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 $P\rightarrow\infty$ 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. 
N.L. Ter-Isaakyan