Glycosyl Oxocarbenium Ions: Structure, Conformation, Reactivity, and Interactions.

Carbohydrates (glycans, saccharides, and sugars) are essential molecules in all domains of life. Research on glycoscience spans from chemistry to biomedicine, including material science and biotechnology. Access to pure and well-defined complex glycans using synthetic methods depends on the success of the employed glycosylation reaction. In most cases, the mechanism of the glycosylation reaction is believed to involve the oxocarbenium ion. Understanding the structure, conformation, reactivity, and interactions of this glycosyl cation is essential to predict the outcome of the reaction. In this Account, building on our contributions on this topic, we discuss the theoretical and experimental approaches that have been employed to decipher the key features of glycosyl cations, from their structures to their interactions and reactivity.We also highlight that, from a chemical perspective, the glycosylation reaction can be described as a continuum, from unimolecular SN1 with naked oxocarbenium cations as intermediates to bimolecular SN2-type mechanisms, which involve the key role of counterions and donors. All these factors should be considered and are discussed herein. The importance of dissociative mechanisms (involving contact ion pairs, solvent-separated ion pairs, solvent-equilibrated ion pairs) with bimolecular features in most reactions is also highlighted.The role of theoretical calculations to predict the conformation, dynamics, and reactivity of the oxocarbenium ion is also discussed, highlighting the advances in this field that now allow access to the conformational preferences of a variety of oxocarbenium ions and their reactivities under SN1-like conditions.Specifically, the ground-breaking use of superacids to generate these cations is emphasized, since it has permitted characterization of the structure and conformation of a variety of glycosyl oxocarbenium ions in superacid solution by NMR spectroscopy.We also pay special attention to the reactivity of these glycosyl ions, which depends on the conditions, including the counterions, the possible intra- or intermolecular participation of functional groups that may stabilize the cation and the chemical nature of the acceptor, either weak or strong nucleophile. We discuss recent investigations from different experimental perspectives, which identified the involved ionic intermediates, estimating their lifetimes and reactivities and studying their interactions with other molecules. In this context, we also emphasize the relationship between the chemical methods that can be employed to modulate the sensitivity of glycosyl cations and the way in which glycosyl modifying enzymes (glycosyl hydrolases and transferases) build and cleave glycosidic linkages in nature. This comparison provides inspiration on the use of molecules that regulate the stability and reactivity of glycosyl cations.

Modular Design of Supramolecular Ionic Peptides with Cell-Selective Membrane Activity.

The rational design of materials with cell-selective membrane activity is an effective strategy for the development of targeted molecular imaging and therapy. Here we report a new class of cationic multidomain peptides (MDPs) that can undergo enzyme-mediated molecular transformation followed by supramolecular assembly to form nanofibers in which cationic clusters are presented on a rigid ß-sheet backbone. This structural transformation, which is induced by cells overexpressing the specific enzymes, led to a shift in the membrane perturbation potential of the MDPs, and consequently enhanced cell uptake and drug delivery efficacy. We envision the directed self-assembly based on modularly designed MDPs as a highly promising approach to generate dynamic supramolecular nanomaterials with emerging membrane activity for a range of disease targeted molecular imaging and therapy applications.

Crystal Packing-Guided Construction of Hetero-Oligomeric Peptidic Ensembles as Synthetic 3-in-1 Transporters.

Strategies to generate heteromeric peptidic ensembles via a social self-sorting process are limited. Herein, we report a crystal packing-inspired social self-sorting strategy broadly applicable to diverse types of H-bonded peptidic frameworks. Specifically, a crystal structure of H-bonded alkyl chain-appended monopeptides reveals an inter-chain separation distance of 4.8 Šdictated by the H-bonded amide groups, which is larger than 4.1 Šseparation distance desired by the tightly packed straight alkyl chains. This incompatibility results in loosely packed alkyl chains, prompting us to investigate and validate the feasibility of applying bulky tert-butyl groups, modified with an anion-binding group, to alternatively interpenetrate the straight alkyl chains, modified with a crown ether group. Structurally, this social self-sorting approach generates highly stable hetero-oligomeric ensembles, having alternated anion- and cation-binding units vertically aligned to the same side. Functionally, these hetero-oligomeric ensembles promote transmembrane transport of cations, anions and more interestingly zwitterionic species such as amino acids.

