Surface characterization of structural and stereo isomeric amino acids using sorption techniques and Raman spectroscopy

Abstract

Event Abstract Back to Event Surface characterization of structural and stereo isomeric amino acids using sorption techniques and Raman spectroscopy Daniel Burnett1, Majid Naderi1, Anett Kondor1 and Manaswini Acharya1 1 Surface Measurement Systems, Ltd., United States Introduction: Amino acids are found in proteins and enzymes, which perform essential roles in nature. Different forms of compounds are known to have different physiological and therapeutic effects and therefore various effective methods are required to provide insight in their physical properties.[1] Considering that hydration is a major factor in determining the structural and isomeric forms of amino acids, the interaction with water vapor would provide fundamental information. Materials and Methods: Alanine samples were obtained from Across Organics. A unique combination of a fiber optic Raman probe with Dynamic gravimetric Vapor Sorption (DVS) was used to monitor the real-time transformation of different forms of Alanine. The sample was exposed to a particular RH while monitoring the change in mass. In-situ Raman spectra were collected during sorption experiments. Total surface free energies of different forms of Alanine were determined using IGC-SEA (Surface Measurement Systems Ltd.). IGC-SEA provides surface energy profiles of particulate/fibrous samples unlike conventional techniques which only yield a single average valued property. Results and Discussion: The combined water sorption/Raman spectroscopy results for ß-Alanine and DL-Alanine show significant characteristics in the region 800 – 1600 cm-1 for both materials. There are no changes in the Raman bands of ß-Alanine during the sorption process up to 90% RH. However, after the 90% RH stage and during the desorption stage the Raman Spectra show irreversible changes in ß-Alanine structure. The water sorption data for DL-Alanine indicates a relatively low uptake and confirms a stable material in agreement with the Raman spectrum. The difference of 125× in moisture uptake levels is attributed to water absorption into the bulk of ß-Alanine, which causes a phase change in ß-Alanine, whereas in the case of DL-Alanine there is no phase or structural changes. The dispersive and specific surface energy profiles at 0% RH indicate ß-Alanine has the higher total surface energy, thus is the most active and have higher affinity for water. At 90% RH, the acid-base surface energy of both samples is increased while the dispersive surface energy did not change significantly. Thus, the polarity, for both alanine samples increases with the relative humidity due to an increase in the acid-base surface energy of both samples. The wettability as the measure of the intermolecular interaction between the solid sample and water can is determined by the thermodynamic work of adhesion with water molecules (Figure 1). The higher work of adhesion shows higher the wettability (more hydrophilic). It is clearly observed that the wettability of both alanine samples increases with the RH, but the increase is more significant in the case of the ß-Alanine. Figure 1. Wettability of (a) DL-Alanine and (b) -Alanine at 0% RH (blue) and 90% RH (red). The work of adhesion values were calculated at 0.005 surface coverage (n/nm). Conclusions: IGC-SEA was able to measure the significant differences in surface chemistry for the ß-Alanine and DL-Alanine at 0 and 90% RH. These results correlate directly with water sorption behavior. The combination of gravimetric vapor sorption with Raman spectroscopy and IGC-SEA allows for better understanding of vapor-induced structural changes of biologically active ingredients.