  The free energy of globular protein chain is considered to be a functional
defined on smooth curves in three dimensional Euclidean space. From the
requirement of geometrical invariance, together with basic facts on
conformation of helical proteins and dynamical characteristics of the protein
chains, we are able to determine, in a unique way, the exact form of the free
energy functional. Namely, the free energy density should be a linear function
of the curvature of curves on which the free energy functional is defined. This
model can be used, for example, in Monte Carlo simulations of exhaustive
searching the native stable state of the protein chain.
