Helical Conjugated Polyelectrolyte Aggregation Induced by Biotin-Avidin Interaction

Intercalation of Alkynylplatinum(II) Terpyridine Complexes into a Helical Poly(phenylene ethynylene) Sulfonate: Application to Protein Sensing

The interactions of two anionic poly(phenylene ethynylene) sulfonate-conjugated polyelectrolytes (mPPESO₃^- and pPPESO₃^-) with two alkynylplatinum(II) terpyridine complexes (Pt²⁺ and Pt³⁺) were studied. The Pt(II) complexes interact with helical mPPESO₃^- by intercalation within the polymer helix to form a "guest-host" ensemble. Titration of Pt(II) complexes into an aqueous solution of mPPESO₃^- gives rise to efficient quenching of the polymer's fluorescence; meanwhile, triplet metal-metal-to-ligand charge transfer (³MMLCT) state emission from the intercalated Pt(II) complexes appears when the ensembles are excited into the polymer's absorption band. The ³MMLCT state emission implies that the Pt(II) complexes aggregate or dimerize on the mPPESO₃^- scaffold. The responses of the mPPESO₃^- and Pt(II) complex ensembles to various proteins were examined by monitoring the mPPESO₃^- fluorescence change. Negatively charged proteins recover the mPPESO₃^- fluorescence more than the positively charged proteins under physiological pH, indicating that electrostatics play an important role in the protein-ensemble interaction.

Pyrophosphate Sensor Based on Principal Component Analysis of Conjugated Polyelectrolyte Fluorescence

The pyrophosphate anion (PPi) plays an important role in biochemical processes. Therefore, a simple but reliable analytical technique is essential for selective detection of PPi in biochemical systems. Here, we present a principal component analysis (PCA) method for analytical determination of PPi concentration using a fluorescent conjugated polyelectrolyte (CPE) combined with a polyamine modifier. The CPE has anionic side chains and dissolves molecularly in water, as indicated by its structured fluorescence emission spectrum. However, addition of tris(3-aminoethyl)amine (tetraamine or N4) quenches the CPE fluorescence emission. Tetraamine, which is a polycation at neutral pH, binds multiple anionic CPE chains, leading to aggregate formation, resulting in aggregation-induced fluorescence quenching. Addition of PPi to the polymer-amine aggregate reverses the process, resulting in fluorescence recovery. The relatively higher concentration of PPi compared to that of the polymer allows it to effectively compete to bind the amine, thus releasing molecularly dissolved polymer chains. Fluorescence correlation spectroscopy of the P1/N4 complex and of P1/N4/PPi confirms the change in size of the CPE aggregates that occurs upon reversible aggregation. Application of PCA to the fluorescence emission data set of standard samples yields two principal components, which are used to create a predictive model for PPi analysis. The PCA method is able to directly determine the concentration of PPi with approximately 95% accuracy within the concentration range from 100 μM to 3 mM, without the need for a reference state as is typically needed for ratiometric fluorescence assays.

Chiral random grain boundary phase of achiral hockey-stick liquid crystals

A disordered chiral conglomerate, the random grain boundary (RGB) phase, has been observed below the smectic A liquid crystal phase of an achiral, hockey-stick molecule. In cells, the RGB phase appears dark between crossed polarizers but decrossing the polarizers reveals large left- and right-handed chiral domains with opposite optical rotation. Freeze-fracture transmission electron microscopy reveals that the RGB phase is an assembly of randomly oriented blocks of smectic layers, an arrangement that distinguishes the RGB from the dark, chiral conglomerate phases of bent-core mesogens. X-ray diffraction indicates that there is significant layer shrinkage at the SmA-RGB phase transition, which is marked by the collapse of layers with long-range order into small, randomly oriented smectic blocks.

Protein induced aggregation of conjugated polyelectrolytes probed with fluorescence correlation spectroscopy: application to protein identification

The interaction of a series of water-soluble conjugated polyelectrolytes with varying backbone structure, charge type (cationic and anionic), and charge density with a set of seven different proteins is explored by using fluorescence correlation spectroscopy (FCS). The FCS method affords the diffusion time for a particular CPE/protein pair, and this diffusion time is a reflection of the aggregation state of the polymer/protein in the solution. The diffusion time is larger for oppositely charged CPE/protein combinations, reflecting the tendency toward the formation of CPE/protein aggregates in these systems. However, by careful analysis of the data, other factors emerge, including possible effects of hydrophobic interaction in specific CPE/protein systems. The final diffusion time for each CPE/protein mixture varies and the diffusion time response pattern created by the six-CPE array for a typical protein is unique, and this effect was leveraged to develop a sensor array for protein identification by using linear-discriminant analysis (LDA) methods. By application of multimode linear discrimination analysis, the unknown protein samples have been successfully identified with a total accuracy of 93%.