RANDOM HYDROPHILIC-HYDROPHOBIC COPOLYMERS

We study a single statistical amphiphilic copolymer chain AB in a sele ctive solvent (e.g. water). Two situations are considered. In the anne aled case, hydrophilic (A) and hydrophobic (B) monomers are at local c hemical equilibrium and both the fraction of A monomers and their loca tion along the chain can vary, whereas in the quenched case (which is relevant to proteins), the chemical sequence along the chain is fixed by synthesis. In both cases, the physical behaviour depends on the ave rage hydrophobicity of the polymer chain. For a strongly hydrophobic c hain (large fraction of B), we find an ordinary continuous theta colla pse, with a large conformational entropy in the collapsed phase. For a weakly hydrophobic, or a hydrophilic chain, there is an unusual first -order collapse transition. In particular, for the case of Gaussian di sorder, this discontinuous transition is driven by a change of sign of the third virial coefficient. The entropy of this collapsed phase is strongly reduced with respect to the theta collapsed phase.