orals 5th Asia-Pacific NMR Symposium 2013

ESR Study of Protein Surface/Internal Water Under Nanoconfinement  (#51)

Yun-Wei Chiang 1
  1. Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan

We investigate protein/peptide structures as well as their surface hydration dynamics using an approach combining spin-label ESR techniques with nanochannels. In addition to the Bax protein, several spin-labeled peptide variants, derived from a 26-mer-long synthetic prion protein peptide, are studied. Under nanoconfinement the formation of crystalline ice is suppressed, allowing the study of hydration dynamics at subfreezing temperatures. We report the temperature-dependent investigations (50 ~ 260 K) of the protein/peptide structures and surface hydration dynamics. The existence of surface hydration and bulk shells are demonstrated by cw-ESR, DEER, ESEEM, and two-pulse ESR experiments. Water in the immediate vicinity of the nitroxide label at the protein-water interface is verified to be non-crystalline under nanoconfinement, and the water accessibility changes little with the nanochannel dimension. Nevertheless, this water accessibility for the nanochannel studies is only half the value for the bulk solvent, even though the protein/peptide structures remain largely the same as those immersed in the bulk solvents. The result demonstrates that while the protein/peptide are confined but structurally unaltered in the nanochannels, their surrounding water exhibits density heterogeneity along the peptide surface normal. A transition related to the protein structural flexibility is found to coincide with the fragile-to-strong crossover (FSC) around 220 K and is demonstrated to be coupled with the FSC of the surface hydration, implying that no real protein transition occurs around 220 K.