We present a formalism which turns out to be very successful in the description of the polypeptide folding.We consider this process as a first-order phase transition and develop a theory which is free of model parameters and is based solely on fundamental physical principles. It describes essential thermodynamical properties of the system such as heat capacity, the phase transition temperature and others from the analysis of the polypeptide potential energy surface calculated within ab initio density functional theory and parameterized by two dihedral angles. This problem is viewed as a major breakthrough in the theoretical understanding of the protein folding process. Our conclusion is based on the comparison of the predictions of our theory with the results of several independent experiments.