  We analyze the problem of the helix-coil transition in explicit solvents
analytically by using spin-based models incorporating two different mechanisms
of solvent action: explicit solvent action through the formation of
solvent-polymer hydrogen bonds that can compete with the intrinsic
intra-polymer hydrogen bonded configurations (competing interactions) and
implicit solvent action, where the solvent-polymer interactions tune biopolymer
configurations by changing the activity of the solvent (non-competing
interactions). The overall spin Hamiltonian is comprised of three terms: the
background \emph{in vacuo} Hamiltonian of the "Generalized Model of Polypeptide
Chain" type and two additive terms that account for the two above mechanisms of
solvent action. We show that on this level the solvent degrees of freedom can
be {\sl explicitly} and {\sl exactly} traced over, the ensuing effective
partition function combining all the solvent effects in a unified framework. In
this way we are able to address helix-coil transitions for polypeptides,
proteins, and DNA, with different buffers and different external constraints.
Our spin-based effective Hamiltonian is applicable for treatment of such
diverse phenomena as cold denaturation, effects of osmotic pressure on the cold
and warm denaturation, complicated temperature dependence of the hydrophobic
effect as well as providing a conceptual base for understanding the behavior of
Intrinsically Disordered Proteins and their analogues.
