Effects of manganese and calcium on conformational stability of concanavalin A: a differential scanning calorimetric study

Physicochemical Studies on Amino Acid Based Metallosurfactants in Combination with Phospholipid

Dicarboxylate metallosurfactants (AASM), synthesized by mixing N-dodecyl aminomalonate, -aspartate and -glutamate with CaCl2, MnCl2 and CdCl2, were characterized by XRD, FTIR, and NMR spectroscopy. Layered structures, formed by metallosurfactants, were evidenced from differential scanning calorimetry and thermogravimetric analyses. Solvent-spread monolayer of AASM in combination with soyphosphatidylcholine (SPC) and cholesterol (CHOL) were studied using Langmuir surface balance. With increasing mole fraction of AASM mean molecular area increased and passed through maxima at ~60 mol% of AASMs, indicating molecular packing reorganization. Systems with 20 and 60 mol% AASM exhibited positive deviations from ideal behavior signifying repulsive interaction between the AASM and SPC, while synergistic interactions were established from the negative deviation at other combinations. Dynamic surface elasticity increased with increasing surface pressure signifying formation of rigid monolayer. Transition of monolayer from gaseous to liquid expanded to liquid condensed state was established by Brewster angle microscopic studies. Stability of the hybrid vesicles, formed by AASM+SPC+CHOL, were established by monitoring their size, zeta potential and polydispersity index values over 100 days. Size and spherical morphology of hybrid vesicles were confirmed by transmission electron microscopic studies. Biocompatibility of the hybrid vesicles were established by cytotoxicity studies revealing their possible applications in drug delivery and imaging.

Synthesis and development of poly(N-hydroxyethyl acrylamide)-ran-3-acrylamidophenylboronic acid polymer fluid for potential application in affinity sensing of glucose

BACKGROUND

In previous work, we described viscosity and permittivity microelectromechanical systems (MEMS) sensors for continuous glucose monitoring (CGM) using poly[acrylamide-ran-3-acrylamidophenylboronic acid (PAA-ran-PAAPBA). In order to enhance our MEMS device antifouling properties, a novel, more hydrophilic polymer-sensing fluid was developed.

METHOD

To optimize sensing performance, we synthesized biocompatible copolymers poly(N-hydroxyethyl acrylamide)-ran-3-acrylamidophenylboronic acid (PHEAA-ran-PAAPBA) and developed its sensing fluid for viscosity-based glucose sensing. Key factors such as polymer composition and molecular weight were investigated in order to optimize viscometric responses.

RESULTS

Compared with PAA-ran-PAAPBA fluid of a similar binding moiety percentage, PHEAA-ran-PAAPBA showed comparable high binding specificity to glucose in a reversible manner and even better performance in glucose sensing in terms of glucose sensing range (27-468 mg/ml) and sensitivity (within 3% standard error of estimate). Preliminary experiment on a MEMS viscometer demonstrated that the polymer fluid was able to sense the glucose concentration.

CONCLUSIONS

Our MEMS systems using PHEAA-ran-PAAPBA will possess enhanced implantable traits necessary to enable CGM in subcutaneous tissues.

Development of boronic acid grafted random copolymer sensing fluid for continuous glucose monitoring

We have previously presented a microelectromechanical system (MEMS) based viscometric sensor for continuous glucose monitoring using protein Concanavalin A (Con A). To address its drawbacks, including immunotoxicity and instability issues, we have synthesized stable, biocompatible copolymers poly(acrylamide-ran-3-acrylamidophenylboronic acid) (PAA-ran-PAAPBA) for viscosity based glucose sensing. We found that PAA-ran-PAAPBA showed very high binding specificity to glucose. Several key factors such as polymer compositions, polymer molecular weights and polymer concentrations have been investigated to optimize viscometric responses. This polymer is able to detect glucose under physiological pH conditions in a reversible manner. Therefore, it has the potential to enable a highly reliable, continuous monitoring of glucose in subcutaneous tissue using the MEMS device.

Development of novel glucose sensing fluids with potential application to microelectromechanical systems-based continuous glucose monitoring

BACKGROUND

We have previously presented a microelectromechanical systems (MEMS) viscometric sensor for continuous glucose monitoring. The sensing fluid used therein was based on protein concanavalin A, which is known to have significant drawbacks, such as immunotoxicity and instability. To address this issue, a stable, biocompatible polymeric sensing fluid has been developed.

METHODS

In the polymeric sensing system, glucose reversibly formed strong ester bonds with the phenylboronic acid moiety on the poly(acrylamide-ran-3-acrylamidophenylboronic acid) (PAA-ran-PAAPBA) polymer backbone, resulting in cross-linking of the copolymers and an increase in the solution viscosity. The copolymers were synthesized via classic free radical copolymerization processes. The viscosity of the PAA-ran-PAAPBA, dissolved in phosphate-buffered saline buffer and in the presence of glucose at physiologically relevant concentrations, was measured by an Ubbelodhe viscometer and a prototype MEMS viscometric device.

