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Salt bridge induced changes in the secondary structure of ionic polypeptides
Author(s) -
Ismail Ashraf A.,
Mantsch Henry H.
Publication year - 1992
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360320907
Subject(s) - chemistry , salt bridge , ionic bonding , bridge (graph theory) , salt (chemistry) , biophysics , ion , biochemistry , organic chemistry , anatomy , medicine , biology , mutant , gene
The carboxylate‐containing homopolypeptides poly( L ‐glutamate) [poly(Glu)] and poly( L ‐aspartate) [poly(Asp)] were found to form different types of ordered structures in the presence of poly( L ‐lysine) [poly(Lys)]. Mixing poly(Glu) with poly(Lys) in aqueous solution at neutral pH results in the instantaneous formation of a gel‐like precipitate. The secondary structure of the gel precipitate can be best described as intermolecular antiparallel β‐strands, involving the backbone amide groups, as evidenced by the presence of characteristic amide I bands in the ir spectrum at 1684 and 1612 cm −1 . Mixing poly (Asp) with poly(Lys) under identical conditions results in the formation of a fine precipitate with a different morphology. Examination of the ir spectrum of the precipitate revealed that unlike poly(Glu), poly (Asp) did not yield any discrete secondary structure upon precipitation with poly(Lys). Addition of solutions containing Ca 2+ or Mg 2+ to the poly (Glu)/poly (Lys) aggregates resulted in complete dissolution of the gel, with the disappearance of the ir bands characteristic of the intermolecular hydrogen‐bonded network. The results demonstrate the importance of salt bridges in establishing strong hydrogen bonds between the backbone amide groups. Reaggregation occurred upon heating the poly (Glu)/poly (Lys) mixture in the presence of Ca 2+ , but not in the presence of Mg 2+ ions. In the presence of Ca 2+ ions, aggregation and formation of an extended hydrogen‐bonded network occurred upon heating. The aggregates formed upon heating poly (Glu)/poly (Lys) in the presence of Ca 2+ were attributed solely to complexation of Ca 2+ to the carboxylate groups of poly (Glu) with poly (Lys) remaining free in solution. Dissolution of the aggregate could be accomplished through addition of Mg 2+ at room temperature. © 1992 John Wiley & Sons, Inc.