  The ratio between the anomalous magnetic moments of proton and neutron has
been recently parametrized by the ratio of proton momentum fractions
$M_{2}^{q_{val}}$. This ratio is evaluated using different constituent quark
models, starting from the CQM density distributions and calculating the next-to
leading order distributions. We show that this momentum fractions
$M_{2}^{q_{val}}$ ratio is a sensitive test for SU(6)-breaking effects and
therefore it is useful to distinguish among different CQMs. We investigate also
the possibility of getting constraints on the formulation of quark structure
models.
