Dynamic dendrimer at electrified interface: potential dependent adsorption-desorption and reorientation of a 4-pyridyl-modified PAMAM dendrimer

Potential-dependent dynamic adsorption-desorption and reorientation of a 4-pyridyl modified PAMAM G2 dendrimer at a Au(111) electrode has been demonstrated.     

PAMAM dendrimer interactions with supported lipid bilayers: a kinetic and mechanistic investigation

The interaction kinetics of polyamidoamine (PAMAM) dendrimers with supported lipid bilayers of 1,2-sn-glycero-dimyristoylphosphocholine prepared by the vesicle deposition has been probed by optical waveguide lightmode spectroscopy and atomic force microscopy (AFM). In particular, the influence of PAMAM dendrimer generation (G2, G4, and G6) and concentration (1 to 100 nM) on the levels of adsorption and lipid bilayer removal have been determined as a function of time; hence interaction kinetics and mechanisms have been further elucidated. Dendrimer interaction kinetics with the lipid bilayer are concentration dependent in a complex manner, with net bilayer removal at 1 and 100 nM and net adsorption at 10 nM; these effects are irrespective of dendrimer generation. The pseudo first order rate constant for bilayer removal (at 1 and 100 nM) follows the order G6 > G4 > G2. In contrast, the pseudo first order rate constant for adsorption at 10 nM follows the order G2 > G4 > G6. AFM has confirmed expansion of lipid bilayer defects, hole formation, and adsorption to the bilayer or bilayer defects, and their concentration and generation dependence. These findings have implications when designing dendrimers for specific biopharmaceutical activities, e.g., as drugs, drug delivery vehicles, transfection agents, or antimicrobials.     

Novel charge-transfer complexes of 4-hexylamino-1,8-naphthalimide-labelled PAMAM dendrimer with some acceptors: a spectrophotometric study

Poly(amidoamine) dendrimers are very interesting macromolecules with highly branched structures and globular-shaped branched polymeric architectures. They are widely used for drug and gene delivery applications. In order to provide important insight into the interactions of poly(amidoamine) dendrimers with some organic acceptors, the binding of small molecules to 4-hexylamino-1,8-naphthalimide-labelled PAMAM dendrimer (PD) have been studied by spectrophotomeric method. The acceptors used in this research include chloranilic acid (CLA), p-chloranil (CHL), 2,6-dichloroquinone-4-chloroimide (DCQ), 2,6-dibromoquinone-4-chloroimide (DBQ), 7,7?,8,8?-tetracyanoquinodimethane (TCNQ), picric acid (PA), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and iodine monobromide (IBr). The spectrophotometric measurements proved that all the charge-transfer (CT) complexes are formed via a stoichiometry (PD: acceptor) of 1:2 (except for IBr acceptor). Accordingly the obtained complexes could be formulated as [(PD)(CLA)₂], [(PD)(DCQ)₂], [(PD)(DBQ)₂], [(PD)(TCNQ)₂], [(PD)(PA)₂], [(PD)(CHL)₂], [(PD)(DDQ)₂] and [(PD)(IBr)₄]. Benesi–Hildebrand and its modification methods were applied to estimate the spectroscopic and physical data.     

Synthesis and catalytic activity of gold-silver binary nanoparticles stabilized by PAMAM dendrimer

Gold-silver binary nanoparticles, which feed atomic ratios of gold to silver were 3:1, 1:1, and 1:3, were prepared. These particles were stabilized by amine-terminated (generation (G) 3.0 and 5.0) and carboxyl-terminated (G 3.5 and 5.5) poly(amidoamine) (PAMAM) dendrimers in water. UV-vis spectra indicate that the particles are not mere physical mixtures of monometallic particles or core/shell type but alloy. According to transmission electron microscope (TEM) observation, the mean diameters of the particles were 7-10 nm for silver particles and 3-4 nm for both gold and alloy particles, respectively. Catalytic activities for reduction of p-nitrophenol were investigated by monitoring the absorbance at 400 nm during the reaction. They were proportional to the feed ratio of gold in the particles and showed a maximum at the ratio of Au:Ag=3:1.     

