Twist-4 contribution to unpolarized structure functions FL and F2 from instantons

We compute in the instanton vacuum the nucleon matrix elements of the twist-4 QCD operators describing power corrections to the second moments of the unpolarized structure functions, FL and F2 . Our approach takes into account the leading contribution in the packing fraction of the instanton medium, ρ-bar/R-bar<<1 . Parametrically leading are the matrix elements of a twist-4 quark-gluon operator, which are of the order of the inverse instanton size, 1/ρ-bar2=(600 MeV)2 . The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (ρ-bar/R-bar)4 and numerically small. These results are in agreement with the pattern of phenomenological 1/Q2 -corrections to R=σL/σT and F2 found in QCD fits to the data. In particular, the rise of R at low Q2 can be obtained from instanton-type vacuum fluctuations at a low scale