Coil‐Globule Collapse in Flexible Macromolecules

The transition of a solvated flexible macromolecular chain from random coil behavior in the θ‐state to a globular compact form in the collapsed state has been the subject of extensive theoretical and experimental studies. Most of the coil‐globule transition studies of macromolecules have concentrated on the prototypical polystyrene‐cyclohexane system. However, chain contractions reported in this system have been around 75% of those in the unperturbed θ‐state. This relatively small decrease in size does not satisfy the criterion for a densely packed, collapsed globule. Experimentally, the collapse from a coil to a true compact globular state has now been established for two flexible macromolecules: poly(N‐isopropylacrylamide) in water and poly(methyl methacrylate) in various solvents. In this contribution, we review recent theoretical studies covering phenomenological and Langevin models as well as computer simulations. In addition, we outline recent experimental studies of the coil‐globule transition of various flexible polymers, copolymers, and polyelectrolytes.

Expansion factor, αequation/tex2gif-stack-1.gif, versus temperature for NaPSS in 4.17 M aqueous NaCl solution. (•): NaPSS‐1, (○): NaPSS‐2.     

Smart,Naturally-Derived Macromolecules for Controlled Drug Release

A series of troxerutin-based macromolecules with ten poly(acrylic acid) (PAA) or poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) homopolymer side chains were synthesized by a supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) approach. The prepared precisely-defined structures with low dispersity (M_w/M_n < 1.09 for PAA-based, and M_w/M_n < 1.71 for PDMAEMA-based macromolecules) exhibited pH-responsive behavior depending on the length of the polymer grafts. The properties of the received polyelectrolytes were investigated by dynamic light scattering (DLS) measurement to determine the hydrodynamic diameter and zeta potential upon pH changes. Additionally, PDMAEMA-based polymers showed thermoresponsive properties and exhibited phase transfer at a lower critical solution temperature (LCST). Thanks to polyelectrolyte characteristics, the prepared polymers were investigated as smart materials for controlled release of quercetin. The influence of the length of the polymer grafts for the quercetin release profile was examined by UV–VIS spectroscopy. The results suggest the strong correlation between the length of the polymer chains and the efficiency of active substance release, thus, the adjustment of the composition of the macromolecules characterized by branched architecture can precisely control the properties of smart delivery systems.     

Defined Picogram Dose Inclusion and Release of Macromolecules using Polyelectrolyte Microcapsules

Summary: Reversible pH‐induced swelling of (PAH/PSS) polyelectrolyte microcapsules is accompanied by increased porosity, making them permeable to poly(acrylic acid) (PAA) at pH values higher than 11.2. This pH‐switchable permeability was used to encapsulate the polyanion in alkaline conditions. Relationship between starting PAA concentration in solution and amount finally being encapsulated has been established and can be used further as calibration curve. A desired amount of encapsulated polymer in the picogram range per capsule can be achieved. The loaded capsules were then used as microreactors by forming a complex between the PAA and Ca²⁺ ions.

General scheme for pH‐induced encapsulation of (PAA) in alkali condition by switching their permeability.     

Organized Shells on Clay Nanotubes for Controlled Release of Macromolecules

The use of tubular halloysite clay as a nanotemplate for layer‐by‐layer (LbL) shell assembly and its utilization for controlled release of drug macromolecules are studied. The LbL nanoshell allowed additional control for the sustained release of drug loaded halloysite tubes. The number of polymeric layers in the shell and molecular weight of the assembled polymers influences the drug release rate. Three bilayer shells of chitosan and gelatin of 15 nm thicknesses gave the best encapsulation and retardation in the release rate of dexamethasone. An encapsulation of the macromolecules inside the lumen of the biocompatible clay nanotubes coupled with the polyelectrolyte shell formation provides a novel formulation for the controlled release of bioactive agents.

     

W/O Emulsion Destabilization and Release of a Polysaccharide Entrapped in the Droplets

This study was conducted as part of European project VEGEPHY to develop a product for the crop protection purposes. It concerns first the destabilization of a W/O emulsion containing water droplets in which the polysaccharide carboxymethylcellulose (CMC) is trapped, the droplets being dispersed in a vegetable oil made of rapeseed methyl ester. Polyglycerolpolyricinoleate (PGPR) is used as surfactant and glycerol is added to enhance the dispersion of the CMC. The second part is dealing with the study of the release and dilution of the CMC obtained by dilution of the destabilized emulsion in water in order to obtain the required final amount of CMC for practical purposes. The destabilization of the emulsion by the demulsifier (cynthiorex PMH 1125) has been followed by differential scanning calorimetry (DSC) that permits by studying the freezing of the dispersed water to detect the presence of bulk water. The release and dilution in water of the CMC versus time was followed and quantified by measuring the conductivity of the sample. The release kinetic was modeled using a first-order empirical model. The results showed that the release process of the W/O emulsion depends on the concentration of the demulsifier, on the stirring rate, and on the temperature. The optimum amount of the non-ionic surfactant was found to be 10% and the full release of the CMC has been obtained in 600 seconds.     

