Hindered Rotations of Protein Asparagine/Glutamine Side-Chain NH₂Groups: Impact of Hydrogen Bonding with DNA

Hindered rotation about an sp2 C-N bond is known to occur in arginine (Arg), asparagine (Asn), and glutamine (Gln) side chains of proteins. However, very little is known about the rotational dynamics of Asn and Gln side-chain NH2 groups. Here, using a unique NMR method, we quantitatively characterized the hindered rotations of protein Asn/Gln side-chain NH2 groups. This NMR method yields simple NH2-selective spectra that allow for an accurate determination of the kinetic rate constants for the hindered rotations. Through the NMR measurements at different temperatures, we investigated the energy barriers that restrict the C-N bond rotations of protein side-chain NH2 groups. Through a comparison of the kinetic data for the free and DNA-bound states of the Antp homeodomain, we also examined the impact of hydrogen bonding on the hindered rotations of the side-chain NH2 groups. Our data suggest that the hydrogen bonding increases the energy barriers by 1-6 kJ/mol.