Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the
pentapeptide Met-enkephalin and the amino acids, which constitute it.
Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR)
in combination with fast field-cycling, thus enabling measuring NMRD curves
for all individual protons. As in earlier works, Papers I–III, pronounced
effects of intramolecular scalar spin-spin interactions, J-couplings, on spin
relaxation were found. Notably, at low fields J-couplings tend to equalize the
apparent relaxation rates within networks of coupled protons. In Met-
enkephalin, in contrast to the free amino acids, there is a sharp increase in
the proton T1-relaxation times at high fields due to the changes in the regime
of molecular motion. The experimental data are in good agreement with theory.
From modelling the relaxation experiments we were able to determine motional
correlation times of different residues in Met-enkephalin with atomic
resolution. This allows us to draw conclusions about preferential conformation
of the pentapeptide in solution, which is also in agreement with data from
two-dimensional NMR experiments (rotating frame Overhauser effect
spectroscopy). Altogether, our study demonstrates that high-resolution NMR
studies of magnetic field-dependent relaxation allow one to probe molecular
mobility in biomolecules with atomic resolution