  We investigate the low-temperature magnetic properties of the molecule-based
chiral spin chain [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O (pym = pyrimidine).
Electron-spin resonance, magnetometry and heat capacity measurements reveal the
presence of staggered $g$ tensors, a rich low-temperature excitation spectrum,
a staggered susceptibility and a spin gap that opens on the application of a
magnetic field. These phenomena are reminiscent of those previously observed in
non-chiral staggered chains, which are explicable within the sine-Gordon
quantum-field theory. In the present case, however, although the sine-Gordon
model accounts well for the form of the temperature-dependence of the heat
capacity, the size of the gap and its measured linear field dependence do not
fit with the sine-Gordon theory as it stands. We propose that the differences
arise due to additional terms in the Hamiltonian resulting from the chiral
structure of [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O, particularly a uniform
Dzyaloshinskii-Moriya coupling and a four-fold periodic staggered field.