Fabrication and investigation of gold (III) ion-imprinted functionalized silica particles.

Au (III) ion-imprinted mesoporous silica particles (Au-Si-Py) was manufactured by the condensation reaction of (3-Aminopropyl)triethoxysilane (AT)and 2-pyridinecarboxaldehyde (Py). The obtained AT-Py Schiff base ligand was then coordinate with the template gold ions and the polymerizable gold-complex was allowed to gel in presence of tetraethoxysilane (TEOS) and then the coordinated gold ions were leached out of the obtained silica matrix using acidified thiourea solution. During the synthetic steps, the obtained materials were investigated utilizing advanced instrumental and spectral methods. Moreover, the morphological structure of both Au (III) ions imprinted Au-Si-Py and non-imprinted NI-Si-Py silica particles were visualized using scanning electron microscope (SEM). Various adsorption experiments had been carried out using both Au-Si-Py and NI-Si-Py to examine their potential for selective extraction of gold ions under different conditions.

Proton Transfer vs Complex Formation Channels in Ionized Formic Acid Dimer: A Direct Ab Initio Molecular Dynamics Study.

Photoirradiation to a hydrogen-bonded system plays an important role in the initial DNA and enzyme damage processes. The formic acid (FA) dimer is a model compound of double proton transfer systems, such as DNA base pairs. In the present study, the reactions of the FA dimer cation, formed upon ionization of the neutral dimer, have been investigated by the direct ab initio molecular dynamics method. Two reaction channels were identified for the FA dimer cation: complex formation and proton transfer (PT). In the complex formation channel, the carbonyl oxygen atoms of the two FA monomers were bound symmetrically, and a face-to-face complex was formed. In the PT channel, the proton of FA+ was transferred to FA, forming the H+(HCOOH)--HCO2 radical cation as product. At low temperature, the complex channel was dominant, whereas the PT channel increased with increasing temperature. The asymmetric spin distribution on the FA dimer cation exhibited a strong correlation with the PT channel.

Synthesis and Characterization of Ion Pairs between Alkaline Metal Ions and Anionic Anti-Aromatic and Aromatic Hydrocarbons with π-Conjugated Central Seven- and Eight-Membered Rings.

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2-• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop- was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit-to which the Rh(I) center is coordinated-and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.

Ferric Ion Driven Assembly of Catalase-like Supramolecular Photosensitizing Nanozymes for Combating Hypoxic Tumors.

A facile approach to assemble catalase-like photosensitizing nanozymes with a self-oxygen-supplying ability was developed. The process involved Fe3+ -driven self-assembly of fluorenylmethyloxycarbonyl (Fmoc)-protected amino acids. By adding a zinc(II) phthalocyanine-based photosensitizer (ZnPc) and the hypoxia-inducible factor 1 (HIF-1) inhibitor acriflavine (ACF) during the Fe3+ -promoted self-assembly of Fmoc-protected cysteine (Fmoc-Cys), the nanovesicles Fmoc-Cys/Fe@Pc and Fmoc-Cys/Fe@Pc/ACF were prepared, which could be disassembled intracellularly. The released Fe3+ could catalyze the transformation of H2 O2 enriched in cancer cells to oxygen efficiently, thereby ameliorating the hypoxic condition and promoting the photosensitizing activity of the released ZnPc. With an additional therapeutic component, Fmoc-Cys/Fe@Pc/ACF exhibited higher in vitro and in vivo photodynamic activities than Fmoc-Cys/Fe@Pc, demonstrating the synergistic effect of ZnPc and ACF.

Self-Assembly of siRNA/PEG- b-Catiomer at Integer Molar Ratio into 100 nm-Sized Vesicular Polyion Complexes (siRNAsomes) for RNAi and Codelivery of Cargo Macromolecules.

