The next step is to fully specify all of the relaxation interactions active on the spins of interest. Firstly the magnetic dipole-dipole interaction is defined between directly bonded nitrogens and protons:
[firstnumber=192] # Define the magnetic dipole-dipole relaxation interaction. interatom.define(spin_id1='@N', spin_id2='@H', direct_bond=True) interatom.define(spin_id1='@NE1', spin_id2='@HE1', direct_bond=True) interatom.set_dist(spin_id1='@N*', spin_id2='@H*', ave_dist=1.02 * 1e-10) interatom.unit_vectors()
The regular expression `@N*' and `@H*' cannot be used with the dipole_pair. define user function as otherwise @N spins will be connected to @HE1 spins of the same tryptophan residue and @H spins to @NE1 spins. The average interatomic distance is set to 1.02 Ångstrom (though the dipole_pair.set_dist user function expects the units of meters). The dipole_pair.unit_vectors is used to calculate the averaged unit vector between the two atoms.
Secondly the chemical shift anisotropy (CSA) relaxation mechanism is defined via the single command:
[firstnumber=198] # Define the chemical shift relaxation interaction. value.set(-172 * 1e-6, 'csa', spin_id='@N*')
If your system does not experience CSA relaxation, the value can be set to zero.
The relax user manual (PDF), created 2019-06-14.