in the regional lymph node of the hamsters. The treatment promoted a decrease in the thickness of infected footpad (P=0.0001) and reduction in the parasitic load in the regional lymph node (P=0.0007) of the animals from group treated with MB+LED. PDT using MB+LED in ACL caused by L. amazonensis shows a strong photodynamic effect. This therapy is very promising, once it is an inexpensive system and the own patient can apply it in their wound and in their house without the need of technical assistance.

Temperature and ionic strength dependent light scattering of DMPG dispersions

The temperature dependence of the intensity of light scattered by aqueous dispersions of the anionic lipid DMPG (dimyristoyl phosphatidylglycerol) was studied at different ionic strengths. The lipid main transition, gel-liquid crystal, is clearly detected by a sharp decrease in light scattering. As expected, the temperature of the main transition (Tm) was found to increase with the increase of the ionic strength. For low ionic strength, a DMPG second temperature transition, termed the ‘post-transition’, can be monitored by both an increase in light scattering and a decrease in conductivity. Zimm plot analysis indicates that below Tm the liposomes tend to aggregate, and show a negative second virial coefficient A2, and particles of large molecular weight. At the main phase transition, parallel to the decrease in light scattering, there is an increase in the sample conductivity, A2 becomes positive and the particle molecular weight decreases, indicating that the vesicles become disaggregated. Moreover, at the post-transition (Tpost) A2 becomes very small, perhaps slightly negative, and the molecular weight increases again. Both the main and the post transitions were found to be reversible. In contrast to the main transition, the post-transition could not be detected by spin labels placed either at the membrane surface, or in the bilayer core. While the mechanism of increased ionization at Tm could be related to the melting of the hydrocarbon chains, the mechanism for the possible counterions recondensation at Tpost is far less clear.

Peptide:lipid ratio and membrane surface charge determine the mechanism of action of the antimicrobial peptide BP100. Conformational and functional studies

The cecropin-melittin hybrid antimicrobial peptide BP100 (H-KKLFKKILKYL-NH2) is selective for Gram-negative bacteria, negatively charged membranes, and weakly hemolytic. We studied BP100 conformational and functional properties upon interaction with large unilamellar vesicles, LUVs, and giant unilamellar vesicles, GUVs, containing variable proportions of phosphatidylcholine (PC) and negatively charged phosphatidylglycerol (PG). CD and NMR spectra showed that upon binding to PG-containing LUVs BP100 acquires α-helical conformation, the helix spanning residues 3-11. Theoretical analyses indicated that the helix is amphipathic and surface-seeking. CD and dynamic light scattering data evinced peptide and/or vesicle aggregation, modulated by peptide:lipid ratio and PG content. BP100 decreased the absolute value of the zeta potential (ζ) of LUVs with low PG contents; for higher PG, binding was analyzed as an ion-exchange process. At high salt, BP100-induced LUVS leakage requires higher peptide concentration, indicating that both electrostatic and hydrophobic interactions contribute to peptide binding. While a gradual release took place at low peptide:lipid ratios, instantaneous loss occurred at high ratios, suggesting vesicle disruption. Optical microscopy of GUVs confirmed BP100-promoted disruption of negatively charged membranes. The mechanism of action of BP100 is determined by both peptide:lipid ratio and negatively charged lipid content. While gradual release results from membrane perturbation by a small number of peptide molecules giving rise to changes in acyl chain packing, lipid clustering (leading to membrane defects), and/or membrane thinning, membrane disruption results from a sequence of events - large-scale peptide and lipid clustering, giving rise to peptide-lipid patches that eventually would leave the membrane in a carpet-like mechanism.

Photophysical properties of N-alkylnaphthalimides and analogs

Abstract Photophysical studies of 1.8 ( I ) and 2.3 ( II ) N-butylnaphthalimides were realized. UV/Vis absorption and fluorescence properties for I and II as a function of solvent polarity and proticity show a higher π orbital interaction between the imide and naphthalene rings for I . A comparison with the spectral data for their respectives anhydrides, amides and carboxylates corroborates this “pattern”. The distinct electronic energy configuration which arises from 1.8 and 2.3 containing conjugated π electrons on the naphthalene ring, is so dramatic that it governs processes spanning from nanoseconds to seconds.

