The shape it adopts on binding

Energy calculations and structural work showed the p53-derived segment folding into essentially the same shape that part of p53 takes when it binds HDM-2 normally, with the penetratin leader pointing away from the complex and lining the pore. So this is not a crude detergent effect: the peptide is recognised as p53 would be, then its tail does the structural work. That explains why the intact 32 residue construct is required and why fragments behave differently.