Absorption flattening as one cause of distortion of circular dichroism spectra of Delta-RuPhen(3) . H(2)TPPS complex

To extend the model that explains why and how much absorption flattening (AF) influences circular dichroism (CD) signals, we have investigated the interesting case of exciton CD in the Soret region of a noncovalent complex formed by (DRuPhen3) 21 and the tetraanionic porphyrin H2TPPS. Different concentrations have been studied by using an AF emulator and spectra simulation. The CD spectra of this compound occasionally show distortions in the solution sampling mode with the increase of concentration; the inhomogeneous distribution in the cell volume is due to aggregation and is the source of the AF effect. On the basis of these results, we conclude that AF is an important cause of distortions in CD spectra for D-RuPhen3 H2TPPS complexes and might affect the CD bands of other aggregated systems as wel

Supramolecular structure of extrinsically chiral porphyrin hetero-assemblies and achiral analogues

Supramolecular structures of porphyrin hetero-assemblies and the achiral analogous are determined through energy dispersive X-ray diffraction. In presence of phenylalanine, CuT4 cationic and H2TTPS anionic porphyrins yield a suprachiral alpha-helix persisting even after removing the chiral template. The analogous synthesis performed without the template-imprinting agent provides an amorphous arrangement, hence an achiral structure

Single-Stranded Nucleic Acids As Templates For Porphyrin Assembly Formation

Resonance light scattering (RLS) and circular dichroism (CD) spectroscopies have been used to investigate the interactions of several cationic porphyrins with poly(dA) and poly(rA). Neither tetrakis(N-methylpyridinium-4-yl)porphin nor its zinc(II) derivative (H₂T4 and ZnT4) show any tendency to form extended porphyrin assemblies in the presence of poly(dA). However, the poly(dA) complex of CuT4 involves considerable porphyrin self-stacking. In the presence of poly(rA), only ZnT4 of these three porphyrin derivatives, fails to assemble. Differences in the interactions of trans-bis(N-methyl-pyridinium-4-yl)diphenylporphine (t-H₂Pagg) with these two single-stranded nucleic acid polymers are described. Whereas the porphyrin is capable of forming extended assemblies with either poly(dA) or poly(rA), increasing the salt concentration in the latter system results in a reversal of the induced circular dichroism spectrum in the Soret region indicating a conformational change of the porphyrin assembly

Kinetic investigation of porphyrin interaction with chiral templates reveals unexpected features of the induction and self-propagation mechanism of chiral memory

Kinetics of the porphyrin aggregation leading to a chiral memory system shows a remarkable "catalytic" effect of the noncovalent templates explaining their self-replication ability

Hierarchical self-assembly of water-soluble porphyrins

Hierarchy is a powerful and promising tool to rationally synthesize non-covalent complex species. In this paper we show two different examples of hierarchically driven aggregation processes allowing for the control of aggregation state or "shape" of the final species. In the first case we propose that water-soluble porphyrins presenting various protonation steps can follow kinetic routes alternative to the thermodynamic pathway; namely for each protonation step is allowed, in principle, an alternative kinetically driven homo-self-aggregation process. Our results indicate that the thermodynamic route leads to a monomeric, protonated final state, but the kinetic pathway ends with self-assemblies of the title porphyrin. The bias between the two final states can be easily pre-determined by going quickly or slowly throughout the thermodynamic-kinetic junction. The second case deals with the so-called "chiral memory" phenomenon. Also in this case the role of the kinetic control over the assembly process is remarkable. In particular, we have observed that self-aggregation of opposite charged, achiral porphyrins does not lead to induced chirality even if a chiral template is added after their aggregation. The final aggregates are kinetically inert, mostly because stabilized by a network of electrostatic interactions between net charges. However, if the two supramolecular components are mixed in the presence of a chiral template, then an induced circular dichroism signal (ICD) appears in the Soret region (the main absorption feature of porphyrins in the visible region whose maximum and intensity are strongly affected by the aggregation state). Removal of the template does not significantly affect the intensity of the ICD: the inert aggregates have memorized the chiral template shape. Remarkably, these species - now inherently chiral - act as very efficient templates for self-aggregation of additional non-chiral porphyrins. © 2004 Elsevier B.V. All rights reserved

Insights into cytochrome c-cardiolipin interaction. Role played by ionic strength

The finding that cytochrome c (cyt c) plays a role in programmed cell death after its release from the mitochondrion has recently renewed interest in this protein. The structural changes in cytochrome c observed at early stages of the apoptotic process have been related to changes occurring in the protein when it forms a complex with phospholipid vesicles; Among the lipids constituting the membrane, cardiolipin is the one thought to bind to cyt c. In this paper, we have investigated the influence exerted by ionic strength on cytochrome c-cardiolipin interaction and found that formation of the cytochrome c-cardiolipin complex occurs via two distinct transitions, implying a high-affinity site and a low-affinity site. Ionic strength significantly influences complex stability; sodium chloride dissociates the complex through two distinct transitions, the second of which occurs at a very high anion concentration. ATP also dissociates the complex, but under the conditions that were investigated, its action is limited to the high-affinity site. The dissociation process is characterized by a very slow kinetic rate constant (k(obs) = 4.2 x 10(-3) s(-1)) and requires several minutes to be completed. We ascribe it to the high activation barrier met by the protein when restoring the native Fe(III)-M80 axial bond. The peroxidase activity shown by cardiolipin-bound cytochrome c is indicative of a less packed protein tertiary conformation in the complex. In line with earlier reports, these data highlight the manifold functions of cytochrome c besides the well-known role it plays in oxidative phosphorylation, shedding more light on the properties of the cytochrome c-cardiolipin complex, involved in the progression of early stages of apoptosis