Fractal aggregation of dyes such as porphyrins and related compounds under stacking

Changes in Optical Properties upon Dye-Clay Interaction: Experimental Evaluation and Applications

The development of hybrid materials with unique optical properties has been a challenge for the creation of high-performance composites. The improved photophysical and photochemical properties observed when fluorophores interact with clay minerals, as well as the accessibility and easy handling of such natural materials, make these nanocomposites attractive for designing novel optical hybrid materials. Here, we present a method of promoting this interaction by conjugating dyes with chitosan. The fluorescent properties of conjugated dye–montmorillonite (MMT) hybrids were similar to those of free dye–MMT hybrids. Moreover, we analyzed the relationship between the changes in optical properties of the dye interacting with clay and its structure and defined the physical and chemical mechanisms that take place upon dye–MMT interactions leading to the optical changes. Conjugation to chitosan additionally ensures stable adsorption on clay nanoplatelets due to the strong electrostatic interaction between chitosan and clay. This work thus provides a method to facilitate the design of solid-state hybrid nanomaterials relevant for potential applications in bioimaging, sensing and optical purposes.

Inhibitory effect of melittin on endonuclease-like activity of centrin

The xeroderma pigmentosum group C protein (XPC) and centrin2 are the primary initiators of global genome nucleotide excision repair (NER). Centrin, acts as a member of the EF-hand super family of calcium-binding proteins, playing roles in reconstitution of the vitro NER reaction. To understand the possible molecular and structural properties of the multiprotein process, the interactions of Euplotes octocarinatus centrin (EoCen), melittin, and DNA are described. EoCen shares a sequence identity of 66% with centrin2. Melittin possesses inverse direction hydrophobic triads-leucine-leucine-tryptophan (LLW) which are responsible for centrin binding. It is applied as a natural peptide to mimic centrin target peptide. As a result, it is proved that the integrated protein shows an endonuclease-like activity to DNA. Melittin is capable of interaction with both EoCen and DNA. More importantly, it is found that melittin displays an inhibitory effect on the endonuclease-like activity of centrin when it co-exists with EoCen and DNA in solution. Meanwhile, the DNA-melittin-EoCen ternary complex forms in the process. Quantitative analyses demonstrated by extensive biophysical assays reveal that binding of the peptide to DNA or centrin modulates the binding properties of it to another component. Furthermore, a possible positioning model of DNA and EoCen on melittin is proposed. This finding may constitute a model for that existing between centrin and its target peptide in NER process.

Modulation effect of double strand DNA on the self-assembly of N-terminal domain of Euplotes octocarinatus centrin

Centrin is a member of the EF-hand super family of calcium-binding proteins, which can behave as a part of damage detector initiated the initiation of nucleotide excision repair (NER). Its self-assembly plays a causative role in fiber contraction associated with the cell division cycle and ciliogenesis. To explore the possible role of DNA in the process of centrin self-assembly, the aggregation properties of N-terminal domain of Euplotes octocarinatus centrin (N-EoCen) in the presence of DNA with or without metal ions are investigated. It is verified that metal ions, such as Ca²⁺ and Tb³⁺, can bind to N-EoCen with 2:1 stoichiometry by isothermal titration calorimetry (ITC). Importantly, this study reports that double strand DNA (dsDNA) is capable of binding N-EoCen, changing conformation of protein and modulating centrin aggregation, as demonstrated by extensive biophysical assays. Interestingly, the open conformation of protein induced by metal ions may be favour of the interaction of protein with dsDNA. Nevertheless, the randomly coiled single strand DNA (ssDNA) is completely inefficient to the aggregation regulation. Furthermore, results reveal that hydrophobic site could play important role in the process. This finding may link to the potent roles of centrin in the NER process.

Modulation of XPC peptide on binding Tb3+ to Euplotes octocarinatus centrin

Centrins are Ca²⁺-binding proteins found throughout eukaryotic organisms. Xeroderma pigmentosum group C protein (XPC), a dominant component of the nuclear excision repair (NER) pathway, is a critical target protein of centrins. A 22-residue peptide (K842-R863) from XPC was used to investigate the effect of metal ions (Ca²⁺ and Tb³⁺) on the peptide binding of Euplotes octocarinatus centrin (EoCen) by isothermal titration calorimetry (ITC) and fluorescence spectroscopy. ITC and tryptophan spectrofluorimetric titrations revealed that metal ions (Ca²⁺ and Tb³⁺) could enhance the affinity between EoCen and the XPC peptide, and the enhanced effects were closely related to the ion potential of metal ions. Since the ion potential of Tb³⁺ (e/r = 0.0325) is larger than that of Ca²⁺ (e/r = 0.0202), the conformational change in the protein induced by Tb³⁺ is larger than that induced by Ca²⁺, and the enhanced affinity of Tb³⁺ is stronger than that of Ca²⁺. This interaction was driven by enthalpy in the presence of EDTA and enthalpy and entropy in the presence of Ca²⁺ or Tb³⁺. Similar to that observed in the presence of EDTA, the N-terminal domain did not participate in the interaction with the XPC peptide even in the presence of metal ions. Resonance light scattering (RLS) and the band shift in native polyacrylamide gel electrophoresis (PAGE) suggested that peptide binding resulted in the dissociation of EoCen aggregates and complex formation via the monomer-peptide form. Tb³⁺-Sensitized emission suggested that peptide binding in turn also had an impact on the Tb³⁺ binding of the protein: the C-terminal domain was slightly strengthened and the N-terminal domain was weakened about 225 fold. RLS and native PAGE indicated that the self-assembly induced by Tb³⁺ binding to the N-terminal domain of EoCen was inhibited in the presence of the XPC peptide. This study elucidates the molecular mechanism of EoCen function in the cellular context.

