Conformation of poly( L ‐homoarginine)

Poly( L ‐arginine) assumes the α‐helix in the presence of the tetrahedral‐type anions or some polyanions by forming the “ringed‐structure bridge” between guanidinium groups and anions which is stabilized by a pair of hydrogen bonds and electrostatic interaction [Ichimura, S., Mita, K. & Zama, M. (1978) Biopolymers 17 , 2769–2782; Mita, K., Ichimura, S. & Zama, M. (1978) Biopolymers 17 , 2783–2798]. This paper describes the parallel CD studies on the conformational effects on poly ( L ‐homoarginine) of various mono‐, di‐, polyvalent anions and some polyanions, as well as alcohol and sodium dodecylsulfate. The random coil to α‐helix transition of poly( L ‐homoarginine) occurred only in NaClO 4 solution or in the presence of high content of ethanol or methanol. The divalent and polyvalent anions of the tetrahedral type (SO   4 2− , HPO   4 2− , and P 2 O   7 4− ), which are strong α‐helix‐forming agents for poly( L ‐arginine), failed to induce the α‐helical conformation of poly( L ‐homoarginine). By complexing with poly( L ‐glutamic acid) or with polyacrylate, which is also a strong α‐helix‐forming agent for poly( L ‐arginine), poly( L ‐homoarginine) only partially formed the α‐helical conformation. Monovalent anions (OH − , Cl − , F − , and H 2 PO   4 − ) did not change poly( L ‐homoarginine) to the α‐helix, and in the range of pH 2–11, the polypeptide remained in an unordered conformation. In sodium dodecylsulfate, poly( L ‐homoarginine) exhibited the remarkably enlarged CD spectrum of an extended conformation, while poly( L ‐arginine) forms the α‐helix by interacting with the agent. Thus poly( L ‐homoarginine), compared with poly( L ‐arginine), has a much lower ability to form the α‐helical conformation by interacting with anions. The stronger hydrophobicity of homoarginine residue in comparison with the arginine residue would provide unfavorable conditions to maintain the α‐helical conformation.