PHASE-TRANSITIONS IN A MANY-LETTER RANDOM HETEROPOLYMER

A ''many-letter'' sequence model of random heteropolymers is introduced and studied. We examine the case where there is a tendency for phase separation with respect to certain types of interactions as in the case of hydrophobic and hydrophyllic interactions in proteins. It is shown 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 number of these letters is independent of the length of the chain. It is also found that a microphase separation with respect to a stronger type of interaction prevents microphase separation with respect to any other weaker type.