Encapsulation and Release Mechanisms in Coordination Polymer Nanoparticles

The interplay of guest encapsulation and release mechanisms in nanoscale metal–organic vehicles and its effect on the drug‐delivery kinetics of these materials were investigated through a new multidisciplinary approach. Two rationally‐designed molecular guests were synthesized, which consist of a red‐fluorescent benzophenoxazine dye covalently tethered to a coordinating catechol group and a protected, non‐coordinating catechol moiety. This allowed loading of the guests into compositionally and structurally equivalent coordination polymer particles through distinct encapsulation mechanisms: coordination and mechanical entrapment. The two types of particles delivered their fluorescent cargo with remarkably different kinetic profiles, which could be satisfactorily modeled considering degradation‐ and diffusion‐controlled release processes. This demonstrates that careful selection of the method of guest incorporation into coordination polymer nanoparticles allows selective tuning of the rate of drug delivery from these materials and, therefore, of the time window of action of the encapsulated therapeutic agents.     

pH响应型纳米药物载体的释药机制及性能研究进展

pH响应型纳米载体因具有智能的酸敏或碱敏释药性能,已成为当前一类重要的多功能纳米载体,并得到了研究人员的广泛关注。特别是酸敏性纳米载体,可用于肿瘤弱酸微环境的药物控释,因而对药物的定点释放和癌症的靶向治疗等生物医学应用发挥了积极作用。本文综述了近年来各类pH响应型纳米载体的典型合成方法,系统地介绍了共价键、分子间作用力以及物理结构变化3种方式引发的pH响应释药机制。深入阐述了pH响应型纳米载体的载药性能、体外释药性能、体外细胞毒性、体内抗癌性能及体内分布性能,并详细列举了近年来pH响应型纳米载体的各类实验参数,进而为pH响应型纳米载体的深入研究提供了方法学的借鉴和性能参考。     

Hydrogen‐Release Mechanisms in Lithium Amidoboranes

Hydrogen storage : In lithium amidoboranes an initial molecule of H₂ is released by the formation of LiH, followed by a redox reaction of the dihydrogen bond formed between LiH^δ? and NH^δ+. In this dehydrogenation process, an intermolecular N? B bond forms through the catalytic effect of a Li cation. After releasing the first molecule of H₂, a Li cation binds to a nitrogen atom, lowering the energy barrier for the second H₂ loss per lithium amidoborane dimer (see figure).

     

Excluded Volume in Macromolecules

Abstract

The excluded volume in macromolecules has been discussed in an elementary way, and has been compared with that of the real gases. The excluded volume has been classified into two categories, namely, inter-and intramolecular excluded volumes, and have been discussed in greater length in terms of their origin and their influence on configurational and hydrodynamic properties. It is concluded that the calculation of inter-molecular excluded volume is not difficult, while the intramolecular excluded volume requires detailed knowledge about the nature and the various interactions between the polymer segments.     

Penetration and Crosslinking of Macromolecules

One of the topical problems of macromolecular chemistry is the question of the extent to which macromolecules in solution or in the amorphous solid state can engage in mutual interpenetration. The present article is concerned primarily with recent investigations on cross-linking copolymerization and with inter-and intramolecular reactions of polymers as a contribution to the study of molecular shape.     

Characterization and Properties of Macromolecules

Abstract

The effect of solvent on the numerical results obtained from gel permeation chromatography has been evaluated. Columns packed with porous glass materials were evaluated, using three solvents. The narrow molecular weight distribution polystyrene standards were chromatographed in each solvent to calibrate the columns. The chromatograms of the standards were then analyzed to yield the molecular weight averages and their molecular weight distributions. The results indicate solvents of high viscosity should be avoided, especially for high molecular weight samples.     

