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We report on the facile preparation of chemically cross-linked microgels in mild conditions by using the reversed microemulsion technique. Sodium alginate has been modified by partially grafting phenol groups to the backbone, on the basis of which microgels have been prepared by the irradiation of visible light in the presence of catalyst Ru(II) complex at room temperature. The irradiation of visible light instead of UV light or gamma rays brings many advantages. The mean diameters of the microgels are 15−40 μm in aqueous solution and 5−15 μm in the dried state. Although the size of the microgel is sensitive to the environment change, it presents excellent size stability in a broad range that covers the physiological condition. The applications of this biocompatible and biodegradable microgel in biology are greatly anticipated. [ABSTRACT FROM AUTHOR]

The article provides an answer to a question on whether primary structures of biomacromolecules have been selected in a Darwinian fashion to adapt to the surrounding environments of the Earth.

DNA displays a richness of biologically relevant supramolecular structures, which depend on both sequence and ambient conditions. The effect of dragging double-stranded DNA (dsDNA) from water into poor solvent on the double-stranded structure is still unclear because of condensation. Here, we employed single molecule techniques based on atomic force microscopy and molecular dynamics (MD) simulations to investigate the change in structure and mechanics of DNA during the ambient change. We found that the two strands are split apart when the dsDNA is pulled at one strand from water into a poor solvent. The findings were corroborated by MD simulations where dsDNA was dragged from water into poor solvent, revealing details of the strand separation at the water/poor solvent interface. Because the structure of DNA is of high polarity, all poor solvents show a relatively low polarity. We speculate that the principle of spontaneous unwinding/splitting of dsDNA by providing a low-polarity (in other word, hydrophobic) micro-environment is exploited as one of the catalysis mechanisms of helicases. [ABSTRACT FROM AUTHOR]

In this paper, we measure the single chain elasticity of an oligomer single-stranded DNA (ssDNA) in both aqueous and nonaqueous, apolar liquid environments by AFM-based single molecule force spectroscopy. We find a marked deviation between the force-extension relations recorded for the two conditions. This difference is attributed to the additional energy required to break the H-bond-directed water bridges around the ssDNA chain in aqueous solutions, which are nonexistent in organic solvents. The results obtained in 8 M guanidine-HCI solution provide more evidence that water bridges around ssDNA originate the observed deviation. On the basis of the results obtained by an ab initio quantum mechanics calculation, a parameter-free freely rotating chain model is proposed. We find that this model is in perfect agreement with the experimental force-extension curve obtained in organic solvents, which further corroborates our assumption. On the basis of the experimental results, it is suggested that the weak H-bonding between ssDNA and water molecules may be a precondition for stable double-stranded DNA to exist in water. [ABSTRACT FROM AUTHOR]

The interactions between monovalent counterions and polyelectrolyte are important in chemical and biological systems. The condensation and screening effect of counterions complicate the polyelectrolyte solutions. By means of single-molecule AFM, the single-chain mechanics of a strong polyelectrolyte, poly(sodium styrenesulfonate) (PSSNa), in KCl aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KCl solutions with a parameter of single-chain modulus (K0). Along with the increase of the concentration of KCl from zero to almost the saturation concentration, a reentrant variation of K0 of single PSSNa chain can be observed. When [K+] is between 0.01 to 3 M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K0 has a positive correlation with the net charge of the polymer chain, the increased K0 at very high KCl concentrations (≥3.5 M) indicates that the chain is charged again. Due to the negative charges on the backbone of PSSNa, only the positively charged counterions (K+) can be adsorbed on the chain. Thus, the PSSNa chain should be positively charged when KCl concentrations ≥3.5 M. That is, the charge inversion occurs in this case, which is induced by a monovalent salt. This finding may lay the foundation for the future applications of drug delivery and gene therapy. [ABSTRACT FROM AUTHOR]

Water, the dominant component underthe physiological condition,is a complicated solvent which greatly affects the properties of solutemolecules. Here, we utilize atomic force microscope-based single-moleculeforce spectroscopy to study the influence of water on the single-moleculeelasticity of an unstructured single-stranded RNA (poly(U)). In nonpolarsolvents, RNA presents its inherent elasticity, which is consistentwith the theoretical single-chain elasticity calculated by quantummechanics calculations. In aqueous buffers, however, an additionalenergy of 1.88 kJ/mol·base is needed for the stretching of thessRNA chain. This energy is consumed by the bound water rearrangement(Ew) during chain elongation.Further experimental results indicate that the Ewvalue is uncorrelated to the salt concentrations and stretchingvelocity. The results obtained in an 8 M guanidine·HCl solutionprovide more evidence that the bound water molecules around RNA giverise to the observed deviation between aqueous and nonaqueous environments.Compared to synthetic water-soluble polymers, the value of Ewof RNA is much lower. The weak interferenceof water is supposed to be the precondition for the RNA secondarystructure to exist in aqueous solution. [ABSTRACT FROM AUTHOR]

Polymers with a carbon-carbon (C-C) backbone are an important class of polymers, which can be regarded as the derivatives of polyethylene (PE). To investigate the effect of side chains on the single-chain enthalpic elasticity (SCEE) of polymers with a C-C backbone, several polymers with pendants or side chains of different lengths and shapes have been studied by single-molecule AFM. We find that both length and shape of the side chains count: only the side chains that are both long and bulky (i.e., bulky dendrons of second or higher generation as side chains) affect the SCEE. Thus, only rare polymers have special SCEE. For the vast majority of polymers, the SCEE is identical to that of PE, which means that the SCEE is determined by the nature of the C-C backbone. It is expected that this conclusion can also be popularized to all polymers with various backbones. This study is an important update to the understanding of polymers at the single-chain level. [ABSTRACT FROM AUTHOR]

Natural cellulose (NC) is the mostabundant biomacromolecule. Thefact that each pyranose ring of NC has three hydroxyl groups impliesthat NC should be highly hydrophilic. The paradoxical water insolubilityof NC is usually explained by the strong tendency to form the hydrogen-bondingnetwork by the high content of hydroxyl groups. In this study, wepresent the first experimental evidence that NC is rather hydrophobic,even if the chains are molecularly dispersed. By single-molecule forcespectroscopy, the single-chain mechanics of NC has been studied invarious liquid environments. In a common nonpolar solvent, octane,NC shows the elastic force–extension (F–E) curve. We find that this kind of F–Ecurve can be fitted well by the QM-FJC model, in whichthe single-chain elasticity obtained from quantum mechanical (QM)calculations is integrated into the freely jointed-chain (FJC) model.However, the result of NC obtained in water is different, which showsa long plateau in the F–Ecurve. Further study shows that the height of the plateau is temperaturedependent. However, the plateau disappears when an 8 M urea solutionis used as the liquid environment. AFM images obtained in water showthat single NC chains exist in a compact globule conformation on thesample surface. According to the molecular structure, methylcellulose(MC) should be more hydrophobic than NC. However, no plateau can beobserved from the MC samples in water. On the basis of all the resultsabove, we can draw a conclusion that both of the hydrophobic effectand the crystallization of NC contribute to the plateau in the F–Ecurve obtained in water. Theexperimental observation of the hydrophobic nature of NC at the single-chainlevel provides new insights into the understanding of NC. [ABSTRACT FROM AUTHOR]

Highly stable covalently attached multilayer films were constructed by visible-light irradiation of hydrogen-bonding directed multilayer films of poly(allylamine) and poly(4-vinylphenol). [ABSTRACT FROM AUTHOR]