In previous studies employing dDNP as the polarization technique, the doubly 13 C-labelled isotopomer [1,4-13 C2]fumarate was used. Here, the [1- 13 C] isotopomer was used, which means the maximum achievable 13 C signal is lower by a factor of 2. There are unitary bounds on the achievable polarization transfer from proton singlet order into 13 C magnetization, which depend on the symmetry of the spin system . Assuming an initial state of pure proton singlet order, the maximum possible transfer into X-spin ( 13 C) polarization for the [1-13 C] isotopomer, an AA'X spin system (Pople notation), is 1, corresponding to 100% 13 C polarization. However, for the [1,4-13 C2] isotopomer, an AA'XX' spin system, the 13 C polarization of each is limited to 50%. This therefore gives the same observable 13 C signal enhancement as using the [1-13 C] isotopomer. This limitation cannot be overcome without further reducing the symmetry of the spin system. It could still be beneficial to use the [1,4-13 C2] isotopomer, because the relaxation properties of the spins should differ based on the symmetry, which might yield higher overall signal enhancements.