  We estimate the twist-4 spin-1 nucleon matrix element f_2 in an
instanton-based description of the QCD vacuum. In addition to the
flavor-nonsinglet we compute also the flavor-singlet matrix element, which
appears in next-to-leading order of the (1/N_c)-expansion. The corresponding
twist-3 spin-2 matrix elements d_2 are suppressed in the packing fraction of
the instanton medium, (\bar \rho)/(\bar R) << 1. We use our results to estimate
the leading (1/Q^2) power corrections to the first moment of the proton and
neutron spin structure functions G_1, as well as the intrinsic charm
contribution to the nucleon spin.
