Conformations of 13C Silk-TBP and13C Silk-TBP adsorbed on the surface of TiO2 nanoparticles in the solid state.
To obtain further insights into the binding mechanisms of the SF-minTBP-1 proteins, we synthesized the 13C -labeled model peptide for solid state NMR analyses. Figure 6 shows13C CP/MAS NMR spectra (10-70 ppm) of (a) 13C Silk-TBP (AGSG[1-13C]AGGRKLPD[3-13C]AGGAGSGAG) and (b) 13C Silk-TBP adsorbed on the surface of TiO2 nanoparticles together with the assignment (Asakura et al., 2005; Asakura & Yao, 2002; Yu Suzuki et al., 2010; Y. Suzuki, Shindo, & Asakura, 2016; Wishart, 2011; M. Yang & Asakura, 2005; Yao et al., 2004). The chemical shifts and the assignments are summarized in Table 1. According to the chemical shifts of the peaks, Ala Cβ and Ala Cα carbons, the conformation of the 13C Silk-TBP sample in a solid state is considered to be random coil. This is due to the sample preparation by freeze-dry treatment (Asakura et al., 2005; Yao et al., 2004). After adsorption treatment of the peptides on the surface of the TiO2 nanoparticles, a small13C labeled methyl peak of [3-13C]Ala13 residue in the partial sequence (RKLPD[3-13C]A) in the peptide could be still observed. Interestingly, the Ala13Cβ chemical shift changed from a random coil (16.3 ppm) to an α-helix (14.4 ppm) (Figure 6). Namely, the Ala13 residue adsorbed on the TiO2 particle took an α-helix form. The α-helix peak also became remarkably sharper compared to the Ala Cβ random coil peak of the peptide before adsorption. This could indicate that the interaction between the minTBP-1 and the surfaces of TiO2 is governed by reversible interactions. In addition, relatively sharp peaks at 21.0 and 25.6 ppm could be observed, which are assigned to the side chain carbons of natural abundant Leu and Lys residues in the sequence RKLPDA. Namely, the peak at 21.0 ppm is assigned to the Leu Cδ1,Cδ2, and Lys Cγ carbons, and the peak at 25.6 ppm to the Lys Cδ carbon (according to the chemical shifts). Thus, the Leu and Lys residues are considered to be attached to the surface of TiO2 particles, although the backbone peaks of these residues could not be observed.