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| - Measurement of Side-Chain Carboxyl pKa Values of Glutamateand Aspartate Residues in an Unfolded Protein byMultinuclear NMR Spectroscopy
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| - A triple-resonance NMR pulse scheme is presented for measuring aspartic and glutamic acidside-chain pKa values in unfolded protein states where chemical shift overlap is limiting. The experimentcorrelates side-chain carboxyl carbon chemical shifts of these residues with the backbone amide protonchemical shift of the following residue. The methodology is applied to an 15N, 13C labeled sample of theN-terminal SH3 domain of the Drosophila protein drk, which exists in equilibrium between folded (Fexch)and unfolded (Uexch) states under nondenaturing conditions. Residue-specific pKa values of side-chaincarboxyl groups are presented for the first time for an unfolded protein (drk Uexch state), determined froma pH titration. Results indicate that deviations from pKa values measured for model compounds are likelydue to local effects, while long-range electrostatic interactions appear to be of minor importance for thisprotein.
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