Side-Chain Dynamics of the Trifluoroacetone Cysteine Derivative Characterized by 19F NMR Relaxation and Molecular Dynamics Simulations.

19F NMR spectroscopy is a powerful tool for the study of the structures, dynamics, and interactions of proteins bearing cysteine residues chemically modified with a trifluoroacetone group (CYF residue). 19F NMR relaxation rates for the fluoromethyl group of CYF residues are sensitive to overall rotational tumbling of proteins, fast rotation about the CF3 methyl axis, and the internal motion of the CYF side-chain. To develop a quantitative understanding of these various motional contributions, we used the model-free approach to extend expressions for 19F- T2 NMR relaxation to include side-chain motions for the CYF residue. We complemented the NMR studies with atomic views of methyl rotation and side-chain motions using molecular dynamics simulations. This combined methodology allows for quantitative separation of the contributions of fast pico- to nanosecond dynamics from micro- to millisecond exchange processes to the 19F line width and highlights the utility of the CYF residue as a sensitive reporter of side-chain environment and dynamics in proteins.