Proteins are essential parts of organisms and participate in virtually every process within cells. There are many non-covalent interactions in protein which are critical to protein structure and function. Van der Waals force is a kind of non-covalent interaction which cannot be ignored.
Van der Waals forces include attraction and repulsions between atoms, molecules, and surfaces, they are caused by correlations in the fluctuating polarizations of nearby particles. They are weaker but numerous, so they are non-negligible in proteins.
The J-coupling transmitted across the van der Waals interaction has been studied computationally, however, no such measurement has been reported in biomolecules.
Protein GB3, which has 56 amino acids, was used as a test protein for the measurement. The researchers used a 1H-13C HMQC sequence modified from an HSQC method which has a more favorable relaxation property for the methyl group. To slow down the spin relaxation, the methyl groups of isoleucine (I), leucine (L), and valine (V) were [13C,1H] labeled and their neighboring aliphatics were [12C,2H] labeled whereas all other residues were [13C,2H] labeled.
Using this method, they built up the antiphase Carbon (methyl)？ Carbon (aliphatic) (CmetCali) magnetization through a long 2T delay . A total of 11 vdwJCmetCali with values ～0.1？0.6 Hz were extracted for unambiguously assigned nonoverlapping CmetCali peaks. Quantum mechanical calculations suggested that the J-coupling was correlated with the exchange-repulsion term of van der Waals interaction.
This work is the first study detecting the J-coupling across the CH/CH van der Waals interaction in proteins. Nevertheless, the direct detection of JCC coupling provides an opportunity to study van der Waals interaction in greater detail in small and medium sized proteins. NMR detection of vdwJCC-coupling offers a new tool to characterize such interactions in proteins.
This work was supported by National Key R&D Program of China and the National Natural Science Foundation of China.
Figure 1. Detection of through-space vdwJCmetCali coupling between carbons of methyl and aliphatic groups. (Image by LI Jingwen)
