Anisotropie Chemical Shift Perturbation Induced by Ions in Conducting Channels
Anisotropie chemical shifts observed from solid-state NMR spectroscopy of uniformly aligned samples can be influenced by three primary factors: a change in orientation of the nuclear site, a change in dynamics, or a change in the chemical shift tensor element magnitudes or orientation to the molecular frame. These features are particularly attractive for characterizing the influence of ions in ion conducting channels. Cation binding results in far more subtle effects than had previously been imagined. Prior to the analysis of the first solid-state NMR characterizations of ion binding [1,2] the experimental data were primarily in the form of a few water-soluble protein structures in the Protein Data Bank to which monovalent ions were bound [e.g. 3,4]. Such binding sites showed optimized solvation for the ions associated with strong binding. Computational modeling efforts on ion channels, for the most part, also showed substantial structural deformation upon ion binding [5–7]. We now realize that much better models for how ions interact with channels can be realized from the characterization of substrate binding to enzymes, for which we have a great deal of information represented in every biochemistry textbook. A delicate balance of molecular interactions and thermodynamic parameters has evolved for enzymes, so that substrates are attracted to the active sites of proteins while not compromising the primary function of these proteins to conduct chemistry on the substrates and to release the products efficiently. Similarly, ions must be attracted to the channel and yet the primary function of these proteins is to facilitate the transfer of ions from one side of the membrane to the other.
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