A protein was purified to homo- geneity. Determination of the mass by gel-filtration chro- matography yields 60 kDa. Chromatography in the pres- ence of 6 M urea yields a 30-kDa species. When the chro- matography is repeated in the presence of 6 M urea and 10 mM b-mercaptoethanol, a single molecular species of 15 kDa results. Describe the structure of the molecule.

In this question, the given protein mass (60 kd) is derived from the process of gel filtration chromatography. This process hat is used, differentiate proteins based on its size. The configuration of the protein could be tertiary or quarternary, thus, the protein uses more time to pass through the gel, and be noticed as a big protein of 60 kd.  

When the procedure is undertaken in the presence of 6 M urea, a 30 kd species is produced, due to urea acting as a denaturing component to transform the proteins to be in its original state.

If the actual technique is done in the presence of 6 M urea and 10 mM Beta-mercaptoethanol, a 15 kd size of protein is produced. This is mainly due to the reaction of beta-mercaptoethanol separating or excluding disulfide bonds that exist in the amino acids.  

Therefore, it can be generalized that the protein possessed disulfide bonds that interplay to link two polypeptide chains of 15 kd each.