The dynamics of GATG glycodendrimers by NMR diffusion and quantitative (13)C relaxation

NMR Studies of Two Lysine Based Dendrimers with Insertion of Similar Histidine-Arginine and Arginine-Histidine Spacers Having Different Properties for Application in Drug Delivery

In this paper we study two lysine-based peptide dendrimers with Lys-His-Arg and Lys-Arg-His repeating units and terminal lysine groups. Combination of His and Arg properties in a dendrimer could be important for biomedical applications, especially for prevention of dendrimer aggregation and for penetration of dendrimers through various cell membranes. We describe the synthesis of these dendrimers and the confirmation of their structure using 1D and 2D Nuclear Magnetic Resonance (NMR) spectroscopy. NMR spectroscopy and relaxation are used to study the structural and dynamic properties of these macromolecules and to compare them with properties of previously studied dendrimers with Lys-2Arg and Lys-2His repeating units. Our results demonstrate that both Lys-His-Arg and Lys-Arg-His dendrimers have pH sensitive conformation and dynamics. However, properties of Lys-His-Arg at normal pH are more similar to those of the more hydrophobic Lys-2His dendrimer, which has tendency towards aggregation, while the Lys-Arg-His dendrimer is more hydrophilic. Thus, the conformation with the same amino acid composition of Lys-His-Arg is more pH sensitive than Lys-Arg-His, while the presence of Arg groups undoubtedly increases its hydrophilicity compared to Lys-2His. Hence, the Lys-His-Arg dendrimer could be a more suitable (in comparison with Lys-2His and Lys-Arg-His) candidate as a pH sensitive nanocontainer for drug delivery.

The self-diffusion of polymethylsilsesquioxane (PMSSO) dendrimers in diluted solutions and melts

Recently developed non-functional derivatives of polymethylsilsesquioxane (PMSSO) dendrimers of the first to fifth generation were characterized by 1H, 13C and 29Si NMR spectroscopy. The self-diffusion and NMR relaxation of PMSSO dendrimers in dilute solutions of toluene and melts were investigated in a wide temperature range (-50-80 °C). The hydrodynamic radii of dendrimers were determined from the self-diffusion coefficients measured in diluted solutions according to the Stokes-Einstein equation. The hydrodynamic radius of PMSSO dendrimers as a function of molecular mass follows a power law with the scaling exponent of 0.32 ± 0.02 in the investigated temperature range. The temperature dependences of the self-diffusion coefficients of dendrimers were described by the Arrhenius-type equation. The activation energies of self-diffusion of dendrimers in diluted toluene solutions are identical for different generations while the dependence of activation energy for dendrimers in melts shows a maximum for the third generation (G3) dendrimer. Taking into account the absence of specific interactions in PMSSO dendrimer melts the observed behavior was ascribed to the manifestation of interpenetration of dendrimer molecules. For low generations (G1 and G2) the short length of the branches does not considerably affect the translational diffusion while for higher generations (G4 and G5) the densification of the structure prevents significant interpenetration.

NMR studies of excluded volume interactions in peptide dendrimers

Peptide dendrimers are good candidates for diverse biomedical applications due to their biocompatibility and low toxicity. The local orientational mobility of groups with different radial localization inside dendrimers is important characteristic for drug and gene delivery, synthesis of nanoparticles, and other specific purposes. In this paper we focus on the validation of two theoretical assumptions for dendrimers: (i) independence of NMR relaxations on excluded volume effects and (ii) similarity of mobilities of side and terminal segments of dendrimers. For this purpose we study 1H NMR spin-lattice relaxation time, T1H, of two similar peptide dendrimers of the second generation, with and without side fragments in their inner segments. Temperature dependences of 1/T1H in the temperature range from 283 to 343 K were measured for inner and terminal groups of the dendrimers dissolved in deuterated water. We have shown that the 1/T1H temperature dependences of inner groups for both dendrimers (with and without side fragments) practically coincide despite different densities of atoms inside these dendrimers. This result confirms the first theoretical assumption. The second assumption is confirmed by the 1/T1H temperature dependences of terminal groups which are similar for both dendrimers.

