SynopsisThe theory of long-range interactions in macromolecules is proposed for the region where the attraction forces predominate (that is below 8-point). In an approximate analytical variant of the theory the free energy of segment-segment interactions is calculated without taking into account their bonding in the single chain, that is, by means of strict and approximate equation for the free energy of interaction of real gas molecules. The theory predicts that under some conditions the macromolecule undergoes below the s-point, during the change of temperature or solvent composition, the first-order phase transition from the statistically coiled state to the compact (globular) state. The transition predicted by the theory is similar to the. condensation of the real gas cloud to the liquid drop. This result is confirmed by the more strict Monte Carlo calculation which takes into account the bonding of the segments in a single chain. This calculation not only confirms the existence of a drastic compression of the macromolecule, when the attractive forces between its segments increase, but show also that, in the transition region, the macromolecules divide into poorly coiled chains (statistical coils) and highly coiled chains (globula), which directly prove the phase character of the transition. In common macromolecules the "intramolecular condensation" predicted by the theory must occur near to the point of intermolecular condensation so it is difficult to observe it experimentally. However, in .rnacromolecules containing both lyophobic and lyophilic groups the intramolecular condensation may well occur due to the mutual attraction of lyophobic groups and lyophilic groups covering the surfaces of obtained globulas may prevent them from aggregation. In these macromolecules one can observe the intermolecular globula-coil phase transitions by changing the temperature or the solvent composition. It is most likely that the denaturation (and renaturation) of the globular proteins are the examples of such transitions.

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