A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices ...designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics-based interactions via competitive binding with ionic surfactants. To this aim, free-standing micrometer-size wires were synthesized using oppositely charged poly(diallyldimethylammonium chloride) and poly(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the quartz crystal microbalance technique and that of the wires (>1 μm) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process.
When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species, stable complexes are found to form spontaneously. The mechanism is based on ...electrostatics and on the compensation between the opposite charges. Electrostatic complexes exhibit a core−shell microstructure. In the core, the polyelectrolyte blocks and the oppositely charged species are tightly bound and form a dense coacervate microphase. The shell is made of the neutral chains and surrounds the core. In this paper, we report on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite γ-Fe2O3) and cationic-neutral copolymers. The copolymers investigated are poly(trimethylammonium ethylacrylate methyl sulfate)-b-poly(acrylamide), with molecular weights 5000-b-30000 g mol-1 and 110000-b-30000 g mol-1. The mixed copolymer−nanoparticle aggregates were characterized by a combination of light scattering and cryo-transmission electron microscopy. Their hydrodynamic diameters were found in the range 70−150 nm, and their aggregation numbers (number of nanoparticles per aggregate) from tens to hundreds. In addition, Magnetic Resonance Spin−Echo measurements show that the complexes have a better contrast in Magnetic Resonance Imaging than single nanoparticles and that these complexes could be used for biomedical applications.
A general method is presented for analysis of the resolution of pyridoxal-P-requiring enzymes by carbonyl reagents. The method
is useful for accurately determining the very small equilibrium ...constants (KP) which characterize the dissociation of cofactor
from many pyridoxal-P-requiring enzymes. The analysis also establishes the minimum number and relative stabilities of distinct
enzymic species involved in the resolution process. Analysis of the resolution of D-serine dehydratase by L-and D-cysteine
resulted in the establishment of an enzyme bound thiazolidine derivative as an intermediate in the pathway for resolution.
The over-all equilibrium constant (KR) for the reaction, D-serine dehydratase + cystein in equilibrium KR thiazolidine derivative
+D-serine apodehydratase was determined. At pH 7.80, T/2 0.33, 25 degrees, KR equal to 1.08 times 10-minus 3. A value of 7.0
nM for the equilibrium constant for the dissociation of D-serine dehydratase to apoenzyme and free pyridoxal-P was determined
from the ratio KR/KT, where KT is the equilibrium constant for the formation of a thiazolidine derivative from free pyridoxal-P
and cysteine. An estimate of 14 nM for KP was also obtained from partial resolution of D-serine dehydratase by high dilution.
The difficulties associated with this direct determination of KP from the dependence on the enzyme concentration of the activity
of very dilute solutions of enzyme are discussed.
The equilibrium constant (KX) for the reaction D-serine dehydratase + pyridoxamine-P in equilibrium KX D-serine apodehydratase:
pyridoxamine-P + pyridoxal-P was determined. At 25 degreees, pH 7.80, ...KX increases from 5.4 times 10-minus 5 to 21 times 10-minus
5 as T/2 is increased from 0.33 to 0.66. A value of 1.3 times 10-minus 4 M at 25 degrees, pH 7.80, T/2 0.33 for the equilibrium
constant (KPMP) for dissociation of pyridoxamine-P from D-serine apodehydratase was determined from the ratio of the equilibrium
constant for dissociation of pyridoxal-P from holoenzyme to KX. Pyridoxamine-P and the thiazolidine, formed from pyridoxal-P
and cysteine, were found to have similar affinities for D-serine apodehydratase. Using the affinities of these derivatives
as a measure of the noncovalent interactions between cofactor and protein, it was possible to estimate the contribution of
the Schiff base linkage to the stability of the complex formed between pyridoxal-P and protein. The covalent Schiff base linkage
in the holoenzyme was found to be no more stable than the Schiff base linkage formed between 6-aminocaproic acid and pyridoxal-P.
The contribution of noncovalent interactions to the stability of the cofactor-protein complex was shown to be at least 20
to 40 times greater than the contribution of the covalent Schiff base linkage.
A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices ...designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics based interactions via competitive binding with ionic surfactants. To this aim free-standing micron-size wires were synthesized using oppositely charged poly(diallyldimethylammonium chloride) and poly(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the Quartz Crystal Microbalance technique, and that of the wires (> 1 micron) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process.
Schiff base formation, transaldimination, and reaction with aminothiols are important reactions which occur on the surface
of pyridoxal-P-requiring enzymes. As a first step in assessing the role of ...the protein in these reactions, models for these
reactions were studied in the absence of enzyme at 25 degrees, gamma/2 0.28. The reaction of 6-aminocaproic acid with pyridoxal-P
to form the Schiff base N6-(P-pyridoxylidene)-aminocaproic acid was studied as a model for formation of Schiff base on the
holoenzyme...
The immune-signaling dyad CD40/CD40L promotes atherogenesis, and patients with unstable angina have elevated plasma levels of soluble CD40L (sCD40L) and membrane-bound CD40L. It is unknown, however, ...whether elevations of circulating sCD40L precede the onset of acute cardiovascular symptoms.
In a prospective, nested case-control evaluation of healthy middle-aged women, mean concentrations of sCD40L at baseline were significantly higher among 130 participants who subsequently developed myocardial infarction, stroke, or cardiovascular death (cases), compared with 130 age- and smoking-matched women who remained free of cardiovascular disease (controls) during a 4-year follow-up (2.86 ng/mL for cases versus 2.09 ng/mL for controls; P=0.02). Women with concentrations above the 95th percentile of the control distribution (>3.71 ng/mL) had a significantly increased relative risk (RR) of developing future cardiovascular events (RR, 3.3; 95% CI, 1.2 to 8.6; P=0.01) that remained after adjustment for usual cardiovascular risk factors (multivariate RR, 2.8; 95% CI, 0.9 to 8.0; P=0.05).
High plasma concentrations of sCD40L may be associated with increased vascular risk in apparently healthy women.
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