A highly conserved region of 21 amino acids flanked by cysteine residues, contained within a larger repeated domain, has been proposed to be the antibodybinding site in the ovarian cancer biomarker CA125 (MUC16). In this study solid-phase peptide synthesis with Fmoc protection chemistry was used to assemble a 21-mer peptide corresponding to the most frequently occurring antibody binding sequence in CA125. Potentially significant sequence variants were also synthesized. Peptide secondary structure was investigated using Fourier transform infrared spectroscopy, revealing the consensus sequence peptide to be largely unstructured at physiological pH whether the cysteine residues were reduced or were oxidized to form an intramolecular disulfide bond. Substitution of serine for proline at position 8 (P8S) results in β-sheet formation in peptides involved in intramolecular disulfide bonds. This β-sheet structure does not persist in peptides incapable of intramolecular disulfide bonding because of sequence nor in peptides treated with the reducing agent dithiothreitol. In CA125, P8S is predicted to occur in ∼25% of repeat domains, suggesting that this structural motif is a non-negligible contributor to overall structure and function. These findings suggest that future structural characterization efforts of CA125 should be especially mindful of the amino acid sequence and oxidation state of the protein.

Polymorphism in hybrid inorganic-organic materials has not been explored as extensively as that in organic compounds, yet differences in solid-state structure can significantly affect the physical properties central to application of these materials. A new polymorph of Ni(en) 3 MoS 4 (en = ethylenediamine), a hydrodesulfurization catalyst precursor, has been synthesized solvothermally and structurally characterized by single-crystal X-ray diffraction. The new structure (2) assumes the orthorhombic Pcab space group with a=14.020(5) A ˚, b=14.821(7) A ˚, and c=16.230(6) A ˚. The structure of a polymorph that had been found previously (1) was redetermined at 100 K, confirming the orthorhombic Pna2 1 structure with a=15.916(13) A ˚, b=7.610(3) A ˚, and c=14.093(6) A ˚. Solvothermal reaction conditions including temperature, solvent water content, and nickel source were important in controlling polymorph formation. Differential scanning calorimetry and solvent-mediated conversion studies were used to compare the stabilities of the two nickel-containing polymorphs. The system was characterized as enantiotropic, with 2 favored at ambient temperature and 1 favored at 120 °C. However, kinetic factors are influential in the intermediate temperature range, and conversion is kinetically hindered under certain conditions. The new structures Co-(en) 3 MoS 4 (3) and Mn(en) 3 MoS 4 (4) were determined through single-crystal methods to be isostructural to 2. Compounds 1-4 were also characterized by elemental analysis, infrared spectroscopy, variable-temperature magnetic susceptibility measurements, and thermogravimetric analysis.