Vesicular polyion complexes (PICs) were fabricated through self-assembly of rigid cylindrical molecules, small interfering RNAs (siRNAs), with flexible block catiomers of poly(ethylene glycol) (2 kDa) and cationic polyaspartamide derivative (70 units) bearing a 5-aminopentyl side chain. 100 nm-sized siRNA-assembled vesicular PICs, termed siRNAsomes, were fabricated in specific mixing ranges between siRNA and block catiomer. The siRNAsome membrane was revealed to consist of PIC units fulfilling a simple molar ratio (1:2 or 2:3) of block catiomer and siRNA. These ratios correspond to the minimal integer molar ratio to maximally compensate the charge imbalance of PIC, because the numbers of charges per block catiomer and siRNA are +70 and -40, respectively. Accordingly, the ζ-potentials of siRNAsomes prepared at 1:2 and 2:3 were negative and positive, respectively. Cross-section transmission electron microscopic observation clarified that the membrane thicknesses of 1:2 and 2:3 siRNAsomes were 11.0 and 17.2 nm, respectively. Considering that a calculated long-axial length of siRNA is 5.9 nm, these thickness values correspond to the membrane models of two (11.8 nm) and three (17.7 nm) tandemly aligned siRNAs associating with one and two block catiomers, respectively. For biological application, siRNAsomes were stabilized through membrane-cross-linking with glutaraldehyde. The positively charged and cross-linked siRNAsome facilitated siRNA internalization into cultured cancer cells, eliciting significant gene silencing with negligible cytotoxicity. The siRNAsome stably encapsulated dextran as a model cargo macromolecule in the cavity by simple vortex mixing. Confocal laser scanning microscopic observation displayed that both of the payloads were internalized together into cultured cells. These results demonstrate the potential of siRNAsomes as a versatile platform for codelivery of siRNA with other cargo macromolecules.

One-Step Preparation of Tough and Self-Healing Polyion Complex Hydrogels with Tunable Swelling Behaviors.

Polyion complex (PIC) hydrogels formed by charge attraction of opposite charged polymers have received unique research interest. Their conventional preparation method, with a large amount of residual salt after polymerization, requires a long-term dialysis treatment to remove the salt and toughen the gel. Here, a promising strategy for the one-step preparation of tough PIC hydrogels without dialysis after polymerization is provided. Bicarbonate and proton ions are selected as the counter ions of the cationic monomer and anionic polymers, respectively. By a CO2 -generating reaction between the counter ions, the residual salt is removed before polymerization, and thus, a PIC hydrogel with tough mechanical performance can be obtained instantly without dialysis. Due to the absence of dialysis, the tough hydrogel can be formed with a wide range of ratios for the oppositely charged polymer with distinct swelling behaviors from non-swelling to super-swelling. This tunable swelling behavior shows the possibility for shape-morphing systems from this one-step method.

Polymerization of hexamethylene diisocyanate in solution and a 260.23 m/z [M+H]+ ion in exposed human cells.

Hexamethylene diisocyanate (HDI) is an important industrial chemical that can cause asthma, however pathogenic mechanisms remain unclear. Upon entry into the respiratory tract, HDI's N=C=O groups may undergo nucleophilic addition (conjugate) to host molecules (e.g. proteins), or instead react with water (hydrolyze), releasing CO2 and leaving a primary amine in place of the original N=C=O. We hypothesized that (primary amine groups present on) hydrolyzed or partially hydrolyzed HDI may compete with proteins and water as a reaction target for HDI in solution, resulting in polymers that could be identified and characterized using LC-MS and LC-MS/MS. Analysis of the reaction products formed when HDI was mixed with a pH buffered, isotonic, protein containing solution identified multiple [M+H]+ ions with m/z's and collision-induced dissociation (CID) fragmentation patterns consistent with those expected for dimers (259.25/285.23 m/z), and trimers (401.36/427.35 m/z) of partially hydrolyzed HDI (e.g. ureas/oligoureas). Human peripheral blood mononuclear cells (PBMCs) and monocyte-like U937, but not airway epithelial NCI-H292 cell lines cultured with these HDI ureas contained a novel 260.23 m/z [M+H]+ ion. LC-MS/MS analysis of the 260.23 m/z [M+H]+ ion suggest the formula C13H29N3O2 and a structure containing partially hydrolyzed HDI, however definitive characterization will require further orthogonal analyses.

Ionic complex of a rhodamine dye with aggregation-induced emission properties.

