Water vapor adsorption and desorption isotherms of biologically active proteins

Surfactant-like Peptide Self-Assembled into Hybrid Nanostructures for Electronic Nose Applications

An electronic nose (e-nose) utilizes a multisensor array, which relies on the vector contrast of combinatorial responses, to effectively discriminate between volatile organic compounds (VOCs). In recent years, hierarchical structures made of nonbiological materials have been used to achieve the required sensor diversity. With the advent of self-assembling peptides, the ability to tune nanostructuration, surprisingly, has not been exploited for sensor array diversification. In this work, a designer surfactant-like peptide sequence, CG₇-NH₂, is used to fabricate morphologically and physicochemically heterogeneous "biohybrid" surfaces on Au-covered chips. These multistructural sensing surfaces, containing immobilized hierarchical nanostructures surrounded by self-assembled monolayers, are used for the detection and discrimination of VOCs. Through a simple and judicious design process, involving changes in pH and water content of peptide solutions, a five-element biohybrid sensor array coupled with a gas-phase surface plasmon resonance imaging system is shown to achieve sufficient discriminatory capabilities for four VOCs. Moreover, the limit of detection of the multiarray system is bench-marked at <1 and 6 ppbv for hexanoic acid and phenol (esophago-gastric biomarkers), respectively. Finally, the humidity effects are characterized, identifying the dissociation rate constant as a robust descriptor for classification, further exemplifying their efficacy as biomaterials in the field of artificial olfaction.

Effects of disulfide bridges and backbone connectivity on water sorption by protein matrices

Understanding the water sorption behavior of protein powders is important in applications such as the preservation of protein-based pharmaceuticals. Most globular proteins exhibit a characteristic sigmoidal water adsorption isotherm at ambient conditions. However, it is not well understood how water sorption behavior is influenced by intrinsic factors that are related to structural properties of proteins. We investigate computationally how structural constraints on proteins influence the water sorption isotherms of amorphous protein powders. Specifically, we study the effects of non-local disulfide linkages and backbone connectivity using pheromone ER-23 and lysozyme as model proteins. We find that non-local disulfide linkages can significantly restrict structural changes during hydration and dehydration, and this in turn greatly reduces the extent of hysteresis between the adsorption and desorption branches. Upon removing the backbone connectivity by breaking all peptide bonds in lysozyme, we find that the hysteresis shifts towards the lower humidity regime, and the water uptake capacity is significantly enhanced. We attribute these changes to the higher aggregation propensity of the constraint-free amino acids in dehydrated condition, and the formation of a spanning water network at high hydration levels.

Microscopic Origin of Hysteresis in Water Sorption on Protein Matrices

Despite the importance of water sorption isotherms for a fundamental understanding of protein-water interactions, the microscopic origin of hysteresis between the adsorption and desorption branches is not well understood. Using our recently developed simulation technique, we compute the water sorption isotherms of two proteins, lysozyme and Trp-cage, a miniprotein. We explicitly compare protein-water interactions in adsorption and desorption processes, by analyzing local hydration in terms of hydrogen bonding, water density, and solvent-accessible surface area. We find that significant differences in hydration behavior between adsorption and desorption manifest themselves at the individual amino acid level, in particular around polar or charged residues. We confirm this observation by demonstrating that Trp-cage's hysteresis can be significantly reduced by mutating charged residues to alanine, a neutral and nonpolar amino acid.

Development of solid super desiccants based on a polymeric superabsorbent hydrogel composite

The innovative design of cross-linked sodium polyacrylate based solid super-desiccant is demonstrating 276 ± 20% 24 h vapor absorption capacity at 99% RH.

Behavior of Moisture Gain and Equilibrium Moisture Contents (EMC) of Various Drug Substances and Correlation with Compendial Information on Hygroscopicity and Loss on Drying

The behavior of moisture gain and equilibrium moisture content (EMC) was determined for 30 drug substances at relative humidities (RH) ranging from 11% to 93%. Based on the results, the drugs were categorized into different classes, following the classification system proposed by Callahan et al. About 23 fell under Class-I (nonhygroscopic), four under Class-II (slightly hygroscopic), and three under Class-III (moderately hygroscopic). Most of the Class-III drugs converted from solid to liquid state at > 75% RH, and the moisture increase was more than 40% above 90% RH. However, the moisture increase was comparatively much smaller at humidities < 40%-50%. Hence, no drug could be categorized in Class-IV (very hygroscopic), where gain of moisture is generally higher even at lower humidities. The results were correlated to the statements given in the compendia on hygroscopicity, and also the values of loss on drying (LOD). The study suggests that there is a need to rationalize the pharmacopoeial information.

