Circular dichroic and immunological properties of human choriogonadotropin‐β carboxyl terminal peptides

Circular dichroic spectra have been obtained in aqueous solution and in trifluoro-ethanol for several synthetic (non-glycosylated) human choriogonadotropin carboxyl terminal peptides of the beta-subunit ranging in size from 10 residues to 40 residues. These was no evidence for formation of alpha-helicity or beta-structure, but the spectra in 90% (v/v) trifluoroethanol were consistent with the occurrence of beta-turns. The Chou-fasman predictive rules also suggest a high probability of beta-turns in these peptides which could result in the occurrence of repeating kinks. Disulfide-linked dimers were also investigated by circular dichroism, and there was evidence of stabilization of particular skewness of the disulfide dihedral angle depending upon the location of the disulfide bond. The single phenylalanyl residue at position 115 in the beta-subunit also contributed to the circular dichroic spectra above 250 nm. Antibodies raised to a peptide consisting of residues 111-145 have been shown to contain two immunological determinants, but the sum of antibodies raised to separate determinant sequences do not equal those raised to the full length peptide. These data could reflect the existence of a conformation-related determinant on the 111-145 peptide or stearic hindrance of immunoglobulin binding of two antibodies to the same peptide.