Spin dependent structure function $g_2(x)$ in quark-parton model. Possible interpretation and numerical estimates

It is shown that in the special infinite momentum frame where photon has pure transverse components at $P\to \mathrm{\infty }$ the spin-dependent deep inelastic structure function $g_2(x)$ has a reasonable interpretation in terms of quark-parton wave functions, whereas in the conventional frame where photon has pure z-component the parton model fails for $g_2(x)$.The spin dependent structure functions $g_1(x)$ and $g_2(x)$ have been calculated in the relativistic quark model constructed in such frame. The results indicate significant twist-3 contribution.