An AIE-active rhodamine based luminogen was prepared via a complexation reaction between non-emissive rhodamine hydrazide (RdH) and bulky camphorsulfonic acid (CSA). Besides acting to open the spirolactam ring of RdH, CSA also imposes a rotational restriction on the resultant ionic complex, RdH(CSA)x. Without CSA, the analogous complex RdH(HCl)3 is a luminogen with aggregation-caused quenching (ACQ) properties. The ionic bonds of RdH(CSA)3 are sensitive to several external stimuli and therefore it is a luminescent sensor for metal ions, organic amines and the blood protein bovine serum albumin (BSA). Besides being a sensor for BSA, the ionic RdH(CSA)3 is also a denaturant capable of uncoiling the peptide chain of BSA.

Fabrication of Dendrimer-Based Polyion Complex Submicrometer-Scaled Structures with Enhanced Stability under Physiological Conditions.

Submicrometer-scaled (subµ-) self-assembled materials have been developed based on polyion complex (PIC) formation, in particular for biomedical-applications. However, sufficient stability under physiological conditions is required for their practical use. In this study, PIC formation behavior is examined using a block aniomer, poly(ethylene glycol)-b-poly(aspartic acid), and homocatiomers, poly(l-lysine) (LPK) and dendritic poly(l-lysine) (DPK) with different generations, to elucidate the contribution of the dendritic architecture to stability enhancement. LPK-based PIC shows a subµ-vesicular structure only at 25 °C in the absence of NaCl; in contrast, DPK-based PIC forms a subµ-structure under physiological salt concentration and temperature conditions, even when the number of charges of a single molecule is much smaller than that of LPK. Moreover, the formation of subµ-vesicular and -spherical micellar structures is dependent on DPK generation. Thus, the molecular backbone architecture of the PIC component plays an important role not only in expanding the preparation conditions and enhancing stability, but also in controlling the self-assembled structures, mainly due to the spatially restricted structures of dendrimers.

Influence of alkoxy groups on rates of acetal hydrolysis and tosylate solvolysis: electrostatic stabilization of developing oxocarbenium ion intermediates and neighboring-group participation to form oxonium ions.

The hydrolysis of 4-alkoxy-substituted acetals was accelerated by about 20-fold compared to that of sterically comparable substrates that do not have an alkoxy group. Rate accelerations are largest when the two functional groups are linked by a flexible cyclic tether. When controlled for the inductive destabilization, an alkoxy group can accelerate acetal hydrolysis by up to 200-fold. The difference in rates of acetal hydrolysis between a substrate where the alkoxy group was tethered to the acetal group by a five-membered ring compared to one where it was tethered by an eight-membered ring was less than 100-fold, suggesting that fused-ring intermediates were not formed. By comparison, the difference in rates of solvolysis of structurally related tosylates were nearly 10(6)-fold between the five- and eight-membered ring series. This observation implicates neighboring-group participation in the solvolysis of tosylates but not in the hydrolysis of acetals. The acceleration of acetal hydrolysis by an alkoxy group is better explained by electrostatic stabilization of intermediates that accumulate positive charge at the acetal carbon atom.

Characterization of the phase behaviour of a novel polymerizable lyotropic ionic liquid crystal.

The development of new polymerizable lyotropic liquid crystals (LLCs) utilizing charged amphiphilic molecules such as those based on long chain imidazolium compounds, is a relatively new design direction for producing robust membranes with controllable nano-structures. Here we have developed a novel polymerizable ionic liquid based LLC, 1-hexadecyl-3-methylimidazolium acrylate (C16mimAcr), where the acrylate anion acts as the polymerizable moiety. The phase behaviour of the C16mimAcr upon the addition of water was characterized using small and wide angle X-ray scatterings, differential scanning calorimetry and polarized optical microscopy. We compare the phase behaviour of this new polymerizable LLC to that of the well known LLC chloride analogue, 1-hexadecyl-3-methylimidazolium chloride (C16mimCl). We find that the C16mimAcr system has a more complex phase behaviour compared to the C16mimCl system. Additional lyotropic liquid crystalline mesophases such as hexagonal phase (H1) and discontinuous cubic phase (I1) are observed at 20 °C for the acrylate system at 50 and 65 wt% water respectively. The appearance of the hexagonal phase (H1) and discontinuous cubic phase (I1) for the acrylate system is likely due to the strong hydrating nature of the acrylate anion, which increases the head group area. The formation of these additional mesophases seen for the acrylate system, especially the hexagonal phase (H1), coupled with the polymerization functionality offers great potential in the design of advanced membrane materials with selective and anisotropic transport properties.