NMR relaxation of the orientation of single segments in semiflexible dendrimers

We study the orientational properties of labeled segments in semiflexible dendrimers making use of the viscoelastic approach of Dolgushev and Blumen [J. Chem. Phys. 131, 044905 (2009)]. We focus on the segmental orientational autocorrelation functions (ACFs), which are fundamental for the frequency-dependent spin-lattice relaxation times T1(ω). We show that semiflexibility leads to an increase of the contribution of large-scale motions to the ACF. This fact influences the position of the maxima of the [1/T1]-functions. Thus, going from outer to inner segments, the maxima shift to lower frequencies. Remarkably, this feature is not obtained in the classical bead-spring model of flexible dendrimers, although many experiments on dendrimers manifest such a behavior.

In situ nanofabrication of hybrid PEG-dendritic-inorganic nanoparticles and preliminary evaluation of their biocompatibility

An in situ template fabrication of inorganic nanoparticles using carboxylated PEG-dendritic block copolymers of the GATG family is described as a function of the dendritic block generation, the metal (Au, CdSe) and metal molar ratio. The biocompatibility of the generated nanoparticles analysed in terms of their aggregation in physiological media, cytotoxicity and uptake by macrophages relates to the PEG density of the surface of the hybrids.

Molecular dynamics simulation of spin–lattice NMR relaxation in poly-l-lysine dendrimers: manifestation of the semiflexibility effect

Spin–lattice NMR relaxation rate vs. reversed temperature for terminal and inner CH₂ groups of n = 2 and 4 generation poly-l-lysine dendrimers.

Investigation of glycofullerene dynamics by NMR spectroscopy

Mannose residues linked to flexible spacers on slowly diffusing glycofullerenes may facilitate efficient rebinding to receptors.

GATG dendrimers and PEGylated block copolymers: from synthesis to bioapplications

Dendrimers are synthetic macromolecules composed of repetitive layers of branching units that emerge from a central core. They are characterized by a tunable size and precise number of peripheral groups which determine their physicochemical properties and function. Their high multivalency, functional surface, and globular architecture with diameters in the nanometer scale makes them ideal candidates for a wide range of applications. Gallic acid-triethylene glycol (GATG) dendrimers have attracted our attention as a promising platform in the biomedical field because of their high tunability and versatility. The presence of terminal azides in GATG dendrimers and poly(ethylene glycol) (PEG)-dendritic block copolymers allows their efficient functionalization with a variety of ligands of biomedical relevance including anionic and cationic groups, carbohydrates, peptides, or imaging agents. The resulting functionalized dendrimers have found application in drug and gene delivery, as antiviral agents and for the treatment of neurodegenerative diseases, in diagnosis and as tools to study multivalent carbohydrate recognition and dendrimer dynamics. Herein, we present an account on the preparation and recent applications of GATG dendrimers in these fields.

Stepwise filtering of the internal layers of dendrimers by transverse-relaxation-edited NMR

The characteristic distribution of transverse relaxation times (T2) within dendrimers (shorter values at the core than the periphery) can be exploited in T2-edited 1D and 2D NMR experiments for the stepwise filtering of internal nuclei according to their topology within the dendritic structure. The resulting filtered spectra, which can be conceived as corresponding to virtual hollow dendrimers, benefit from reduced signal overlap, thus facilitating signal assignment and characterization. The generality of the method as a powerful tool in structural and end-group analysis has been confirmed with various dendritic families and nuclei ((1)H, (13)C, (31)P).