RESULTS

Experimental results showed that the polymer molecular weight and composition depended on the solvent quantity, while the sensing fluid viscosity was determined by the polymer molecular weight and percentage composition of PAAPBA. The study of the temperature effect on viscosity showed that the polymer sensed glucose effectively under physiological conditions, although the high temperature lowered its sensitivity. Through proper adjustment of these parameters, a distinctive viscosity increase was observed when the glucose concentration increased from 0 to 450 mg/dl, which was detectable by our prototype MEMS device.

CONCLUSIONS

We have successfully developed a stable, biocompatible polymeric system for the sensitive detection of glucose. MEMS experiments demonstrated that the sensing fluid was able to sense glucose at different concentrations. This sensing system can potentially enable highly reliable, continuous monitoring of glucose in interstitial fluid from subcutaneous tissue.

Stretched-Exponential Analysis of Heat-Induced Aggregation of Apo-Concanavalin A

The kinetics of heat-induced aggregation of apo-concanavalin A (aConA) was investigated as a function of temperature and protein concentration by circular dichroism and turbidity. Heat-induced aggregation, as well as conformational change, of aConA was fitted to stretched-exponential equations. The exponent of the conformational change maintained 0.5 despite the protein concentration and temperature, indicating the presence of a common intermediate during the conformational change. After the process, aggregates grew with increasing temperature and initial protein concentration. The reaction order of aggregation was 1.5, indicating that the rate-limiting steps of aConA aggregation involve both conformational change and aggregation.

Conformation of concanavalin A and its fragments in aqueous solution and organic solvent-water mixtures

The conformations of concanavalin A (con A), an all-beta protein, and its three CNBr-cleaved fragments were studied by CD. Con A in buffer showed a 197 nm maximum and a 223 nm minimum, which were red-shifted by 6-7 nm from those of regular all-beta proteins and beta-sheet of (Lys)n. Fragment 1 (residue 1-42) resembled an unordered form with a CD maximum at 200 nm, but fragments 2 (residues 43-129) and 3 (residues 130-237) showed a regular CD spectrum with two extrema at 192-193 nm (+) and 214-216 nm (-). Equimolar mixture of the three fragments showed some degree of interaction, but did not reconstitute the conformation of native con A, probably because of the loss of bound Ca2+ and Mn2+ ions in the fragments. In ethanol-, methanol-, and dioxane-water mixed solvents, con A and its fragments remained as beta-sheet. In contrast, addition of trifluoroethanol and sodium dodecyl sulfate induced alpha-helix at the expense of beta-sheet for con A and its fragments in aqueous solution. In 80% trifluoroethanol, the induced helicities exceeded their sequence-predicted helix-potentials, but in 10 mM sodium dodecyl sulfate the helicities agreed well with corresponding predictions.

Conformational change and aggregation of concanavalin A at high temperatures

Increase of the beta sheet content and aggregation of concanavalin A (con A) induced at about 60 C were followed with circular dichroism (c.d.) and scattered light intensity (I90) on both metal-complexed and demetallized species. The conversion occurred at a higher temperature for metal-complexed species than for demetallized one. A concentration-independent conversion curve of metal-complexed species, obtained for a concentration range below around 6 microgram/ml (6 x 10(-3) kg m-3) with a midpoint at 57 degrees C, was well described in terms of a conformational equilibrium between two conformers. However, aggregation did exist even at a low concentration of 1 microgram/ml. Aggregation also occurred without the conformational change as found at the initial stage or in Tris buffer, which suggested the absence of direct coupling between the conformational change and the aggregation. Changes of c.d. at 222 nm, expected to represent the main chain conformation, differed from those at 290 nm reflecting the environment of side chain chromophores. Time courses of three properties examined, c.d. at 222 nm, at 290 nm, and I90, always exhibited a lag in the case of metal-complexed species while the lag was not observed in the case of demetallized species, however. Lag became longer in c.d. but it became shorter in I90 as the protein concentration increased.

Stability of enzyme inhibitors and lectins in foods and the influence of specific binding interactions

Proteins with actual or potential antinutrient or toxicant activity found in foodstuffs include (1) enzyme inhibitors, especially those specific for serine proteinases and alpha-amylases, and (2) lectins (hemagglutinins). These inhibitors and lectins must be inactivated during processing or food preparation, usually by heat, to avoid possible undesirable effects. Knowledge of their heat stabilities thus helps determine conditions required for their inactivation or denaturation. Many are heat-stable proteins, and their conformations can be stabilized or destabilized by interactions with other constituents present in the food or the digestive tract. Differential scanning calorimetric (DSC) results show that specific binding interactions can lead to substantial increases in kinetic thermal stability of proteins. Examples of such stabilization include serine proteinase-proteinase inhibitor, alpha-amylase-amylase inhibitor, and metal ion-lectin complexes. The extent of thermal stabilization of proteinases in complexes with inhibitors is correlated with the equilibrium association constant. Presence of more than one denaturing unit revealed by DSC in complexes involving multiheaded inhibitors can be interpreted in relation to domain structures of the inhibitors. Basic information on stability of the enzyme inhibitors and lectins is relevant to food processing, quality, and safety.