Contribution of thermal energy to initial ion production in matrix-assisted laser desorption/ionization observed with 2,4,6-trihydroxyacetophenone.

RATIONALE: Although several reaction models have been proposed in the literature to explain matrix-assisted laser desorption/ionization (MALDI), further study is still necessary to explore the important ionization pathways that occur under the high-temperature environment of MALDI. 2,4,6-Trihydroxyacetophenone (THAP) is an ideal compound for evaluating the contribution of thermal energy to an initial reaction with minimum side reactions. METHODS: Desorbed neutral THAP and ions were measured using a crossed-molecular beam machine and commercial MALDI-TOF instrument, respectively. A quantitative model incorporating an Arrhenius-type desorption rate derived from transition state theory was proposed. Reaction enthalpy was calculated using GAUSSIAN 03 software with dielectric effect. Additional evidence of thermal-induced proton disproportionation was given by the indirect ionization of THAP embedded in excess fullerene molecules excited by a 450 nm laser. RESULTS: The quantitative model predicted that proton disproportionation of THAP would be achieved by thermal energy converted from a commonly used single UV laser photon. The dielectric effect reduced the reaction Gibbs free energy considerably even when the dielectric constant was reduced under high-temperature MALDI conditions. With minimum fitting parameters, observations of pure THAP and THAP mixed with fullerene both agreed with predictions. CONCLUSIONS: Proton disproportionation of solid THAP was energetically favorable with a single UV laser photon. The quantitative model revealed an important initial ionization pathway induced by the abrupt heating of matrix crystals. In the matrix crystals, the dielectric effect reduced reaction Gibbs free energy under typical MALDI conditions. The result suggested that thermal energy plays an important role in the initial ionization reaction of THAP.

Dimer ion formation and intermolecular fragmentation of 1,2-diacylglycerols revealed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry for more comprehensive lipid analysis.

RATIONALE: The ionization of neutral diacylglycerols (DAGs) by electrospray ionization mass spectrometry (ESI-MS) is challenging compared with other lipid classes which possess ionic head group conjugations. Although ESI-MS is the method of choice in lipidomic analysis, it is questionable whether all lipid classes can be efficiently ionized by this method. Actually, various lipids were not efficiently detected (due to poor ionization) in many studies which claimed to comprehensively describe lipid profiles. Since neutral lipids are precursors for the biosynthesis of most other lipid classes, the necessity for improved or alternative ionization and identification schemes becomes obvious. METHODS: We identified the 1,2-diacylglycerol (DAG) dimer ion formation in the gas phase by ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization ((-)ESI) mode. The geometry of the dimer ion was investigated by accurate density functional theory (DFT) calculations at the B3LYP/6-311+G(d)//B3LYP/LANL2DZ level of theory. Fragmentation of the dimer ions of many investigated DAGs has been achieved via collision-induced dissociation (CID) experiments with several elevated collision energies (0-12 eV). RESULTS: We revealed the possibility to ionize neutral DAGs as dimer ions in the negative ESI mode. Quantum mechanical calculations revealed a polar head-to-head intermolecular interaction between one charged DAG and one DAG neutral. This represents an energy minimum structure for the DAG dimer ions. We could furthermore detect CID fragmentation product ions that can only result from intermolecular reactions in this head-to-head conformation (SN2 nucleophilic substitution reactions inside the dimer DAG ion). CONCLUSIONS: Here, we present for the first time the opportunity to ionize and identify DAGs as dimer ions. This new finding provides a new alternative for investigations of important diacylglycerol lipids and provides the opportunity to obtain complementary and more comprehensive results in future lipidomic studies.

Formation of curcumin molecular ion under electrospray ionisation conditions in the presence of metal cations.

