  The experimental data on F2 structure functions of the proton and deuteron
were used to construct their moments. In particular, recent measurements
performed with CLAS detector at Jefferson Lab allowed to extend our knowledge
of structure functions in the large-x region. The phenomenological analysis of
these experimental moments in terms of the Operator Product Expansion permitted
to separate the leading and higher twist contributions. Applying nuclear
corrections to extracted deuteron moments we obtained the contribution of the
neutron. Combining leading twist moments of the neutron and proton we found d/u
ratio at x->1 approaching 0, although 1/5 value could not be excluded. The
twist expansion analysis suggests that the contamination of higher twists
influences the extraction of the d/u ratio at x->1 even at Q2-scale as large as
12 (GeV/c)^2.