Interaction between PAMAM 4.5 dendrimer, cadmium and bovine serum albumin: a study using equilibrium dialysis, isothermal titration calorimetry, zeta-potential and fluorescence

Binding of Cd²⁺ by PAMAM 4.5 dendrimer was studied by equilibrium dialysis, isothermal titration calorimetry and zeta-potential measurement. The following binding parameters were obtained: n = 23.8 ± 9.5, K_b = 4.7 ± 0.9 × 10³ in water; and n = 41.3 ± 13.4, K_b = 2.1 ± 0.8 × 10³ in 0.15 mol/l phosphate-buffered saline. The location of the bound Cd²⁺ is discussed. The interactions between bovine serum albumin, PAMAM 4.5 dendrimer and cadmium were analyzed using fluorescence and equilibrium dialysis. The competition between Cd²⁺ binding to BSA and PAMAM 4.5 dendrimer was investigated. It is proposed that PAMAM 4.5 dendrimer could be successfully used for extracting Cd²⁺ from aqueous solutions (environmental protection).     

Synthesis of monofunctional curcumin derivatives, clicked curcumin dimer, and a PAMAM dendrimer curcumin conjugate for therapeutic applications

Curcumin, the primary active ingredient in the spice turmeric, was converted to reactive monofunctional derivatives (carboxylic acid/azide/alkyne). The derivatives were employed to produce a 3 + 2 azide-alkyne "clicked" curcumin dimer and a poly(amidoamine) (PAMAM) dendrimer-curcumin conjugate. The monofunctional curcumin derivatives retain biological activity and are efficient for labeling and dissolving amyloid fibrils. The curcumin dimer selectively destroys human neurotumor cells. The synthetic methodology developed affords a general strategy for attaching curcumin to various macromolecular scaffolds.     

Boron nitride nanocluster as a carrier for lomustine anticancer drug delivery: DFT and thermodynamics studies

Monatshefte für Chemie - Chemical Monthly - Previous reports were shown that boron nitride nanostructures can be biocompatible and nontoxic. Therefore, interaction of lomustine as an...     

Sensor activity, photodegradation and photostabilisation of a PAMAM dendrimer comprising 1,8-naphthalimide functional groups in its periphery

The colouristic and fluorescent characteristics of a new composite material based on a PAMAM dendrimer of second generation whose periphery is modified with 4-N,N-dimethylaminoethylamino-1,8-naphthalimide and polyamide-6 have been investigated. This dendrimer has been investigated with regard to its application as a heterogenic sensor capable of detecting metal cations and protons in aqueous solutions. In the presence of metal cations (Ni²⁺, Fe²⁺, Fe³⁺ and Co²⁺) and protons the fluorescence intensity of the composite increases due to their coordination with dendrimer molecule. The results obtained reveal the capacity of this system to act as a sensitive sensor of environmental pollution by metal cations and protons. It has been shown that in N,N-dimethylformamide solution the metal cations inhibit the processes of photodegradation of the dendrimer.     

Solvent quality changes the structure of G8 PAMAM dendrimer, a disagreement with some experimental interpretations

We have performed approximately 20-40 ns of molecular dynamics (MD) simulations for the generation 8 PAMAM dendrimer in explicit water under varying pH conditions to study the structure of the dendrimer (approximately 156,738 atoms at low pH). This is the first report of such a long MD simulation of a larger generation PAMAM dendrimer including the effect of salt and counterions with explicit water molecules. We find that changing the pH from a high value (approximately 12) to a low value (approximately 3) changes the radius of gyration from Rg = 37.8 to 43.1 A (increasing by 13%). We also find significant back-folding of the primary amines and a large amount of water penetration inside the polymer. The increase in size with decrease in pH is consistent with our earlier studies on G3-G6 and agrees with the Monte Carlo theory by Welch and Muthukumar of G8 (Macromolecules, 1998, 31, 5892) and the experiments on G5 and G8 PAMAM dendrimer by Topp et al. (Macromolecules, 1999, 32, 7232). However, these results disagree dramatically with the interpretations of SANS experiments of G8 PAMAM dendrimers by Nisato et al. (Macromolecules, 2000, 33, 4172) who observe no change in the size of the dendrimer with variations of solution pH and ionic strength. We assume that the disagreement might arise from neglecting nonspherical shape, penetration of water and ions into the core, and aggregation, all of which might depend on pH.     

