  It is shown that the contribution of the instantons to the fragmentation of
quarks leads to the appearance of a imaginary part in diagrams of quark-quark
scattering at large transfer momentum. The imaginary part comes from the
analytical continuation of the instanton amplitudes from Euclidean to Minkowsky
space-time and reflects quasiclassical origin of instanton solution of QCD
equations of motion. This phenomenon and instanton--induced quark spin-flip
give a new nonperturbative mechanism for the observed anomalous single-spin
asymmetries in hadron-hadron and lepton-hadron interactions.
