Nucleon Structure and High Energy Scattering
  If the pomeron is generated by a two gluon exchange there is no a priori reason for a drastic suppression of three gluon exchange with negative parity and charge parity. This would lead to an unacceptably large difference between $p\,p$ and $p\, \bar{p}$ scattering. It is shown that a natural suppression of the $C$=$P$=--1 contribution to high energy scattering is given by a cluster structure of the nucleon. 
Michael Rueter, H.G.Dosch (Institut fuer Theor. Physik, Univ.   Heidelberg)