Singular behavior of the reference interaction site model theory observed for peptide in salt solution

We examine the reference interaction site model theory applied to a peptide–salt solution system, with the assumption that a zwitterionic dipeptide (Ala–Ala) is present in NaCl solution at the infinite-dilution limit. For some sets of the parameters in the peptide–cation and peptide–anion potentials, the theory exhibits singular behavior that can be interpreted as a signal of the ion condensation with a trend of aggregation of peptide molecules. For other sets of the potential parameters, however, no such singularity occurs. The qualitative aspects of the conclusions are somewhat sensitive to the potential parameters employed.