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.