Bhaskar G. Maiya

Studies of third-order optical nonlinearity and nonlinear absorption in tetra tolyl porphyrins using degenerate four wave mixing and Z-scan

We present our experimental results on the measurement of third-order optical nonlinearity in the ns and ps domain, in several Tetra Tolyl Porphyrin molecules (TTP), using degenerate four wave mixing (DFWM) and Z-Scan techniques. Our results indicate a very high value of nonlinearity for these molecules in the ns domain and reasonably high values in the ps domain. They are found to exhibit a strong nonlinear absorption at both 532 nm and 600 nm. The high value of nonlinearity for ns pulses is attributed to higher excited singlet and triplet states. Time-resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in these molecules.

Metallotexaphyrins: a new family of photosensitisers for efficient generation of singlet oxygen

The photophysical properties of a new series of tripyrroledimethine-derived ‘expanded porphyrins’(‘texaphyrins’) are reported; these compounds show strong low energy optical absorptions in the 730–770 nm spectral range as well as a high triplet quantum yield, and act as efficient photosensitizers for the production of singlet oxygen in methanol solution.

Cobalt(III), nickel(II) and ruthenium(II) complexes of 1,10-phenanthroline family of ligands: DNA binding and photocleavage studies

DNA binding and photocleavage characteristics of a series of mixed-ligand complexes of the type [M(phen)2LL]n+ (where M = Co(III), Ni(II) or Ru(II), LL = 1,10-phenanthroline (phen), phenanthroline-dione (phen-dione) or dipyridophenazine (dppz) andn = 3 or 2) have been investigated in detail. Various physico-chemical and biochemical techniques including UV/Visible, fluorescence and viscometric titration, thermal denaturation, and differential pulse voltammetry have been employed to probe the details of DNA binding by these complexes; intrinsic binding constants (Kb) have been estimated under a similar set of experimental conditions. Analysis of the results suggests that intercalative ability of the coordinated ligands varies as dppz>phen>phen-dione in this series of complexes. While the Co(II) and Ru(II) complexes investigated in this study effect photocleavage of the supercoiled pBR 322 DNA, the corresponding Ni(II) complexes are found to be inactive under similar experimental conditions. Results of detailed investigations carried out inquiring into the mechanistic aspects of DNA photocleavage by [Co(phen)2(dppz)]3+ have also been reported.

Binding of Porphyrins by the Tumor-Specific Lectin, Jacalin (Jack Fruit (Artocarpus integrifolia) Agglutinin)

Jacalin (Artocarpus integrifolia agglutinin) specifically recognizes thetumor-associated T-antigenic disaccharide structure,Galβ13GalNAc. Porphyrins and their derivatives are currently used asphotosensitizers in photodynamic therapy to treat malignant tumors. In thisstudy, the interaction of several free base porphyrins and their metalderivatives with jacalin is investigated by absorption and fluorescencespectroscopy. Each lectin subunit was found to bind one porphyrin moleculeand the association constants were estimated to be in the range of2.4×103M−1 to 1.3×105M−1 at room temperaturefor the interaction of different porphyrins with jacalin. These values arein the same range as those obtained for the interaction of monosaccharidesto jacalin. Both free lectin and lectin saturated with the specificsaccharide were found to bind different porphyrins with comparable bindingstrength indicating that porphyrin binding takes place at a site differentfrom the sugar binding site. Further, both anionic and cationic porphyrinswere found to interact with the lectin with comparable affinity, clearlyindicating that the charge on the porphyrin does not play any role in thebinding process and that most likely the interaction is mediated byhydrophobic forces. These results suggest that jacalin and other lectins maypotentially be useful for targeted delivery of porphyrins to tumor tissuesin photodynamic therapy.

Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin.

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.

A supramolecular array assembled via the complementary binding properties of ruthenium(II) and tin(IV) porphyrins

A new porphyrin trimer has been self-assembled by employing the mutually non-interfering coordination properties of the ruthenium(II) and tin(IV) centers to form a multi-metal array. The photo- and electro-chemical properties of this array are also reported.

