  We report on resonance Raman spectroscopy measurements with excitation photon
energy down to 1.16 eV on graphene, to study how low-energy carriers interact
with lattice vibrations. Thanks to the excitation energy close to the Dirac
point at $\mathbf{K}$, we unveil a giant increase of the intensity ratio
between the double-resonant 2D and 2D$^\prime$ peaks with respect to that
measured in graphite. Comparing with fully \textit{ab initio} theoretical
calculations, we conclude that the observation is explained by an enhanced,
momentum-dependent coupling between electrons and Brillouin zone-boundary
optical phonons. This finding applies to two dimensional Dirac systems and has
important consequences for the modeling of transport in graphene devices
operating at room temperature.
