  We investigate the vibrational density of states (vDOS) in harmonic
approximation of a binary mixture of colloidal patchy particles with two and
three patches for different relative compositions $x_2$. At low temperature,
this system forms a thermo-reversible gel, i.e. a fully bonded network of
chains of two-patches particles, in which the branching points are provided by
three-patches particles. For all the compositions we find in the vDOS a
pronounced peak at low frequency whose height grows on increasing the fraction
of two-functional particles, or equivalently with the average length of the
chains. To identify the various spectral features, we compare the vDOS of the
whole system with the one of small representative structures of the network and
with the vDOS of a long linear chain of two-patches particles and we find that
these structures are indeed able to rationalize the various peaks in the vDOS
of the full system. At large $x_2$ the vDOS of the gel and of the long chain
show remarkable similarities. Analyzing the dispersion relations and the
spectrum of the linear chain we show that the excess of low frequency modes,
the analog of the boson peak in glassy disordered systems, arises from the
strong coupling between rotations and translations.
