Statistical study of linear heteropolymers

The understanding of the conformational transitionas of biopolymers is a fundamental problem of statistical physics. Protein as well as nucleic acid folding are the main puzzling problems of biological importance. In order to tackle such problems, we undertook a systematic study of polyampholytes so as to understand the relation between the sequence of charges and the observed conformations. The term polyampholyte (designed as PA hereafter) is a generic name describing a polymer chain bearing positively and negatively charged monomers in a given sequence. A coil-globule transition was observed in an alternate polyampholyte. It was recognized that alternate sequences were highly specific and that random sequences should not exhibit any coil-globule transition. For globally neutral sequences, which is the relevant situation to proteins, our general aim is to understand what, in a given sequence, is relevant to conformational transitions. Note that nucleic acids are no PA but polyelectrolytes (any nucleotide bears the same negative charge), so that the electrostatic interactions do not depend on the sequence of bases but merely on the ionic force. Diblocks and alternate sequences are the two extreme cases among random sequences. For an alternate PA, it is just 0 (except of course for the two monomers in the middle of the chain, but their contribution becomes negligible as the chain length grows). Although long diblocks are far from being the rule among biopolymers (and hereafter it will clearly appear why they don't exist in nature at ordinary temperature) they are nonetheless relevant to the physiccs of biopolymers for the following reason : any sequence of charges generally contains small diblocks spread throughout the chain. The longer the chain, the longer the diblocks. These small diblocks should undergo the earliest transitions (i. e. at the highest temperatures). It seems therefore important to understand the nature of the transitions occurring in diblocks as a function of their size. This is the main issue of the present work.