Competition for hydrogen-bond formation in the helix-coil transition and protein folding

The problem of the helix-coil transition in explicit solvents with the ability for hydrogen bonding, like water, is addressed analytically using spin models. Besides the regular helix-coil transition, an additional coil-helix or reentrant transition is also found at lower temperatures. The reentrant transition arises due to competition between polymer-polymer and polymer-water hydrogen bonds. The balance between the two types of hydrogen bonding can be shifted to either direction through changes not only in temperature, but also by pressure, mechanical force, osmotic stress or other external influences. Both polypeptides and polynucleotides are considered within a unified formalism. Our approach allows to explain the experimental difficulty of reentrant transition observation with the pressure and shows the origin of advantage of pulling experiments for DNA. The results are discussed and compared with a number of recent publications where a significant level of agreement is obtained.