Determination of Sorption Isotherm and Rheological Properties of Lysozyme Using a High-Resolution Humidity Scanning QCM-D Technique

Abstract

The high-resolution humidity scanning QCM-D technique enables investigation of hydration of soft matter films using a quartz crystal microbalance with dissipation monitoring (QCM-D) equipped with a humidity module. Based on a continuous increase of relative humidity, properties of soft matter films can be investigated depending on the water content of the surrounding atmosphere. Determination of complete water sorption isotherms is possible via analysis of the overtone dependence of the resonance frequencies. Rheological properties are monitored via measurement of the dissipation. The glass transition can be identified from the change of viscoelastic properties of the film reflected in changes of the dissipation. A high-resolution water sorption isotherm of lysozyme was measured and compared with results from water sorption calorimetry. Analysis of the rheological behavior during hydration of lysozyme films revealed the presence of two separate sharp transitions at the water activities 0.67 and 0.91, which are connected to the glass transition. In previous works, only the existence of a broad glass transition has been reported so far. Combining the QCM-D data with Raman scattering data presented earlier, a new mechanism of isothermal glass transition in lysozyme is proposed.

Humidity scanning quartz crystal microbalance with dissipation monitoring setup for determination of sorption-desorption isotherms and rheological changes

“…In many cases, the coating of the quartz sensor with a film leads to deviations from the Sauerbrey equation due to the rheological and/or surface properties of the applied film. In this work, the QCM-D data were analyzed according to the single viscoelastic film in air model, 13 which previously has been used for QCM-D hydration studies on pig gastric mucin, 8 lysozyme, 10 and starch films. 11 According to this model, the frequency change, normalized per overtone, corresponding to the total mass of the coated sensor m f , is proportional to the overtone in square n 2 by the following relation:…”

“…In addition, the rheological properties of the film can be evaluated through the ratio between the storage and loss moduli, which describes the elastic behavior (G ′ f ) in relation to the viscous properties (G ′′ f ) of the film. 10,14 This is achieved by incorporating the complex frequency, which is defined as ∆f = f + iΓ, where f is the real part and Γ is the imaginary part. Equation (2) can now be rewritten according to Eq.…”

“…Then, from the molal composition as a function of time, we can derive ρ out (t) for the outgoing LiCl solution. This is achieved by fitting the molal composition to experimental data on density of LiCl solutions as a function of its molal concentration at 25 • C. 10,16 After calculating the variation of the solution density from this fitting procedure, we can finally calculate the flow rate r out according to Eq. (14), which accounts for the fact that the density varies as a function of time,…”

Hydration and the phase diagram of acid hydrolyzed potato starch

“…Page 9 of 31 A c c e p t e d M a n u s c r i p t 9 lab makes use of a continuous dilution of a saturated lithium chloride solution which results in a continuous scanning of water activity above the sample film (Graf and Kocherbitov, 2013).…”

“…Due to its high sensitivity this technique delivers real-time analysis of the thickness, swelling and rheological properties of the sample. We used a newly developed humidity scanning QCM-D method (Graf M a n u s c r i p t 14 and Kocherbitov, 2013) to investigate starch films of various thicknesses which were prepared by drop-coating the sensor with dilute aqueous solutions of acid hydrolyzed starch. As Figure 3 (a) shows, the sorption isotherms obtained with the QCM-D method are in close agreement with the sorption calorimetry data.…”