Multivalent glycoconjugate syntheses and applications using aromatic scaffolds

Glycan-protein interactions are of utmost importance in several biological phenomena. Although the variety of carbohydrate residues in mammalian cells is limited to less than a dozen different sugars, their spatial topographical presentation in what is now associated as the "glycocodes" provides the fundamental keys for specific and high affinity "lock-in" recognition events associated with a wide range of pathologies. Toward deciphering our understanding of these glycocodes, chemists have developed new creative tools that included dendrimer chemistry in order to provide monodisperse multivalent glycoconjugates. This review provides a survey of the numerous aromatic architectures generated for the multivalent presentation of relevant carbohydrates using covalent attachment or supramolecular self-assemblies. The basic concepts toward their controlled syntheses will be described using modern synthetic procedures with a particular emphasis on powerful organometallic methodologies. The large variety of dendritic aromatic scaffolds, together with a brief survey of their unique biophysical and biological properties will be critically reviewed. The distinctiveness of the resulting multivalent glycoarchitectures, encompassing glycoclusters, glycodendrimers and molecularly defined self-assemblies, in forming well organized cross-linked lattices with multivalent carbohydrate binding proteins (lectins) together with their photophysical, medical, and imaging properties will also be briefly highlighted. The topic will be presented in increasing order of aromatic backbone complexities and will end with fullerenes together with self-assembled nanostructures, thus complementing the various scaffolds described in this special thematic issue dedicated to multivalent glycoscience.

Exploring the efficiency of gallic acid-based dendrimers and their block copolymers with PEG as gene carriers

The synthesis of a new family of amino-functionalized gallic acid-triethylene glycol (GATG) dendrimers and their block copolymers with polyethylene glycol (PEG) has recently being disclosed. In addition, these dendrimers have shown potential for gene delivery applications, as they efficiently complex nucleic acids and form small and homogeneous dendriplexes. On this basis, the present study aimed to explore the interaction of the engineered dendriplexes with blood components, as well as their stability, cytotoxicity and ability to enter and transfect mammalian cells. Results show that GATG dendrimers can form stable dendriplexes, protect the associated pDNA from degradation, and are biocompatible with HEK-293T cells and erythrocytes. More importantly, dendriplexes are effectively internalized by HEK-293T cells, which are successfully transfected. Besides, PEGylation has a marked influence on the properties of the resulting dendriplexes. While PEGylated GATG dendrimers have improved biocompatibility, the long PEG chains limit their uptake by HEK-293T cells, and thus, their ability to transfect them. As a consequence, the degree of PEGylation in dendriplexes containing dendrimer/block copolymer mixtures emerges as an important parameter to be modulated in order to obtain an optimized stealth formulation able to effectively induce the expression of the encoded protein.

Original submitted 29 November 2011; Revised submitted 8 March 2012; Published online 20 July 2012

Peripheral functionalization of dendrimers regulates internalization and intracellular trafficking in living cells

GATG (gallic acid-triethylene glycol) dendrimers represent appealing nanostructures for biomedical applications. The incorporation of specific ligands and targeting and imaging agents on their surface has resulted in promising tools in diagnosis and drug delivery. With the aim to further explore the versatility of GATG dendrimers in the biomedical field, in this work we study the effect of peripheral substitution on their uptake and intracellular trafficking in living cells. To this end, peripheral groups with different physicochemical properties and biological relevance have been installed on the surface of GATG dendrimers, and their interactions, uptake efficacy, and specificity for certain cell populations studied by confocal microscopy. Finally, this information was used to design a pH-sensitive drug delivery system for the selective release of cargo molecules inside cells after lysosomal localization. These results along with the easy functionalization and modular architecture of GATG dendrimers reveal these systems as promising nanotools in biomedicine.

Average relaxation time of internal spectrum for carbosilane dendrimers: nuclear magnetic resonance studies

A new theoretical description of the interior mobility of carbosilane dendrimers has been tested. Experiments were conducted using measurements of the (1)H NMR spin-lattice relaxation time, T(1H), of two-, three- and four-generation carbosilane dendrimers with three different types of terminal groups in dilute chloroform solutions. Temperature dependences of the NMR relaxation rate, 1/T(1H), were obtained for the internal CH(2)-groups of the dendrimers in the range of 1/T(1H) maximum, allowing us to directly evaluate the average time of the internal spectrum for each dendrimer. It was found that the temperature of 1/T(1H) maximum is practically independent of the number of generations, G; therefore, the theoretical prediction was confirmed experimentally. In addition, the average time of the internal spectrum of carbosilane dendrimers was found to be near 0.2 ns at room temperature, and this value correlates well with the values previously obtained for other dendrimer structures using other experimental techniques.