US EPA

8417433 

Journal Article 

Protonation of Peptides. II. The Protonation of an Amide and of a Diamide in Dichloroacetic Acid, and the Behavior of Poly-γ-benzyl-L-glutamate in Dichloroacetic Acid and in Some Mixed Solvents 

Steigman, J; Verdini, AS; Montagner, C; Strasorier, L 

1969 

Yes 

Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 

91 

7 

1829-1836 

English 

10.1021/ja01035a039 

Ultraviolet absorption measurements were made of anisamide in dichloroacetic acid containing isopropylamine or sulfuric acid. Nmr measurements were made of the diamide N-benzoylglycine--propylamide in dichloroacetic acid-deuteriochloroform mixtures, and in dichloroacetic acid containing added sulfuric acid. It was concluded that neither amide was protonated in dichloroacetic acid solution, and that as a consequence, polypeptides like poly-γ-benzyl-L-glutamate exist in the coil conformation in dichloroacetic acid because of strong solvation rather than protonation. Optical titrations of poly-γ-benzyl-L-glutamate were carried out in mixtures of dichloroacetic acid with a number of cosolvents which brought about coil-to-helix transitions. The order of decreasing efficiency of these cosolvents in effecting the transitions was: triethylamine > acetic acid ≃ methanol > water ≃ nitrobenzene > nitroethane > formic acid > monochloroacetic acid ∼ cyanoacetic acid ∼ 1,2-di-chloroethane > carbon tetrachloride. The hypothesis was made that this order represented the abilities of the various donors to form hydrogen bonds with either dichloroacetic acid monomeric molecules or with the non-hydrogen-bonded terminal carboxyl group of a chain polymer of the solvent. In either event a shift in solvent polymer-dimer-monomer proportions would take place, reducing the concentration of the chain polymer, and reducing the solvation of the peptide groups of the polymer. © 1969, American Chemical Society. All rights reserved.