PAMAM dendrimer composite membrane for CO2 separation: Formation of a chitosan gutter layer

A poly(amidoamine) (PAMAM) dendrimer composite membrane with an excellent CO₂/N₂ separation factor was developed in-situ. The In-situ Modification (IM) method was used to modify the surface of commercial porous membranes, such as ultrafiltration membranes, to produce a gas selective layer by controlling the interface precipitation of the membrane materials in the state of a received membrane module. Using the IM method, a chitosan layer was prepared on the inner surface of a commercially available ultrafiltration membrane as a gutter layer, in order to affix PAMAM dendrimer molecules on the porous substrate. After chitosan treatment, the PAMAM dendrimer was impregnated into the gutter layer to form a PAMAM/chitosan hybrid layer. The CO₂ separation performance of the resulting composite membrane was tested at a pressure difference of 100 kPa and a temperature of 40 °C, using a mixed CO₂ (5 vol%)/N₂ (95 vol%) feed gas. The PAMAM dendrimer composite membrane, with a gutter layer prepared from ethylene glycol diglycidyl ether and a 0.5 wt% chitosan solution of two different molecular weight chitosans, revealed an excellent CO₂/N₂ separation factor and a CO₂ permeance of 400 and 1.6 × 10⁻⁷ m³ (STP) m⁻² s⁻¹ kPa⁻¹, respectively. SEM observations revealed a defect-free chitosan layer (thickness 200 nm) positioned directly beneath the top surface of the UF membrane substrate. After PAMAM dendrimer treatment, the hybrid chitosan/PAMAM dendrimer layer was observed with a thickness of 300 nm. XPS analysis indicated that the hybrid layer contained about 20–40% PAMAM dendrimer.     

Characterization by fourier transform infrared spectroscopy (FT-IR) and 2D IR correlation spectroscopy of PAMAM dendrimer

FT-IR and 2D correlation spectroscopy were employed to study the microstructural changes occurring during phase transitions of a liquid crystal poly(amidoamine) codendrimer (PAMAM (L1)16(L2)16) generation 3, functionalized on the terminal groups by one-chain promesogenic calamitic units (4-(4'-decyloxybenzoyloxy)salicylaldehyde (L1)) and two-chain promesogenic calamitic units (4-(3',4'-didecyloxybenzoyloxy)salicylaldehyde (L2)). Spectral modifications associated with molecular conformation rearrangements allowing for molecular shape change on going from a liquid-crystalline organization to another were found. The transition temperatures were calculated, and they are in good agreement with the DSC data. Spectral analysis gives evidence of the LC phase transitions and to an additional transition associated with the existence of conformers. Various types of hydrogen bonding have been established.     

Activation of leinamycin by thiols: a theoretical study

Reaction of thiols with the 1,2-dithiolan-3-one 1-oxide heterocycle found in leinamycin (1) results in the conversion of this antitumor antibiotic to a DNA-alkylating episulfonium ion (5). While the products formed in this reaction have been rationalized by a mechanism involving initial attack of thiol on the central sulfenyl sulfur (S2') of the 1,2-dithiolan-3-one 1-oxide ring, the carbonyl carbon (C3') and the sulfinyl sulfur (S1') of this heterocycle are also expected to be electrophilic. Therefore, it is important to consider whether nucleophilic attack of thiol at these sites might contribute either to destruction of the antibiotic or conversion to its episulfonium ion form. To address this question, we have used computational methods to examine the attack of methyl thiolate on each of the three electrophilic centers in a simple analogue of the 1,2-dithiolan-3-one 1-oxide heterocycle found in leinamycin. Calculations were performed at the MP2/6-311+G(3df,p)//B3LYP/6-31G level of theory with inclusion of solvent effects. The results indicate that the most reasonable mechanism for thiol-mediated activation of leinamycin involves initial attack of thiolate at the S2'-position of the antibiotic's 1,2-dithiolan-3-one 1-oxide heterocycle, followed by conversion to the 1,2-oxathiolan-5-one intermediate (3).     