A study of the differences between two amorphous spray-dried samples of cefditoren pivoxil which exhibited different physical stabilities

The objective of this study was to investigate the reasons for the difference in physical stability of two amorphous cefditoren pivoxil samples that had been prepared using spray drying at inlet-air temperatures of 40 degrees C (SD-A) and 100 degrees C (SD-B). The two samples appeared amorphous by powder X-ray diffraction and had indistinguishable glass transition temperatures. Despite the fact that glass transition is often regarded as an indicator of the stability of amorphous forms, crystallisation was observed for SD-A, but not for SD-B, during storage at 60 degrees C and 81% relative humidity (RH). Gravimetric water sorption data demonstrated very similar water sorption until high RH values, at which point SD-A sorbed more water than did SD-B. The values of the dispersive, acidic (K(A)) and basic (K(D)) components of surface energy of the spray-dried samples were obtained using inverse gas chromatography (IGC), in the dry state and after equilibration with different RH environments. The data showed that the two amorphous samples had different surface properties and that the effect of sorbed water on these samples was also different. It is concluded that the two samples did not have long-range order, but had differences in the orientation of molecules at the surface, which were significant enough to alter the stability when the samples were stressed with water vapour and high temperature storage. IGC proved a valuable tool with which to study changes in the surface properties of amorphous materials.

Water vapor adsorption properties of amorphous cefditoren pivoxil evaluated by adsorption isotherms and microcalorimetry

Water vapor adsorption of ground cefditoren pivoxil was studied. The amount of water adsorbed increased with a decrease in the crystallinity of cefditoren pivoxil. It was found from the microcalorimetric measurements that the differential heat of water vapor adsorption at 1.5% adsorbed water increased with decreasing crystallinity of cefditoren pivoxil, suggesting that hygroscopicity of cefditoren pivoxil was enhanced by grinding. These results indicated that hydrophilic adsorption sites in cefditoren pivoxil increased through the grinding process. The results of infrared (IR) spectra examination suggested that the increment of hydrophilic adsorption sites through the grinding process resulted from the change of the environment of the carbonyl groups in two esters and amide.

Moisture effects on protein-excipient interactions in spray-dried powders. Nature of destabilizing effects of sucrose

The preparation of stable solid protein formulations presents significant challenges. Ultimately, the interactions between incorporated excipients and the pharmaceutical protein determine the formulation stability. In this study, moisture was utilized to probe the interactions between a model protein, trypsinogen, and sucrose in the solid state, following spray drying. Through investigation of the physical properties of the spray-dried formulations, we attempted to elucidate the mechanisms underlying the previously observed stabilizing and destabilizing effects of the carbohydrate during spray drying. Both dynamic and equilibrium moisture uptake studies indicated the presence of an optimal protein-sugar hydrogen bonding network. At low sucrose contents, a preferential protein-sucrose hydrogen bonding interaction was dominant, resulting in protein stabilization. However, at high carbohydrate concentrations, preferential sugar-sugar interactions prevailed, resulting in a phase separation within the formulation matrix. The preferential incorporation of the sucrose molecules in a sugar-rich phase reduced the actual amount of the carbohydrate available to interact with the protein and thereby decreased the number of effective protein-sucrose contacts. As a consequence, the protein could not be effectively protected during spray drying. We hypothesize that the observed phase separation at this sucrose concentration regime originates from its exclusion from the protein in solution before spray drying, further accompanied by preferential clustering of the sucrose molecules.

Determining the water sorption monolayer of lyophilized pharmaceutical proteins

The concept of monolayer water coverage is useful in the development of lyophilized protein formulations. Herein, we have explored three different methodologies to determine the water monolayer for pharmaceutical proteins: (1) theoretical prediction based on the amino acid composition and their relative propensities to sorb water; (2) a traditional adsorption isotherm measurement by Karl Fischer water titration of samples held at various relative humidities (created by saturated salt solutions); and (3) an adsorption isotherm measurement with a gravimetric sorption analyzer (GSA), which consists of a microbalance within a computer-controlled humidified environment. Data from the latter two methods were analyzed with the Brunauer-Emmett-Teller (BET) gas adsorption equation to yield experimental monolayers. In our study, we examined six different therapeutic proteins and found that for each case all three approaches yielded similar results for the water monolayer. We also attempted to use the BET equation to determine the water monolayer for a model sugar (trehalose) and polyol (mannitol), which are potential excipients in pharmaceutical protein formulations. We found that calculations from the data obtained by the traditional and GSA methods yielded consistent results for trehalose, which remained amorphous upon lyophilization. Mannitol tended to form anhydrous crystals upon freezedrying, and was thus not amenable to analysis. The utility of both traditional and GSA methods for determining the water monolayer was extended to colyophilized protein:sugar systems as well.