Solvent-induced changes in the photophysical properties of N-alkylphthalimides

Abstract The photophysical properties of N -alkylphthalimides (alkyl being methyl, ethyl, propyl and butyl) are investigated in various solvents. Inversion of excited singlet states of different configurations, resulting in an increase in fluorescence yield, is promoted by hydrogen bonding with the solvent. Although these compounds are non-emissive in aprotic solvents, their fluorescence intensity increases markedly on increase in the medium proticity. Very acidic conditions (concentrated HClO 4 ) result in protonation of the ground state and a decreased fluorescence yield.

Optimization of the ultrasound-assisted synthesis of allyl 1-naphthyl ether using response surface methodology

Allyl 1-naphthyl ethers are useful compounds for different purposes, but reported methods to synthesize them require long reaction times. In this work, we have obtained allyl 1-naphthyl ether in good yield using ultrasonic-assisted methodology in a 1-h reaction. A central composite design was used to obtain a statistical model and a response surface (p<0.05; R(2)=0.970; R(2)(adj)=0.949; R(2)(pred)=0.818) that can predict the optimal conditions to maximize the yield, validated experimentally.

Effect of Anions from the Hofmeister Series and Urea on the Binding of the Charged and Uncharged Forms of the Local Anesthetic Tetracaine to Zwitterionic Micelles

It is widely known that ions in the aqueous phase affect the binding of charged solutes to membranes. Here we report the effect of ions and urea on the interaction of both the charged and uncharged forms of the local anesthetic tetracaine (TTC, an aminoester derivative of ben‐zoic acid) to zwitterionic micelles. Binding was monitored by the increase in TTC fluorescence. Shifts in the emission wavelength maximum (δmax) indicated that the anesthetic was located in an environment of lower polarity. The neutral form of TTC bound to micelles to a larger extent than the protonated form, in agreement with results found for lipid bilayers (Boulanger, Leitch, Schreier and Smith, Can. J. Biochem. 58, 986–995, 1980). When ions from the Hofmeister series and urea were compared in their ability to affect the partitioning of the anesthetic, binding of both the charged and uncharged forms was found to increase upon addition of SO42– and CI− but was seen to decrease in the presence of SCN−, CIO4−‐ and urea. Solubility measurements revealed that the solubility of uncharged TTC increases in solutions containing the additives in the following order: SO42– < CI− < CIO4−‐ < dilute buffer < SCN− < urea. Spin label EPR spectra indicated that, except for CIO4−‐, the ions had little effect on micellar structure. Static light scattering measurements corroborated this result indicating a large increase in micellar molecular weight in the presence of CIO4−‐ and lesser increases for CI.‐ and SCN−. The results show that, besides affecting the binding of ionic species through an electrostatic mechanism, ions also act by altering water structure and, as a consequence, the water solubility and the tendency to partition into the less polar micellar environment of polar charged or uncharged small organic solutes such as the benzoic acid ester derivative. Moreover, evidence suggests that the ions bind directly to the zwitterionic polar groups of the micelles, leading to changes in structure and size.

Long-term aging of vanadium(V) oxide xerogel precursor solutions: structural and electrochemical implications

Abstract Long-term aging of polyvanadic acid precursor solutions gives rise to dramatic effects on the structural and electrochemical characteristics of the corresponding V 2 O 5 · n H 2 O xerogel products. Among these, an 880 mV shift of the redox waves has been observed in the cyclic voltammograms of the modified electrodes, prepared by using 1-year-old precursor solutions. The aging effects have also been evaluated from the systematic changes in the FTIR spectra, especially for the VO and VOV vibrational peaks in the 1130–950 and 900–700 cm −1 range, respectively. Based on light scattering measurements, it has been shown that the polymerization of the polyvanadic acid precursor solutions does not stop even after 1 year. A condensation process takes place in parallel, leading to the formation of aggregates involving approximately four polymeric units. The polymerization and aggregation processes have been correlated with the changes in the X-ray diffraction and EIS patterns, as well as with the changes in the electrochemical behavior of the VXG films before and after Li + ion intercalation.