As pointed out above, one of the most surprising features of the pH-dependence of T S is that the transition from fast to slow relaxation occurs around the pH-value of 11, which is far above the pKa values of the acid-base equilibria for this system. The two spins supporting the nuclear singlet state may find themselves in two different molecular environments A (red) and B (blue). This change may be induced upon binding of an otherwise NMR inactive partner (grey ball). The exchange dynamics are described by an exchange matrix