Characterization of the restricted rotation of the dimethyl groups in chemically N -terminal ¹³C-labeled antifreeze glycoproteins: A temperature-dependent study in water to ice through the supercooled state

Site-specific chemical modification, especially with isotopically enriched groups, allows one to study the structure and dynamics of proteins for which uniform enrichment is difficult. When the N -terminal alanine in antifreeze glycoprotein (AFGP) is replaced with an N,N -dimethyl alanine the methyl groups show signatures of slow rotation about the C-N bond. In order to separate the local dynamics of the N -terminus from the overall protein dynamics, we present a complete characterization of this dynamics. Temperature-dependent nuclear magnetic-resonance experiments from room temperature to subzero temperatures, including the supercooled state and in the presence of ice, are presented. Quantum chemical calculations are also performed on a localized N -terminus of the AFGP. Our results show that in the solution state at room temperature and in the super cooled regime, the dimethyl groups undergo a slow, restricted rotation with an unequal distribution of population between two major conformations. At lower temperatures in the presence of ice, the dynamics become much more complex due to freezing out of several conformational states. Based on these results, we conclude that the segmental dynamics of the N -terminus are local to the first residue and do not affect the overall dynamics of the protein.