Abstract
The interplay between protein concentration and (observation) time has been investigated for the adsorption and crystal growth of the bacterial SbpA proteins on hydrophobic fluoride‐functionalized SiO2 surfaces. For this purpose, atomic force microscopy (AFM) has been performed in real‐time for monitoring protein crystal growth at different protein concentrations. Results reveal that (1) crystal formation occurs at concentrations above 0.08 µM and (2) the compliance of the formed crystal decreases by increasing protein concentration. All the crystal domains observed presented similar lattice parameters (being the mean value for the unit cell: a = 14.8 ± 0.5 nm, b = 14.7 ± 0.5 nm, γ = 90 ° ± 2). Protein film formation is shown to take place from initial nucleation points which originate a gradual and fast extension of the crystalline domains. The Avrami equation describes well the experimental results. Overall, the results suggest that protein‐substrate interactions prevail over protein–protein interactions.Research Highlights AFM enables to monitor protein crystallization in real‐time.AFM high‐resolution determines lattice parameters and viscoelastic properties.S‐layer crystal growth rate increases with protein concentration.Avrami equation models protein crystal growth.
Highlights
Crystal formation, either in nature or in the laboratory, is denoted crystal growth
Depending on the parameters that rule the type of crystallization, the theoretical approaches can be classified into two general cases: those depending on the thermodynamic conditions and those led by kinetics (Meldrum & Cölfen, 2008; Rabe, Verdes, & Seeger, 2011; Xu, Ma, & Cölfen, 2007)
According to the analysis of the results, protein-substrate interactions dominated over protein–protein interactions in SbpA crystal formation
Summary
Crystal formation, either in nature or in the laboratory, is denoted crystal growth. Classically, a crystal is a three dimensional periodic arrangement of atoms (molecules, ions, etc.) in solids, being the texture of the crystals hardly perfect (Kittel, 1996). We have chosen as crystallization model system a bacterial protein that is able to form crystalline nano-arrays in a quite controlled manner. Such protein, SbpA from Lysinibacillus sphaericus CCM2177, has the ability to diffuse from solution toward a surface and to subsequently selfassemble with other neighboring proteins to form a characteristic square (p4) lattice symmetry (Sleytr, Sára, Pum, & Schuster, 2001). SbpA from Lysinibacillus sphaericus CCM2177, has the ability to diffuse from solution toward a surface and to subsequently selfassemble with other neighboring proteins to form a characteristic square (p4) lattice symmetry (Sleytr, Sára, Pum, & Schuster, 2001) This process can be considered merely a kinetic process. Former studies reported that SbpA proteins follow a two-stage nucleation process (Chung, Shin, Bertozzi, & De Yoreo, 2010; Shin et al, 2012), where the protein-substrate and protein–protein interactions may play an
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