  We apply the Lifshitz theory of dispersion forces to find a contribution to
the free energy of peptide films which is caused by the zero-point and thermal
fluctuations of the electromagnetic field. For this purpose, using available
information about the imaginary parts of dielectric permittivity of peptides,
the analytic representation for permittivity of typical peptide along the
imaginary frequency axis is devised. Numerical computations of the
fluctuation-induced free energy are performed at room temperature for the
freestanding peptide films, containing different fractions of water, and for
similar films deposited on dielectric (SiO$_2$) and metal (Au) substrates. It
is shown that the free energy of a freestanding peptide film is negative and,
thus, contributes to its stability. The magnitude of the free energy increases
with increasing fraction of water and decreases with increasing thickness of a
film. For peptide films deposited on a dielectric substrate the free energy is
nonmonotonous. It is negative for thicker than 100 nm films, reaches the
maximum value at some film thickness, but vanishes and changes its sign for
thinner than 100 nm films. The fluctuation-induced free energy of peptide films
deposited on metallic substrate is found to be positive which makes films less
stable. In all three cases, simple analytic expressions for the free energy of
sufficiently thick films are found. The obtained results may be useful to
attain film stability in the next generation of organic microdevices with
further shrinked dimensions.

Applied Physics;Materials Science