PHASE-TRANSITIONS IN A MANY-LETTER RANDOM HETEROPOLYMER

A ''many-letter'' sequence model of random heteropolymers is introduce d and studied. We examine the case where there is a tendency for phase separation with respect to certain types of interactions as in the ca se of hydrophobic and hydrophyllic interactions in proteins. It is sho wn that a freezing transition to a state where a few conformations are thermodynamically dominant can prevent microphase separation even for flexible chains if enough letters are used in the sequence. The numbe r of these letters is independent of the length of the chain. It is al so found that a microphase separation with respect to a stronger type of interaction prevents microphase separation with respect to any othe r weaker type.