SEQUENTIAL COMPACTION OF A RANDOM COPOLYMER OF HYDROPHILIC AND HYDROPHOBIC AMINO-ACID-RESIDUES

The behavior of a high molecular weight(48.8 x 10(3)) synthetic random copolymer composed of Glu, Leu, and Trp residues in molar ratio 82.5: 16:1.5 under compaction and unfolding conditions has been studied usin g circular dichroism, fluorescence spectroscopy, viscometry, and size- exclusion chromatography (FPLC). It has been shown that the decrease o f the pH value leads to two-stage compaction of the copolymer molecule s. This means the formation of an intermediate conformational state. S tructural properties and stability of the revealed conformations (comp act state and less compact intermediate) of the copolymer toward tempe rature and urea-induced unfolding have also been investigated. The com pact conformation of the macromolecules is characterized by high resis tance toward urea action (at 21.5 degrees C), whereas the transition f rom intermediate (less compact) state to coil-like conformation can ea sily be accomplished by the addition of small amounts of urea (at 21.5 degrees C). Melting of the compact state in the presence of urea is a two-stage process as well. This fact also confirms the existence of a folding intermediate.