Abstract
Quartz crystal microbalance with dissipation monitoring (QCM-D) has been employed to study the assembly and recrystallization kinetics of isolated SbpA bacterial surface proteins onto silicon dioxide substrates of different surface wettability. Surface modification by UV/ozone oxidation or by vapor deposition of 1H,1H,2H,2H-perfluorododecyltrichlorosilane yielded hydrophilic or hydrophobic samples, respectively. Time evolution of frequency and dissipation factors, either individually or combined as the so-called Df plots, showed a much faster formation of crystalline coatings for hydrophobic samples, characterized by a phase-transition peak at around the 70% of the total mass adsorbed. This behavior has been proven to mimic, both in terms of kinetics and film assembly steps, the recrystallization taking place on an underlying secondary cell-wall polymer (SCWP) as found in bacteria. Complementary atomic force microscopy (AFM) experiments corroborate these findings and reveal the impact on the final structure achieved.
Highlights
Crystalline bacterial protein layers (S-layers) are arrays ofproteins (Mw of 40 to 200 kDa) forming the outermost envelope of prokaryotes, and represent the simplest biological membrane [1,2]
S-layer formation on secondary cell-wall polymer (SCWP) films. Both the binding of thiolated-SCWP to gold surfaces and its subsequent use as underlying material for the recrystallization of SbpA bacterial protein could be monitored in real time by means of Quartz crystal microbalance with dissipation monitoring (QCM-D) (Figure 2)
The in vitro formation of a SCWP film mimicking the underlying material found by bacterial S-layer proteins in nature (Figure 2a) induced a variation in frequency of 30 Hz, corresponding to a bound mass of about 0.53 μg/cm2 (Figure 2b)
Summary
Crystalline bacterial protein layers (S-layers) are arrays of (glyco)proteins (Mw of 40 to 200 kDa) forming the outermost envelope of prokaryotes, and represent the simplest biological membrane [1,2]. Some fundamental questions are yet to be solved about the S-layer recrystallization process under different physico-chemical conditions of the substrate (i.e., wettability or the presence of specific chemical groups and receptors), such as the different adsorption kinetics, crystal domain sizes and lattice symmetry parameters obtained. The importance of such factors for the morphology and biological function of isolated proteins has been already shown in literature [16]
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