Flavour mixing and spin of the proton

We study the proton matrix elements of the flavour singlet axial current using an extended three-flavour Nambu-Jona-Lasinio model that includes not only an axial anomaly interaction but also a flavour-mixing four-quark interaction in the axial-vector channel. The pseudoscalar meson spectrum gives strong constraints on the strength of this interaction. We investigate its influence on the spin structure of constituent quarks in the proton.