Role of four conserved aspartic acid residues of EF-loops in the metal ion binding and in the self-assembly of ciliate Euplotes octocarinatus centrin

Ciliate Euplotes octocarinatus centrin (EoCen) is an EF-hand calcium-binding protein closely related to the prototypical calcium sensor protein calmodulin. Four mutants (D37K, D73K, D110K and D146K) were created firstly to elucidate the importance of the first aspartic acid residues (Asp37, Asp73, Asp110 and Asp146) in the beginning of the four EF-loops of EoCen. Aromatic-sensitized Tb³⁺ fluorescence indicates that the aspartic acid residues are very important for the metal-binding of EoCen, except for Asp73 (in EF-loop II). Resonance light scattering (RLS) measurements for different metal ions (Ca²⁺ and Tb³⁺) binding proteins suggest that the order of four conserved aspartic acid residues for contributing to the self-assembly of EoCen is Asp37 > Asp146 > Asp110 > Asp73. Cross-linking experiment also exhibits that Asp37 and Asp146 play critical role in the self-assembly of EoCen. Asp37, in site I, which is located in the N-terminal domain, plays the most important role in the metal ion-dependent self-assembly of EoCen, and there is cooperativity between N-terminal and C-terminal domain (especially the site IV). In addition, the dependence of Tb³⁺ induced self-assembly of EoCen and the mutants on various factors, including ionic strength and pH, were characterized using RLS. Finally, 2-p-toluidinylnaphthalene-6-sulfonate (TNS) binding, ionic strength and pH control experiments indicate that in the process of EoCen self-assembly, molecular interactions are mediated by both electrostatic and hydrophobic forces, and the hydrophobic interaction has the important status.

Centrin: another target of monastrol, an inhibitor of mitotic spindle

Monastrol, a cell-permeable inhibitor, considered to specifically inhibit kinesin Eg5, can cause mitotic arrest and monopolar spindle formation, thus exhibiting antitumor properties. Centrin, a ubiquitous protein associated with centrosome, plays a critical role in centrosome duplication. Moreover, a correlation between centrosome amplification and cancer has been reported. In this study, it is proposed for the first time that centrin may be another target of the anticancer drug monastrol since monastrol can effectively inhibit not only the growth of the transformed Escherichia coli cells in vivo, but also the Lu(3+)-dependent self-assembly of EoCen in vitro. The two closely related compounds (Compounds 1 and 2) could not take the same effect. Fluorescence titration experiments suggest that four monastrols per protein is the optimum binding pattern, and the binding constants at different temperatures were obtained. Detailed thermodynamic analysis indicates that hydrophobic force is the main acting force between monastrol and centrin, and the extent of monastrol inhibition of centrin self-assembly is highly dependent upon the hydrophobic region of the protein, which is largely exposed by the binding of metal ions.

Quantitative investigations of thermal and photoinduced J- and H-aggregation of hydrophobic spirooxazines in binary solvent through UV/vis spectroscopy

Photo- and chemo-stimulable architectures.

Study of spectral behaviour and optical properties of cis/trans-bis(N-methylpyridinium-4-yl)diphenyl porphyrin adsorbed on layered silicates

The spectral properties of two dicationic porphyrins, cis- and trans-bis(N-methylpyridinium-4-yl)diphenyl porphyrins, upon adsorption on clay mineral templates were investigated. A series of reduced charge Nanocor montmorillonites was used as host templates. The main impact was given on the influence of the layer charge on the absorption and fluorescence spectra. Adsorption of the porphyrins led to significant changes in their spectra but preserved the photoactivity of the dyes. The changes can be attributed to two phenomena: structural changes (flattening) of the porphyrin molecules and molecular aggregation. Latter one was more influenced by the molecular structure of the dyes and influenced the properties of mainly trans-isomer. The formation of the assemblies was significantly influenced by the layer charge of montmorillonite template. This study presents the example how one is able to influence the spectral properties and possibly also the functionality of adsorbed photoactive molecules via a strategy of appropriate combination of a photoactive component with specific inorganic templates.

J-Aggregation of Anionic Ethyl meso-Thiacarbocyanine Dyes Induced by Binding to Proteins

The effects of ribonuclease A (RNase), lysozyme, trypsin, and bovine serum albumin (BSA) on the J-aggregation behavior of 3,3'-bis[sulfopropyl]-5-methoxy-4',5'-benzo-9-ethylthiacarbocyanine (1), 3,3'-bis[sulfopropyl]-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine (2), and 3,3'-bis[sulfopropyl]-5,5'-dimethoxy-9-ethylthiacarbocyanine (3) were studied in aqueous solution. The formation of J-aggregates at pH 6 is induced by RNase for 1-3, by lysozyme for 1 and 2, and by trypsin for 2. The formation of J-aggregates correlates with decay of the dimers and is supported by induced circular dichroism spectra. The concentration of J-aggregates for lysozyme/1 increases with an increase in the protein/dye concentration ratio, reaches a plateau, and then gradually decreases. J-aggregates are characterized by relatively weak fluorescence; e.g., Phi(f) = 0.01 for lysozyme/1, and by a small Stokes shift of 6-8 nm, indicating almost resonance fluorescence. J-aggregation proceeds in the range of seconds to minutes with sigmoidal type kinetic curves for trypsin/2 and nonsigmoidal kinetics in the other cases. The presence of BSA, in contrast to RNase, lysozyme, and trypsin, results in deaggregation of dimers of 1-3 and formation of bound monomers and exhibits intense fluorescence from the trans-monomer; e.g., Phi(f) = 0.22 for BSA/1. Generally, the binding of 1-3 to the proteins is a cooperative process, where the number of binding sites changes from n = 15 for lysozyme/1 to n = 6 for trypsin/2 and n = 0.3 and 1 for BSA/3.