Esther J. Gibbs

Aggregation Kinetics Of Extended Porphyrin And Cyanine Dye Assemblies

The kinetics of J-aggregate formation has been studied for two chromophores, tetrakis-4-sulfonatophenylporphine in an acid medium and pseudoisocyanine on a polyvinylsulfonate template. The assembly processes differ both in their sensitivity to initiation protocols and in the reaction profiles they produce. The porphyrins assembly kinetics, for example, displays an induction period unlike that of the cyanine dye. Two kinetic models are presented. For the porphyrin, an autocatalytic pathway in which the formation of an aggregation nucleus is rate-determining appears to be applicable; for the pseudoisocyanine dye, an equation derived for diffusion-limited aggregation of a fractal object satisfactorily fits the data. These models are shown to be useful for the analysis of kinetic data obtained for several biologically important aggregation processes.

Depolarized Resonance Light Scattering by Porphyrin and Chlorophyll a Aggregates

A quantum mechanical model is developed for the observed resonance enhancement of light scattering by aggregates of electronically interacting chromophores. Aggregate size, monomer oscillator strength, extent of electronic coupling, and aggregate geometry are all important determinants of intensity in resonance light scattering (RLS) spectra. The theory also predicts the value of the depolarization ratio (rho(v)(90)) of RLS for a given aggregate geometry. These results are used to interpret the RLS depolarization ratios of four aggregates: tetrakis(4-sulfonatophenyl)porphine aggregated at low pH (rho(v)(90) = 0.17 at 488 nm), trans-bis(N-methylpyridinium-4-yl)-diphenylporphinato copper(II) aggregated in 0.2 M NaCl solution (rho(v)(90) = 0.13 at 450 nm) and on calf thymus DNA (rho(v)(90) = 0.20 at 454 nm), and chlorophyll a aggregates in formamide/water (rho(v)(90) = 0.23 and 0.32 at 469 and 699 nm, respectively). The analysis is consistent with a J-aggregate geometry for all four systems. Furthermore, the specific values of rho(v)(90) allow us to estimate the orientation of the monomer transition dipoles with respect to the long axis of the aggregate. We conclude that depolarized resonance light scattering spectroscopy is a powerful probe of the geometric and electronic structures of extended aggregates of strong chromophores.

Interactions Of Porphyrins With Purified DNA And More Highly Organized Structures

Studies of the solution properties of gold(III)tetrakis(4-N-methylpyridyl) porphine and its DNA binding characteristics have been conducted utilizing uv/vis absorption spectroscopy, circular dichroism (CD), Mossbauer spectroscopy, and temperature-jump relaxation techniques. These studies indicate that over the concentration range considered this water soluble gold(III) porphyrin does not aggregate, binds axial ligands only weakly with a preference for soft Lewis bases, and is capable of intercalation into nucleic acids of appropriate base pair content. The interaction of this and several other porphyrins with the synthetic polynucleotide poly(dA-dC).poly(dT-dG) has been studied. Spectroscopic signatures for intercalation were found for those derivatives not having axial ligands. Intercalation into chromatin in vitro can also occur with those porphyrins and metalloporphyrins which do not have axial ligands. Finally, studies utilizing microinjection techniques indicate that once within the cell, tetrakis(4-N-methylpyridyl)porphine tends to localize in the nucleus.

Porphyrin induced Z to B conversion of poly(dG-dC)2 in ethanol.

The water soluble porphyrins H2TMpyP-2, H2TMpyP-4, and CuTMpyP-4 are found to bind to Z-form poly(dG-dC)2 in 60% ethanol (v/v) and to facilitate the conversion of the polymer to the B form. Metalloporphyrins with axial ligands (MnTMpyP-4, ZnTMpyP-4) interact to some degree with the Z form, but do not lead to extensive conversion to the B form. The conversion of the Z form into the B form was determined by CD titration experiments, which were used to quantitate the fraction of poly(dG-dC)2 present in each conformation. Under all conditions each bound porphyrin molecule converts multiple base pairs from Z to B. The kinetics of porphyrin reactions with Z-poly(dG-dC)2 in 60% ethanol were measured using two different detection techniques. Stopped flow spectrophotometry was used to observe the time-dependent spectral changes associated with the porphyrins during the reaction. Time-dependent changes in the poly(dG-dC)2 conformation were observed directly using CD. The porphyrin absorbance changes under the conditions of these experiments have a much shorter half time (t1/2 approximately 0.1 to 2 sec) than the CD changes (t1/2 approximately 10 sec). Thus it could be determined that a complex with spectral characteristics similar to those of the porphyrin intercalated into B-form poly(dG-dC)2 is produced while the polymer is predominantly in the Z form.

Interactions of porphyrins with nucleotides and nucleic acids

Abstract A number of features of porphyrin and metalloporphyrin interactions with nucleic acids are discussed. The role of base pair opening in porphyrin intercalation, the influence of ionic strength on base specificity and the basis for zinc porphyrin/DNA relaxation effects are explored. In addition, preliminary results dealing with the interactions of porphyrins and metalloporphyrins with mononucleotides are presented. For porphyrin derivatives having no axial ligands a stacking type interaction seems likely, but a second type of interaction, perhaps involving hydrogen bonding, may be important with iron(III) and cobalt(III) porphyrin derivatives.

Probing the Mechanism of Insulin Aggregation with Added Metalloporphyrins

The mechanism of inhibition of insulin-based amyloid gel formation by metal derivatives of tetrakis(4-sulfonatophenyl)porphyrin has been investigated. Time-course UV/vis measurements in conjunction with atomic force microscopy (AFM) were used to study the correlation between observed kinetics and amyloid structure for various concentration ranges of added metalloporphyrins. Observed structures include fibrils as well as circular, ring-like structures formed as a result of the interaction of insulin with porphyrin. In addition, binding studies demonstrate that the effectiveness of inhibition of the various metalloporphyrins is directly related to the strength of binding to insulin. It is suggested that both the electron distribution in the porphyrin core and the tendency to form porphyrin dimers affect both the structure of amyloid formed and the kinetic profile of the reaction.

On the kinetics of formation of hemozoin, the malaria pigment.

We report on the kinetics of formation of hemozoin as a function of hemin concentration and reaction medium. Evidence is presented for the critical role played by interfacial regions in the efficient conversion of hemin to the malaria pigment.

Influence of DNA on the rate of porphyrin metallation

Poly(dG–dC)2 and poly(dA–dT)2 have marked influences on the rate of insertion of copper(II) into cationic porphyrins reflecting the interaction mode of the porphyrin with the nucleic acid.