Characterizing viscoelastic properties of synthetic and natural fibers and their coatings with a torsional pendulum.

Characterizing and understanding the viscoelastic mechanical properties of natural and synthetic fibers is of great importance in many biological and industrial applications. Microscopic techniques such as micro/nano indentation have been successfully employed in such efforts, yet these tests are often challenging to perform on fibers and come with certain limitations in the interpretation of the obtained results within the context of the macroscopic viscoelasticity in the fiber. Here we instead explore the properties of a series of natural and synthetic fibers, using a freely-oscillating torsional pendulum. The torsional oscillation of the damped mass-fiber system is precisely recorded with a simple HD video-camera and an image processing algorithm is used to analyze the resulting videos. Analysis of the processed images show a viscoelastic damped oscillatory response and a simple mechanical model describes the amplitude decay of the oscillation data very well. The natural frequency of the oscillation and the corresponding damping ratio can be extracted using a logarithmic decrement method and directly connected to the bulk viscoelastic properties of the fiber. We further study the sensitivity of these measurements to changes in the chemo-mechanical properties of the outer coating layers on one of the synthetic fibers. To quantify the accuracy of our measurements with the torsional pendulum, a complementary series of tests are also performed on a strain-controlled rheometer in both torsional and tensile deformation modes.

Liposomes Entrapped in Biopolymer Hydrogels Can Spontaneously Release into the External Solution.

Hydrogels of biopolymers such as agar and gelatin are widely used in many applications, and in many cases, the gels are loaded with nanoparticles. The polymer chains in these gels are cross-linked by physical bonds into three-dimensional networks, with the mesh size of these networks typically being 10-100 nm. One class of "soft" nanoparticles are liposomes, which have an aqueous core surrounded by a lipid bilayer. Solutes encapsulated in the liposomal core can be delivered externally over time. In this paper, we create liposomes with diameters ∼150 nm from an unsaturated phospholipid (lecithin) and embed them in agar gels (the aqueous phase also contains 0-50% of glycerol, which is an active ingredient in cosmetic products). Upon placing this gel in quiescent water, we find that the liposomes release out of the gel into the water over a period of 1-3 days, even though the gel remains intact. This is a surprising result that runs contrary to our expectation that the liposomes would simply remain immobilized in the gel. We show that the release rate of liposomes can be tuned by several variables: for example, the release rate increases as the agar concentration is lowered and the rate increases steadily with temperature. In addition to agar, release of liposomes also occurs out of other physical gels including those of agarose and gelatin. However, liposomes made from a saturated phospholipid do not release out of any gels. We discuss a possible mechanism for liposomal release, which involves intact liposomes deforming and squeezing through transient large pores that arise in physical networks such as agar. Our findings have relevance to transdermal delivery: they suggest the possibility of systematically delivering liposomes loaded with actives out of an intact matrix.

Light-Mediated Synthesis of 2-(4-Methoxyphenyl)-1-pyrroline via Intramolecular Reductive Cyclization of a Triplet Alkylnitrene.

Irradiation of p-methoxyazidobutyrophenone (1) in methanol yielded 2-(4-methoxyphenyl)-1-pyrroline (2) and several other photoproducts. However, in the presence of tris(trimethylsilyl)silane (TTMSS), 2 is formed selectively. Transient absorption and ESR spectroscopy verify that the irradiation of 1 forms triplet alkylnitrene 31N through intramolecular energy transfer from the triplet ketone (T1K). DFT calculations indicate that 31N abstracts H atoms from TTMSS but not methanol, which explains the selectivity. Thus, triplet alkylnitrenes can undergo selective reductive cyclization via H atom abstraction from TTMSS.

A multidisciplinary approach to probing enthalpy-entropy compensation and the interfacial mobility model.

In recent years, interfacial mobility has gained popularity as a model with which to rationalize both affinity in ligand binding and the often observed phenomenon of enthalpy-entropy compensation. While protein contraction and reduced mobility, as demonstrated by computational and NMR techniques respectively, have been correlated to entropies of binding for a variety of systems, to our knowledge, Raman difference spectroscopy has never been included in these analyses. Here, nonresonance Raman difference spectroscopy, isothermal titration calorimetry, and X-ray crystallography were utilized to correlate protein contraction, as demonstrated by an increase in protein interior packing and decreased residual protein movement, with trends of enthalpy-entropy compensation. These results are in accord with the interfacial mobility model and lend additional credence to this view of protein activity.

Ultrafast spectroscopy and computational study of the photochemistry of diphenylphosphoryl azide: direct spectroscopic observation of a singlet phosphorylnitrene.

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ∼ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ∼ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.

Effect of alkyl substituents on photorelease from butyrophenone derivatives.

Photolysis of 1a yields 4a in argon-saturated methanol, whereas 1b is photostable. Laser flash photolysis of 1a in acetonitrile shows formation of biradical 2a (lambda(max) = 340 nm, tau = approximately 60 ns), which undergoes intersystem crossing to form Z-3a (lambda(max) = 380 nm, tau = 270 ns) and E-3a (lambda(max) = 380 nm, tau = 300 ms). Z-3a regenerates the starting material, whereas E-3b undergoes intramolecular lactonization to release the alcohol moiety and form 4a. Similar laser flash photolysis of 1b shows formation of biradical 2b (lambda(max) = 340 nm, tau = 1.9 micros in acetonitrile), which is longer-lived than 2a is. However, 2b only undergoes intersystem crossing to form Z-3b (lambda(max) = 380 nm, tau = 4.3 micros). Calculations demonstrate that intramolecular pseudo hydrogen bonding between the OH moiety and the radical centered on the isopropyl carbon in 2b and the bulkiness of the isopropyl group prevent the necessary rotation to form E-3b. In comparison, 2a does not form an intramolecular pseudo hydrogen bond between the methylene radical center and the OH group, and as a consequence, it undergoes intersystem crossing to form both E- and Z-3a.