Ionic Mixed Interactions in Macromolecules

Purely ionic interactions in natural and synthetic macromolecules involve the mutual interaction of fixed charges and their interaction with mobile ions. Such charge‐dependent interactions lead to well‐documented effects, including chain expansion of polyelectrolytes, globularization of polyampholytes, distributions of mobile ions according to charge screening, or ion condensation models. A variety of structural features, functions, and applications of these systems is amplified by the superimposition of charge‐independent effects associated with the occurrence of less polar or hydrophobic groups, special salts, surfactants, or complementary protein assemblies. For instance, ionic and hydrophobic attractive interactions stabilize pearls (or rings)‐on‐a‐string conformations, possibly a model for the formation of the chromatin assembly. The attractive interactions due to hydrophobic fatty acid groups attached to polysaccharides promote the formation of vesicles that entrap and slowly release water‐soluble drugs. Intra‐ and intermolecular associations based on ion‐pairing mixed interactions also control the formation of host–guest compounds, protein conformation, and the assembly of layered polyelectrolytes. Metallo‐supramolecular polymers and networks are formed due to the coordination of multivalent cations with bi‐ and trifunctional organic ligands. The association of lithium salts to polymers in the absence of water allows the formation of highly efficient energy sources. It also allows the identification of the ionic species that control charge‐independent contributions to Hofmeister effects. This critical review presents a synthetic classification of systems displaying ionic mixed interactions, and a discussion of underlying molecular mechanisms.     

明胶大分子中的正电子湮没行为

The microstructural properties of gelatin samples derived from the bovine bone as well as skin of fish which inhabit in the cold sea were examined at molecular scale by using the positron annihilation lifetime spectroscopy（PALS）. The longest-lived component of PALS provides information about the free-volume holes in gelatin macromolecules. The results show that both average size and number of free-volume holes in the fish gelatin macromolecules are lower than those in the bovine gelatin macromolecules. Silver halide emulsions were also precipitated by using the balanced double-jet technique,for which two types of media containing fish gelatin and bone gelatin were used in the stage of nucleus formation,respectively. The monodispersibility of silver halide crystallites can be improved and the growth and coalescence of nucleus grains can be restricted when the fish gelatino-peptizer was used as dispersion medium at the nucleus formation. It thus suggested that the function of gelatino-peptizers as protective colloid is correlated with the microstructure character of gelatin macromolecules established by PALS. The gelatino-peptizer derived from the fish skin possesses stronger power governing nucleation and growth of silver halide grains compared with that made from the bovine bone.     

Macromolecules in a blend of poor and amphiphilic solvents

The globular state of the homopolymer macromolecule in a blend composed of a poor solvent and an amphiphilic solvent (substrate), whose molecules tend to be aligned with the solvent concentration gradient in the inhomogeneity region, was theoretically studied. The size of a homogeneous globule and the substrate concentration in its volume were calculated in terms of a bulk approximation. After the transition of the macromolecule from the coil to the globule state, its volume first decreases with a decrease in temperature and then begins to grow due to substrate molecules penetrating the globule. The substrate concentration in the globule insignificantly exceeds that outside the globule at identical second virial coefficients of interaction between monomer units and between substrate molecules. The expression for the free energy functional depending on the volume fractions of the components and on the orientation of substrate molecules was examined in the ground-state approximation. The orientation effect leads to narrowing of the surface layer and to a decrease in the surface tension of the homogeneous globule, thereby increasing its stability with respect to the transition to the unfolded-coil state.     

Stiffening Transition in Semiflexible Cyclic Macromolecules

Conformational changes in cyclic and linear chains triggered by increasing chain stiffness are explored using MC simulations. The transition to the rigid strained objects is continuous and steeper for cycles. Circular macromolecules of contour length equal to ≈5 persistence lengths are characterized as rigid strained objects with a lowered spatial dimensionality. The coil regime is almost omitted in the static structure factor already for semiflexible chains of both architectures and the flattening of stiffer cycles is evidenced by oscillations superimposed on the rod‐like behavior of corresponding linear chains. The orientational correlation function shows qualitative difference in behavior of cyclic and linear chains.

     

Elastically Flexible Crystals have Disparate Mechanisms of Molecular Movement Induced by Strain and Heat

Elastically flexible crystals form an emerging class of materials that exhibit a range of notable properties. The mechanism of thermal expansion in flexible crystals of bis(acetylacetonato)copper(II) is compared with the mechanism of molecular motion induced by bending and it is demonstrated that the two mechanisms are distinct. Upon bending, individual molecules within the crystal structure reversibly rotate, while thermal expansion results predominantly in an increase in intermolecular separations with only minor changes to molecular orientation through rotation.     

Infrared Intensity Studies in Fluorinated Macromolecules

Summary: Functionalized perfluoropolyethers are investigated by infrared spectroscopy with the aim to give a quantitative estimate of the carboxylic acid groups species; since hydrogen bonded moieties have been observed, also the relative distribution of the associated species has been determined. The study is based both on measured and on theoretically predicted infrared intensities, as obtained by density functional theory (DFT) calculations on model systems.