  We investigate the structure of the dilatation operator D of planar N=4 SYM
in the sector of single trace operators built out of two chiral combinations of
the 6 scalars. Previous results at low orders in `t Hooft coupling \lambda
suggest that D has the form of an SU(2) spin chain Hamiltonian with long range
multiple spin interactions. Instead of the usual perturbative expansion in
powers of \lambda, we split D into parts D^(n) according to the number n of
independent pairwise interactions between spins at different sites. We
determine the coefficients of spin-spin interaction terms in D^(1) by imposing
the condition of regularity of a BMN-type scaling limit. For long spin chains,
these coefficients turn out to be expressible in terms of hypergeometric
functions of \lambda, which have regular expansions at both small and large
values of \lambda. This suggest that anomalous dimensions of ``long'' operators
in the two-scalar sector should generically scale as square root of \lambda at
large coupling, i.e. in the same way as energies of semiclassical states in
dual AdS5 x S5 string theory. We speculate that D^(1) may be a Hamiltonian of a
new integrable spin chain.
