Aprotein was purified to homogeneity. determination of the mass by gel filtration chromatography yields 60 kda. chromatography in the presence of 6 m urea yields a 30 kda species. when the chromatography is repeated in the presence of 6 m urea and 10 mm beta-mercaptoethanol, a single molecular species of 15 kda results

In the given case, the mass of the protein mentioned is 60 kd obtained by the procedure of gel filtration chromatography. This procedure distinguishes proteins on the bases of their size. The configuration of the protein in the given case may be either tertiary or quarternary, therefore, the protein consumes more time to go through the gel, and thus gets observed as a large protein of 60 kd.  

When the technique is performed in the existence of 6 M urea, a 30 kd species results, as urea is a denaturing component and it causes the proteins to be in its native state. While when the same procedure is performed in the existence of 6 M urea and 10 mM Beta-mercaptoethanol, a 15 kd size of protein results. This is because beta-mercaptoethanol dissociates disulfide bonds between the amino acids.  

Hence, it can be concluded that the protein exhibited disulfide bonds that associate two polypeptide chains of 15 kd each.