  We present an extension of the Kirkwood-Shumaker (KS) theory of
proton-fluctuation interactions to situations where the perturbation theory,
usually invoked to derive these interactions, fails. In order to do that we
formulate a generalized theory of fluctuation interactions with non-linear
macroion (surface) free energy term that naturally leads to the long-range KS
interactions, but can also be straightforwardly generalized to situations where
the perturbation expansion ceases to be valid. It also allows for other surface
free energy terms to be treated within the same formalism, enabling us to
derive the complete fluctuation interaction with monopolar and dipolar
fluctuations included, leading to the KS theory as a limiting form.
