New Amphiphilic Immunogens by Cu2+-Mediated Complexes of Water-Born Poly(N-Isopropylacrylamide) and Bovine Serum Albumin

Cu2+-mediated complex formation between temperature-responsive poly(N-isopropylacylamide-co-acrylic acid) (CP) and bovine serum albumin (BSA) was investigated in neutral and in physiological salt solutions. Water soluble or insoluble ternary complexes are formed upon addition of divalent copper ions to the solution. Stability of the ternary polycomplexes depends on the composition of the copolymers, concentration of copper ions and ratio of components. Stimuli-sensitive (temperature, pH- and ionic strength-) behavior of CP, CP-Cu2+ and CP-Cu2+-BSA solution was studied. The ternary CP-Cu2+-BSA complexes exhibit reversible temperature-responses in physiological salt solutions at suitable ratios of the components. Thermal collapse of ternary polycomplexes was observed in higher pH solutions than for CP and CP-Cu2+. The negative temperature solubility coefficient in salt containing solutions may be due to cooperative hydrophobic interactions. Cu2+ in 0.154 M NaCl solutions of CP with BSA, in contrast to CP-Cu2+ BSA complexes in water, gave increased BSA-specific immunogenicity. A single immunization of mice with ternary CP1-Cu2+-BSA complexes in physiological salt solutions without adjuvant increased the specific immune response to BSA. The highest level of immunogenic activity was after a second immunization which lasted more than 18 months. The highest immunogenic activity was exhibited by stable ternary complexes, and immunoactive polycomplexes with a negative temperature solubility coefficient. A hypothetical structural model for the formation of ternary polycomplexes and mechanism for the manifestation of immunological activity are presented.