Electrospray ionisation (ESI) mass spectra obtained for solutions containing curcumin, copper cation and other metal cations, namely Co2+, Ni2+, Mn2+ and Zn2+, have shown an abundant curcumin molecular ion at m/z 368. This ion was not formed for solutions containing curcumin and copper cations or for those containing curcumin and other metal cations. To the best of our knowledge, it is the first example of a system in which copper cations and other metal cations promote formation of organic radical cation under ESI conditions.

Mesomorphic [2]rotaxanes: sheltering ionic cores with interlocking components.

Two types of liquid crystalline [2]rotaxanes based on a conventional tetracatenar motif (a rod-shaped molecule with two side chains at each end) have been prepared. Dicationic compounds with ester stoppers and tetracationic materials with pyridinium stoppers are compared to each other and their dumbbell shaped analogs. Since the ionic core contributes about 70% to the overall length and molecular weight of the molecules, sheltering the ionic cores with an interlocked neutral macrocycle has considerable effect on the mesomorphism and thermal stability of the materials. The influence of the sheltering macrocycle, the numbers of charges on the core and the size and nature of the side chains (aliphatic vs siloxane) were probed. [2]Rotaxanes with linear side chains and minimum ratios of chain-to-core volumes of about 0.35 and 0.30 for tetra- and dicationic compounds, respectively, display smectic liquid crystal phases. Larger ratios increase the temperature range of the smectic A phases beyond the decomposition temperatures; a disadvantage for processing because no stable isotropic liquid phase is available. The change from tetra- to dicationic [2]rotaxanes increased not only the fluidity of their smectic A phases but also their thermal and chemical stability. Branched side chains (2-hexyldecyl) disfavor the formation of lamellar mesophases and, instead, induce higher ordered soft crystal phases. No liquid crystal phases but soft crystal phases are observed for the analogous di- and tetracationic compounds without an ion sheltering interlocked macrocycle (dumbbells).

Preparation and characterization of polyion complex micelles with phosphobetaine shells.

A pair of oppositely charged diblock copolymers, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly((3-(methacryloylamino)propyl)trimethylammonium chloride) (PMPC-b-PMAPTAC) and poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) (PMPC-b-PAMPS), was prepared via reversible addition-fragmentation chain transfer radical polymerization using a PMPC-based macro chain transfer agent. The pendant phosphorylcholine group in the hydrophilic PMPC block has anionic phosphate and cationic quaternary amino groups, which are neutralized within the pendant group. Therefore, the mixing of aqueous solutions of PMPC-b-PMAPTAC and PMPC-b-PAMPS leads to the spontaneous formation of simple core-shell spherical polyion complex (PIC) micelles comprising of a segregated PIC core and PMPC shells. The PIC micelles were characterized using (1)H NMR spin-spin (T2) and spin-lattice relaxation times (T1), diffusion-ordered NMR spectroscopy, static light scattering, dynamic light scattering (DLS), and transmission electron microscopy techniques. The hydrodynamic size of the PIC micelle depended on the mixing ratio of PMPC-b-PMAPTAC and PMPC-b-PAMPS; the maximum size occurred at the mixing ratio yielding stoichiometric charge neutralization. The PIC micelles disintegrated to become unimers with the addition of salts.

Preparation of ionically cross-linked pectin nanoparticles in the presence of chlorides of divalent and monovalent cations.

Nanoparticles were prepared by ionotropic gelation of low-methoxylated (LM) and amidated low-methoxylated (AM) pectin with zinc chloride (ZnCl2) in aqueous media. The samples were characterized by atomic force microscopy, dynamic light scattering, turbidimetry, zeta potential, and pH measurements. Pectin nanoparticles could be prepared at a pectin concentration of 0.07% (w/w) and a ZnCl2-to-pectin ratio of 15:85 (w/w) in the presence of sodium chloride, but not in pure water. Interestingly, particles in the nanometer size-range could also be prepared in the absence of the cross-linker ZnCl2. The dynamic light scattering studies revealed that the AM-pectin nanoparticles were much less polydisperse than the LM-pectin nanoparticles. The AM-pectin nanoparticles were therefore considered to be more promising as a potential drug delivery system, and further studies were performed to investigate the colloidal stability and the effect of the pectin concentration on the size, charge, and compactness of these nanoparticles.