title: Synthesis and acid ionization constants of cyclic cystine peptides H-Cys-(Gly)n-Cys-OH (n = 0-4).
abstract: Cyclic peptide disulfides of the general formula H-Cys-(Gly)n-Cys-OH (n = 0-4) were synthesized from the corresponding peptide derivatives [Boc-Cys(Trt)(Gly)-n-Cys(Trt)-OBut] by oxidation with iodine in methanol and by subsequent removal of the terminal groups with trifluoroacetic acid. Acid ionization constants of the obtained peptides were determined by potentiometric titration in aqueous KCl (0.1 mol/L) medium. All compounds have two dissociable hydrogens, corresponding to carboxyl (pK1 = 2.35-2.84) and to terminal amino group (pK2 = 5.61-6.93); pK1 values show first an upward and then a downward trend with the increase in ring size; the opposite is true for pK2 values. These trends could be tentatively attributed to the intramolecular salt bridge (-COO- ----NH+3-) formation.
authors.affiliation: Department of Organic Chemistry and Biochemistry, Rudjer Bosković Institute, Croatia, Yugoslavia.
authors.forename: S
authors.initials: S
authors.lastname: Horvat
authors.forename: B
authors.initials: B
authors.lastname: Grgas
authors.forename: N
authors.initials: N
authors.lastname: Raos
authors.forename: V
authors.initials: V
authors.lastname: Simeon
mesh_terms: D000595:Amino Acid Sequence; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003553:Cystine / Q000032:analysis*; D004220:Disulfides; D005563:Formic Acid Esters / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D008969:Molecular Sequence Data; D010456:Peptides, Cyclic / Q000032:analysis / Q000138:chemical synthesis*
chemical_list: D004220:Disulfides; D005563:Formic Acid Esters; D007202:Indicators and Reagents; D010456:Peptides, Cyclic; C000345:t-butyloxycarbonyl group; D003553:Cystine