Phosphinic derivative of DTPA conjugated to a G5 PAMAM dendrimer: an 17O and 1H relaxation study of its Gd(III) complex

A DTPA-based chelate containing one phosphinate group was conjugated to a generation 5 polyamidoamine (PAMAM) dendrimer via a benzylthiourea linkage. The Gd(III) complex of this novel conjugate has potential as a contrast agent for magnetic resonance imaging (MRI). The chelates bind Gd3+via three nitrogen atoms, four carboxylates and one phosphinate oxygen, and one water molecule completes the inner coordination sphere. The monomer Gd(III) chelates bearing nitrobenzyl and aminobenzyl groups ([Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-) as well as the dendrimeric Gd(III) complex G5-(Gd(DTTAP))63) were studied by multiple-field, variable temperature 17O and 1H NMR. The rate of water exchange is faster than that of [Gd(DTPA)(H2O)]2- and very similar on the two monomeric complexes (8.9 and 8.3 x 10(6) s-1 for [Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-, respectively), while it is decreased on the dendrimeric conjugate (5.0 x 10(6) s-1). The Gd(III) complex of the dendrimer conjugate has a relaxivity of 26.8 mM-1 s-1 at 37 degrees C and 0.47 T (corresponding to 1H Larmor frequency of 20 MHz). Given the contribution of the second sphere water molecules to the overall relaxivity, this value is slightly higher than those reported for similar size dendrimers. The experimental 17O and 1H NMR data were fitted to the Solomon-Bloembergen-Morgan equations extended with a contribution from second coordination sphere water molecules. The rotational dynamics of the dendrimeric conjugate was described in terms of global and local motions with the Lipari-Szabo approach.     

Oxidation of CO by SO2: a theoretical study

The elementary reaction SO(2) + CO --> CO(2) + SO((3)Sigma) (1) and the subsequent reaction SO((3)Sigma) + CO --> CO(2) + S((3)P) (2) have been studied by the application of the Gaussian-3//B3LYP quantum chemical approach to characterize the potential energy surfaces and transition state kinetic analysis to derive rate coefficients. Reaction 1 is found to take place via two transition states (TS), a cis-OSOCO TS and a trans-OSOCO TS. Reaction via the cis-TS is concerted and takes place on a singlet surface. Intersystem crossing to the final products occurs after passage through the barrier on the singlet surface. The trans-TS leads to a very weakly bound singlet OSOCO intermediate that then passes through a second TS (on the triplet surface) to form the products. Reaction 2 takes place on triplet surfaces. There is a concerted reaction through a cis-SOCO TS and a weakly bound trans-SOCO has also been identified. Reaction 2 is analogous to the reaction CO + O(2)((3)Sigma) --> CO(2) + O((3)P) (3), and this reaction has been reinvestigated at a similar level of theory and the rate coefficient derived by quantum chemistry is compared with experiment. The sensitive effects of trace impurities such as H(2), H(2)O, and hydrocarbons on the accurate experimental determination of the rate coefficient of reaction 3 is discussed. Using rate coefficients for reactions 1 and 2 obtained via quantum chemical calculations, we have been unable to model the extent of decomposition of SO(2) measured in a shock tube study of reaction between SO(2) and CO [Bauer, S. H.; Jeffers, P.; Lifshitz, A.; Yadava, B. P. Proc. Combust. Inst. 1971, 13, 417]. In light of the known sensitivity of reaction 3 to trace impurities, we have incorporated trace amounts of H(2), CH(4), or H(2)O, together with our rate coefficients for (1) and (2), in a kinetic model of Alzueta et al. [Combust. Flame 2001, 127, 2234], which is then shown to be able to substantially model the SO(2) data of Bauer et al. In the course of this modeling study we also computed heats of formation for a number of sulfur-containing small molecules: HS, HSO, HSOH, HOSO, HS(2), HSO(2), HOSO(2), HOSOH, and HOSHO.     

