Slow dynamic processes, such as folding or unfolding of proteins, can be witnessed by nuclear spins, provided that nonequilibrium populations of the energy levels can be sustained over time intervals that are on the order of the inverse of the rate constants of the dynamic processes. Large biomolecules have slow diffusion rates and may host exchange processes that involve high-energy barriers, such as concerted breaking and making of multiple hydrogen bonds, [1] or isomerisation in proline residues.[2]The lifetimes of the populations of ordinary Zeeman eigenstates, i.e., the well-known longitudinal relaxation times T 1 , are limited by various anisotropic interactions that are modulated by molecular tumbling. For spins with I = 1 = 2 , the dominant fluctuating interactions are dipole-dipole couplings and anisotropic chemical shifts. An obvious way to limit these magnetic interactions, and thereby extend the lifetimes of coherences, is to use spins with low gyromagnetic ratios ('heteronuclei'), [1] such as nitrogen-15. Methods have been developed to use heteronuclei in deuterated samples for resonance assignment, [3][4][5] opening the way for structural studies of paramagnetic proteins, [6] protein-protein interactions, [7] intrinsically disordered proteins, [8] slow diffusion, [9] protein dynamics, [10] and slow exchange. A more elegant approach is based on 'longlived states' (LLS), which in isolated two-spin systems are immune to relaxation mechanisms that are symmetric with respect to spin-exchange. [11,12] It was demonstrated that such states can be excited by appropriate pulse sequences and sustained either by a suitable radio-frequency (rf) irradiation or by moving the sample out of the static field. The populations of these states have relaxation time constants T LLS that can be much longer than T 1 . Ratios as large as T LLS /T 1 = 37 have been observed in partly deuterated saccharides. The longest lifetime recorded so far is T LLS = 1583 s = 26 min (T LLS /T 1 = 8), observed [13] in 15 N 2 O, using LLS involving 'heteronuclei'. The resulting longlived states offer very useful tools to follow slow exchange, [14] flow or diffusion. [15,16] Applications of long-lived states should become increasingly widespread provided they can be excited in systems featuring diverse spin patterns with more than two coupled spins. Alternatively, since the main (dipolar) interaction is silenced only for the pair of spins participating in the antisymmetric state, extended lifetimes can be obtained by substituting all remaining non-participating protons within a sphere of ca 5 by deuterons. [17] Significant progress has been made towards extending the range of applications of LLS. Long lifetimes have been obtained in symmetric molecules comprising four coupled spins. Thus, Pileio et al. measured ratios T LLS /T 1 = 8 in citric acid. [18] Pileio et al. [19] have also shown how the eigenvalues of the Liouvillian can be analysed to identify long-lived states by diagonalisation, also proposed by Gopalakrishnan et al. [20] This met...