We have realized a new porous metal-organic framework UTSA-76a with pyrimidine groups on the linker, exhibiting high volumetric methane uptake of ~260 cm(3) (STP) cm(-3) at 298 K and 65 bar, and record high working capacity of ~200 cm(3) (STP) cm(-3) (between 5 and 65 bar). Such exceptionally high working capacity is attributed to the central "dynamic" pyrimidine groups within UTSA-76a, which are capable of adjusting their orientations to optimize the methane packing at high pressure, as revealed by computational studies and neutron scattering experiments.