Covalent and noncovalent hybrids of di-amino porphyrin functionalized graphene oxide and their interaction with gold nanoparticles

Porphyrin functionalization of graphene oxide (GO) influenced the plasmonic effect of gold nanoparticles (AuNP). The former was achieved by modification of GO with 5,10-bis(4-aminophenyl)-15,20-diphenylpor-phyrin, P(NH2)2,adj, by noncovalent interactions as well as by covalent association, following standard chemis-try. The success of the chemical functionalization of GO with P(NH2)2,adj, was confirmed by FTIR. Steady-state and time-resolved fluorescence showed a strong fluorescence quenching of porphyrin in the presence of GO, indicative of a photoinduced electron transfer process from porphyrin units to GO, which acts as an electron acceptor. The surface plasmon coupling effect promoted by the AuNP@GO hybrids, proved to be effective only in the case of the noncovalent hybrid, detected through the decrease of the porphyrin fluorescence lifetime and increase in the emission intensity in solution, in good agreement with FLIM results on deposited samples.info:eu-repo/semantics/publishedVersio

Unfolding kinetics of beta-lactoglobulin induced by surfactant and denaturant: a stopped-flow/fluorescence study

The beta ->alpha transition of beta-lactoglobulin, a globular protein abundant in the milk of several mammals, is investigated in this work. This transition, induced by the cationic surfactant dodecyltrimethylammonium chloride (DTAC), is accompanied by partial unfolding of the protein. In this work, unfolding of bovine beta-lactoglobulin in DTAC is compared with its unfolding induced by the chemical denaturant guanidine hydrochloride (GnHCl). The final protein states attained in the two media have quite different secondary structure: in DTAC the alpha-helical content increases, leading to the so-called alpha-state; in GnHCl the amount of ordered secondary-structure decreases, resulting in a random coil-rich final state (denatured, or D, state). To obtain information on both mechanistic routes, in DTAC and GnHCl, and to characterize intermediates, the kinetics of unfolding were investigated in the two media. Equilibrium and kinetic data show the partial accumulation of an on-pathway intermediate in each unfolding route: in DTAC, an intermediate (I-1) with mostly native secondary structure but loose tertiary structure appears between the native (beta) and alpha-states; in GnHCl, another intermediate (I-2) appears between states beta and D. Kinetic rate constants follow a linear Chevron-plot representation in GnHCl, but show a more complex mechanism in DTAC, which acts like a stronger binding species.info:eu-repo/semantics/publishedVersio

Enhanced fluorescence of a dye on DNA-assembled gold nanodimers discriminated by lifetime correlation spectroscopy

The surface plasmon modes of metal nanoparticles provide a way to efficiently enhance the excitation and emission from a fluorescent dye. We have employed DNA-directed assembly to prepare dimers of gold nanoparticles and used their longitudinally coupled plasmon mode to enhance the fluorescence emission of an organic red-emitting dye, Atto-655. The plasmon-enhanced fluorescence of this dye using dimers of 80 nm particles was measured at the single-molecule detection level. The top enhancement factors were above 1000-fold in 71% of the dimers within a total of 32 dimers measured, and in some cases, they reached almost 4000-fold, in good agreement with model simulations. Additionally, fluorescence lifetime correlation analysis enabled the separation of enhanced from nonenhanced emission simultaneously collected in our confocal detection volume. This approach allowed us to recover a short relaxation component exclusive to enhanced emission that is attributed to the interaction of the dye with DNA in the interparticle gaps. Indeed, the frequency of enhancement events is larger than that expected from the volume occupancy of the gap region, thus suggesting that the interaction of the dye with DNA linkers favors the observation of emission enhancement in our dimer particles

Self-aggregation of free base porphyrins in aqueous solution and in DMPC vesicles

Free base porphyrin (PPhe), derivatized with aminosulfonyl groups linked to the aromatic amino acid phenylalanine at the meso-positions, was mixed with DMPC vesicles. The resulting interaction was studied by absorption, steady-state and transient state fluorescence, at different pHs.http://www.sciencedirect.com/science/article/B6TFB-4R68NKV-2/1/aca6e255f297903981898ccea264a38

Self-association of free base porphyrins with aminoacid substituents in AOT reverse micelles

The aggregation properties of aminosulfonyl porphyrins with aminoacid substituents: methionine with the acid function methylated, PMetCH3; phenylalanine, PPhe; attached to the para positions of the meso phenyls of the porphyrin, was followed in organic solvents, water-dioxane mixtures and in reverse micelles of aerosol OT (AOT RM), using absorption, steady-state and time-resolved fluorescence. In AOT RM in the absence of water, both porphyrins are mainly present as monomers with the Soret band around 420 nm and with emission maxima around 649 and 717 nm. In the presence of water accounted for by parameter [omega]0, [omega]0 = [H2O]/[AOT], the solubility of both porphyrins is enhanced and followed by important spectral changes similar to those observed in the solvent mixtures: new maxima in absorption around 405 nm and in emission around 670 nm. A concomitant increase of the aggregates' contribution is found for PMetCH3, whereas for PPhe the effect is much more pronounced until [omega]0 = 10, above which a de-aggregation process occurs, due to competition between intra- and inter-molecular interactions. The small resonant light scattering signals observed points to the existence of aggregates of small dimensions whose nature is discussed in terms of exciton theory.http://www.sciencedirect.com/science/article/B6TGY-4HM7S0D-1/1/0b4fa2b44d0d626b2e1b99609a937ba