Long-lived coherences for magnetic interactions in proteins

Living systems rely on molecular building blocks of low symmetry such as amino-acids and nucleotides, which generally yield short-lived magnetic transitions in response to electromagnetic radiation. This is the first demonstration that in proteins of relatively large size local magnetic symmetry can be induced to enable the detection of interactions based on long-lived coherent transitions of nuclear spins. Long-lived coherences (LLCs) are superpositions of quantum states with singlet and triplet spin-permutation symmetries that feature significantly longer relaxation time constants compared to those of standard nuclear spin coherences. We report in this study that glycine residues in Lysozyme, a 14.3 kDa protein, feature Gly-H2,3 LLCs with relaxation time constants twice as long as the classical counterparts. Using a new excitation method for LLCs in glycines 4, 49, 54, 67, 117, and 126, Lysozyme Gly-H dipolar interactions with neighboring hydrogen spins were mapped in a high magnetic field - at 950 MHz 1H Larmor frequency. As predicted by theory, the positions of nearby atoms in the protein structure on one side or the other of Gly molecular symmetry planes reflecting protons H2,3 determines the signs of LLC magnetic interaction signals. LLC-based transfers therefore yield stereospecific signals from glycine residues to 1H neighboring atoms. The symmetry-encoded sign of the detected signals provides angle constraints, in addition to the distance information. LLC probes based on naturally-abundant 1H spins can be useful for in-cell spectroscopy, circumventing the introduction of heterogenous spin labels for following protein-ligand or protein-protein interactions in the natural environment. This is the first demonstration that magnetisation transfer through space from long-lived coherences can be obtained in proteins. Applications of LLCs were believed to be limited to systems featuring fast rotational motion in solution, mainly small molecules. This new LLC-based method yields stereospecific distance constraints and has the potential to extend the protein-size domain for the study of intra- and intermolecular interactions.