Thermally and pH-responsive gelation of nanoemulsions stabilized by weak acid surfactants.

Nanoemulsions are widely used in applications such as in food products, pharmaceutical ingredients and cosmetics. Moreover, nanoemulsions have been a model colloidal system due to their ease of synthesis and the flexibility in formulations that allows one to engineer the inter-droplet potentials and thus to rationally tune the material microstructures and rheological properties. In this article, we study a nanoemulsion system in which the inter-droplet interactions are modulated by temperature and pH. We develop a nanoemulsion suspension in which the droplets are stabilized by weak acid surfactants whose charged state can be independently controlled by temperature and pH, leading to a responsive electrostatic repulsion. Moreover, the additional poly(ethylene glycol) segment (PEG) on the surfactant gives rise to a temperature responsive attraction between droplets via PEG-PEG association and ion-dipole interaction. The interplay of these three interactions gives rise to non-monotonic trends in material properties and structures as a function of temperature. The underlying mechanism resulting in these trends is obtained by carefully characterizing the nanoemulsion droplets and studying the molecular interactions. Such mechanistic understanding also provides guidance to modulate the inter-droplet potential using pH and ionic strength. Moreover, the molecular understanding of the weak acid surfactant also sheds light on the destabilization of the nanoemulsion droplets triggered by a switch in pH. The control of the competition of attractive and repulsive interactions using external stimuli opens up the possibility to design complex nanoemulsion-based soft materials with controllable structures and rheological properties.

Selective formation of triplet alkyl nitrenes from photolysis of beta-azido-propiophenone and their reactivity.

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

Solid-state photolysis of alpha-azidoacetophenones.

Solid-state photolysis of 1 yields 2 in a crystal-to-crystal reaction. The reaction takes place by alpha-cleavage to form a benzoyl and an azido alkyl radical pair. The azido alkyl radicals rearrange into iminyl radicals and N2. The iminyl and benzoyl radicals are held in close proximity within the crystal lattice, which allows them to combine and form 2. X-ray structure analysis, molecular modeling and trapping studies support this mechanism.

Mechanistic Insights into the Hydrolysis of Organophosphorus Compounds by Paraoxonase-1: Exploring the Limits of Substrate Tolerance in a Promiscuous Enzyme.

We designed, synthesized and screened a library of analogs of the organophosphate pesticide metabolite paraoxon against a recombinant variant of human serum paraoxonase-1. Alterations of both the aryloxy leaving group and the retained alkyl chains of paraoxon analogs resulted in substantial changes to binding and hydrolysis, as measured directly by spectrophotometric methods or in competition experiments with paraoxon. Increases or decreases in the steric bulk of the retained groups generally reduced the rate of hydrolysis, while modifications of the leaving group modulated both binding and turnover. Studies on the hydrolysis of phosphoryl azide analogs as well as amino-modified paraoxon analogs, the former being developed as photo-affinity labels, found enhanced tolerance of structural modifications, when compared with O-alkyl substituted molecules. Results from computational modeling predict a predominant active site binding mode for these molecules which is consistent with several proposed catalytic mechanisms in the literature, and from which a molecular-level explanation of the experimental trends is attempted. Overall, the results of this study suggest that while paraoxonase-1 is a promiscuous enzyme, there are substantial constraints in the active site pocket, which may relate to both the leaving group and the retained portion of paraoxon analogs.

Computational Modeling of Human Paraoxonase 1: Preparation of Protein Models, Binding Studies, and Mechanistic Insights.

The enzyme human paraoxonase 1 (huPON1) has demonstrated significant potential for use as a bioscavenger for treatment of exposure to organophosphorus (OP) nerve agents. Herein we report the development of protein models for the human isoform derived from a crystal structure of a chimeric version of the protein (pdb ID: 1V04) and a homology model derived from the related enzyme diisopropylfluorophosphatase (pdb ID: 1XHR). From these structural models, binding modes for OP substrates are predicted, and these poses are found to orient substrates in proximity to residues known to modulate specificity of the enzyme. Predictions are made with regard to the role that residues play in altering substrate binding and turnover, in particular with regard to the stereoselectivity of the enzyme, and the known differences in activity related to a natural polymorphism in the enzyme. Potential mechanisms of action of the protein for catalytic hydrolysis of OP substrates are also evaluated in light of the proposed binding modes.

Intramolecular H-atom abstraction in gamma-azido-butyrophenones: formation of 1,5 ketyl iminyl radicals.

Photolysis of gamma-azidobutyrophenone derivatives yields 1,4 ketyl biradicals via intramolecular H-atom abstraction. The 1,4 ketyl biradicals expel a nitrogen molecule to form 1,5 ketyl iminyl biradicals, which decay by ring closure to form a new carbon-nitrogen bond. The 1,5 ketyl iminyl biradicals were characterized with transient spectroscopy. In argon/nitrogen-saturated solutions, the biradicals have lambda(max) approximately 300 nm and tau = 15 micros. DFT-TD calculations were used to support the proposed mechanism for formation of the 1,5 ketyl iminyl radicals.

Competition between alpha-cleavage and energy transfer in alpha-azidoacetophenones.

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.