Fluorescence studies on a supramolecular porphyrin bearing anthracene donor moieties

A tetraphenylporhyrin bearing four anthracene donor moieties (5,10,15,20-tetra(3-(9-methyloxyanthracenyl)phenyl)porphyrin, I) was synthesized and fully characterized. Spectroscopic and electrochemical data and results of metallation and intermolecular fluorescence quenching experiments suggest that there is no appreciable interaction between the porphyrin and anthracene moieties in I. Although intermolecular quenching of anthracene fluorescence by 5,10,15,20-(tetraphenyl)porphyrin is not obvious, the fluorescence of the anthracene donors in I is quenched by more than 90%. On the basis of the fluorescence excitation pectral data, the quenching observed is attributed to intramolecular singlet-singlet energy transfer from anthracene to the porphyrin. However, neither Forsters dipole-dipole formalism nor Dexters electron exchange mechanism can explain adequately this intramolecular energy transfer. Thermodynamic considerations and solvent-dependent fluorescence data indicate that photoinduced electron transfer from singlet anthracene to the porphyrin can compete with energy transfer in this supramolecular system.

In vitro PHOTODYNAMIC INACTIVATION OF Herpes simplex VIRUS WITH SAPPHYRINS: 22 π-ELECTRON PORPHYRIN-LIKE MACROCYCLES

Abstract— The photodynamic inactivation of HSV‐1, a virus having a membranous envelope, with both a decaalkyl sapphyrin and its dicarboxy‐substituted analog was studied. The decaalkyl sapphyrin was as efficient in the inactivation of HSV‐1 on a per macrocycle basis as DHE, whereas the efficiency of the dicarboxy‐substituted sapphyrin was approximately two orders of magnitude less. Fluorescence studies of sapphyrins binding to liposomes and VSV suggested that the decaalkylsapphyrin bound monomerically to cholesterol‐rich regions of the viral envelope, whereas its charged analog localized in a more polar environment.

Functional Equality in the Absence of Structural Similarity AN ADDED DIMENSION TO MOLECULAR MIMICRY

The crystal structure ofmeso-tetrasulfonatophenylporphyrin complexed with concanavalin A (ConA) was determined at 1.9 Å resolution. Comparison of this structure with that of ConA bound to methyl α-d-mannopyranoside provided direct structural evidence of molecular mimicry in the context of ligand receptor binding. The sulfonatophenyl group of meso-tetrasulfonatophenylporphyrin occupies the same binding site on ConA as that of methyl α-d-mannopyranoside, a natural ligand. A pair of stacked porphyrin molecules stabilizes the crystal structure by end-to-end cross-linking with ConA resulting in a network similar to that observed upon agglutination of cells by lectins. The porphyrin binds to ConA predominantly through hydrogen bonds and water-mediated interactions. The sandwiched water molecules in the complex play a cementing role, facilitating favorable binding of porphyrin. Seven of the eight hydrogen bonds observed between methyl α-d-mannopyranoside and ConA are mimicked by the sulfonatophenyl group of porphyrin after incorporating two water molecules. Thus, the similarity in chemical interactions was manifested in terms of functional mimicry despite the obvious structural dissimilarity between the sugar and the porphyrin.

Excited state dynamics in tetra tolyl porphyrins studied using degenerate four wave mixing with incoherent light and ps pulses

Abstract We present our experimental results on the excited state dynamics in several tetra tolyl porphyrins (with different central metal-ions) studied using degenerate four wave mixing with nanosecond incoherent light (DFWM-IL) and picosecond time-resolved four wave mixing (DFWM-PS). Incoherent light studies indicate three lifetimes, which have been attributed to different excited states. The shortest component with a decay constant of S n ( T 2 ) , 3–6 ps component is due to the vibrational relaxation of the Franck-Condon states in S 1 ( τ vib ) , and the long component of 20–100 ps is due to the population relaxation ( T 1 ) from S 1 to S 0 . The population relaxation times obtained from time-resolved DFWM studies using a picosecond laser substantiates these results.