Carboxylate-terminated Poly Amidoamine Research Articles

Reagent Anions for Charge Inversion of Polypeptide/Protein Cations in the Gas Phase

Various reagent anions capable of converting polypeptide cations to anions via ion/ion reactions have been investigated. The major charge inversion reaction channels include multiple proton transfer and adduct formation. Dianions composed of sulfonate groups as the negative charge carriers show essentially exclusive adduct formation in converting protonated peptides and proteins to anions. Dianions composed of carboxylate groups, on the other hand, show far more charge inversion via multiple proton transfer, with the degree of adduct formation dependent upon both the size of the polypeptide and the spacings between carboxylate groups in the dianion. More highly charged carboxylate-containing anions, such as those derived from carboxylate-terminated polyamidoamine half-generation dendrimers show charge inversion to give anion charges as high in magnitude as -4, with the degree of adduct formation being inversely related to dendrimer generation. All observations can be interpreted on the basis of charge inversion taking place via a long-lived chemical complex. The lifetime of this complex is related to the strengths and numbers of the interactions of the reactants in the complex. Calculations with model systems are fully consistent with sulfonate groups giving rise to more stable complexes. The kinetic stability of the complex can also be affected by the presence of electrostatic repulsion if it is multiply charged. In general, this situation destabilizes the complex and reduces the likelihood for observation of adducts. The findings highlight the characteristics of multiply charged anions that play roles in determining the nature of charge inversion products associated with protonated peptides and proteins.

Intrinsic Fluorescence of Carboxylate-Terminated Polyamido Amine Dendrimers

The “intrinsic” fluorescence of carboxylate-terminated polyamido amine (PAMAM-CT) dendrimers is studied by two fluorescence techniques—excitation-emission matrices (EEMs) and lifetimes. The EEMs show similar spectral profiles for all dendrimer generations (a broad peak with an excitation and emission maximum of 380 and 440 nm, respectively) and an overall increase in relative fluorescence emission with increasing generation. Three distinct, fairly discrete lifetimes are also recovered. The shortest lifetime (0.2–0.4 ns) is a background signal from the solvent. The two longer lifetimes (1.3–2.5 and 4.2–7.1 ns) are attributed to the dendrimer. There is a general shift to longer lifetimes in τ2 and τ3, with increasing generation (G n). Both lifetimes nearly double in magnitude, going from G2.5 to G7.5, which is indicative of a specific fluorescent component being in a more protected or constrained microenvironment. These results imply that the dendrimer becomes densely packed with increasing generation. The weak, but detectable, fluorescence is most likely due to an n → π transition from the amido groups throughout the dendritic structure. Even though the exact nature of PAMAM-CT fluorescence is not fully understood, it is clear that this property shows the unique aspects of the architecture of these dendrimers and can be utilized in their characterization.

Spectroscopic Investigations of Polyamido Amine Starburst Dendrimers with Reichardt's ET-30 Dye

The association between Reichardt's ET-30 dye and amine- and carboxylate-terminated polyamido amine (PAMAM) dendrimers was investigated and shown to be different from results reported for micelles and traditional polymers. The absorption spectra illustrated that the solvatochromic, microenvironmental polarity probe associated with PAMAM dendrimers in less polar regions with a polarity comparable to 1-decanol. Model compounds that mimic PAMAMs' surface groups and branching moieties were used to better define the associated dye's location and demonstrated that ET-30 penetrated beyond the dendrimer's surface groups, into the interfacial region. Additionally, changing sample preparation methods controlled the nature and extent of the dye/dendrimer association. Under certain experimental conditions, the dye/dendrimer complexes formed large aggregates.

Spectrochemical investigations in dendritic media: evaluation of nitromethane as a selective fluorescence quenching agent in aqueous carboxylate-terminated polyamido amine (PAMAM) dendrimers

Alternant and nonalternant polycyclic aromatic hydrocarbons (PAHs) are employed to compare carboxylate-terminated polyamido amine (PAMAM) dendrimers to typical anionic micelles. Nitromethane is a known, selective, quenching agent of alternant PAHs. However, recent studies by Acree and co-workers have found that nitromethane will also quench the fluorescence emission intensity of nonalternant PAHs in the presence of anionic surfactants above the critical micelle concentration. The quenching of alternant and nonalternant PAHs by nitromethane is used to compare dendritic ‘unimolecular micelles’ to traditional micelles. Experimental results indicate that the PAHs’ association with these dendrimers does not appear to occur in the ‘palisade’ region, as seen in traditional micelles, but rather deeper within the dendrimer structure. Solvent polarity probe studies and quenching in the absence of nitromethane also support this conclusion. Due to the fact that the PAHs do not reside near the negatively charged surface groups, the nitromethane selective quenching rule is obeyed in carboxylate-terminated PAMAM dendrimers (10 mM in surface groups).