In order to compensate for field homogeneity effects, the individual pulses for the M2S and S2M sequence were replaced by composite pulse counterparts. In detail, 180 φ pulses were replaced by 90 90+φ 180 φ 90 90+φ sequences; 90 φ pulses were replaced by 180 97.2+φ 360 291.5+φ 180 97.2+φ 90 φ sequences . The free evolution delays were adjusted to account for the duration of the elongated pulse elements. For example, echo delays were shortened by half the duration of the refocusing element. The 13 C shifts may be interpreted qualitatively by assuming rapid exchange between the protonated and deprotonated forms in figure . If the exchange is assumed to be rapid relative to the 13 C chemical shift changes, the observed C chemical shift corresponds to the mean value for the exchanging species, weighted by their populations. The pH-dependence is weak above pH ∼ 7 since the deprotonated Sq 2-species dominates in this regime.