Conformational studies of alternating copoly(Leu-Lys) and copoly(Leu-Orn) in alcohol-water solvent mixtures

Conformational transitions of alternating copoly(l-leucyl-l-lysine) and copoly(l-leucyl-l-ornithine) in organic solvents and in alcohol-water mixtures were determined by c.d. measurements and the results compared with those from random copoly(Leu^48.3, Lys^51.7). As reported previously^16,17, in salt-free water these alternating copolymers undergo a conformational transition from a disordered to β-structure when the pH is raised or when various salts are added, whereas random copolymers adopt an α-helix conformation under similar conditions. However, both alternating copolymers reveal a tendency to form α-helix in 2,2,2-trifluoroethanol and in alcohol-water mixtures at neutral pH, as does the random copolymer. The alcohol concentration at which the α-helix can be induced is dependent on the kind of alcohol, the α-helix promoting power follows the the series: 2,2,2-trifluoroethanol > isopropanol > ethanol > methanol. In addition, these alternating copolymers in methanol-water mixtures below 50% (by volume) methanol form the β-structure when the pH is raised. On the other hand, above 60% methanol the fraction of α-helix already formed at neutral pH is enhanced at higher pH-values.