IR, Raman and theoretical ab initio RHF study of aminoglutethimide — an anticancer drug

A comparative analysis of the IR and Raman spectra of aminoglutethimide (AG) dissolved in CCl₄, CHCl₃ and CH₃CN was performed. Most of the absorption bands were assigned to characteristic group vibrations with the use of the IR and Raman spectra of deuterated AG, glutethimide, N-methyl glutethimide and glutarimide. The AG samples very weakly interacting with the environment were studied with the use of the Ar matrix isolation IR spectra. For comparison, the IR and Raman spectra of the crystalline samples formed by hydrogen-bonded AG molecules were recorded. The spectra were analyzed also in terms of normal modes and the harmonic approximation with the use of the ab initio restricted Hartree-Fock theory. It was found that increasing the solute concentration in CCl₄ and CHCl₃ leads to formation of the autoassociates. In CH₃CN the solute–solvent AG–CH₃CN dimers occur. Possible structures of the associates were theoretically studied on the model systems: the centrosymmetric glutarimide dimer and the linear AG–CH₃CN dimer. By a comparison of the theoretical and experimental spectra we were able to identify several peaks originating from the solute–solvent interactions.     

Preparation, characterization and photoelectrochemical study of mixed C60-Starburst PAMAM G0.0 dendrimer films anchored on the surface of nanocrystalline TiO2 semiconductor electrodes

Nanocrystalline TiO2 silanized electrodes were prepared and further modified in a sequential fashion with C60 and Starburst PAMAM G0.0 dendrimers, resulting in a novel photoelectrochemical sensitization film that showed particularly high photocurrent (IPCE) and global photoconversion efficiencies (eta).     

Hydrogen bonding, solvation, and hydrolysis of cisplatin: a theoretical study

Density functional calculations on a range of hydrogen bonded clusters of cisplatin are reported. A systematic search of 1:1 cisplatin:water complexes reveals only three stable minima, which contain a number of common, recurring interactions, such as an N-H...O-H...Cl bridging mode. Expanding these clusters by adding water molecules leads to a model of the first solvation shell of cisplatin, which contains the above motifs along with several strong water-water interactions. The strengths of such interactions are rationalized on the basis of electrostatic potentials, and quantified by use of Atoms in Molecules properties. This analysis also allows us to estimate cisplatin's position on Abraham's hydrogen bond acidity and basicity scales, indicating that cisplatin is a strong donor and acceptor of hydrogen bonds due to the dominance of hard, electrostatic interactions. The effects of this explicit solvation on the barrier to hydrolysis, and hence activation, of cisplatin are explored, indicating a slightly higher barrier than in the gas phase, leading to better agreement with experiment than either gas phase or continuum solvation calculations.     

N-nitrosation of amines by NO2 and NO: a theoretical study

Gas-phase nitrosation of amines implies a nonionic pathway different from the nitrosonium nitrosation via acidification of nitrite. Electronic structure calculations discussed in this work suggest a free radical mechanism, in which NO2 abstracts a hydrogen atom from the nitrogen in primary and secondary amines to form an intermediate complex of an aminyl radical and nitrous acid. The aminyl radical intermediate is then quenched by nitric oxide, leading to the formation of nitrosamine. High-level calculations (CBS-QB3) show that alkyl substitutions on amines can activate the H-abstraction reaction. Thus, while H-abstraction from NH3 was found to exhibit a reaction barrier (DeltaH) of 106 kJ/mol, similar calculations indicate that the corresponding barriers decrease to 72 and 45 kJ/mol for methylamine and dimethylamine, respectively. Heterocyclic secondary amines have also been investigated in a similar manner. The five-membered-ring (5-m-r) amine appears to be the most reactive: pyrrolidine (DeltaH=30 kJ/mol), azetidine (DeltaH=44 kJ/mol), piperidine (DeltaH=44 kJ/mol), and aziridine (DeltaH=74 kJ/mol). The reaction barrier for 1H-pyrrole, an aromatic 5-m-r secondary amine, was found to be 59 kJ/mol. The origin of the high activity for the 5-m-r alkylamine stems from a hydrogen-bond-like interaction between the aminyl radical and the nascent nitrous acid molecule. This theoretical study suggests that, in the presence of nitrogen oxides, the gas-phase nitrosation of secondary amines is feasible.