As the temperature increases, protein will become less viscous. At this point the protein molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form and as a results, a peak appeared in the DSC signal (First peak- Denaturaion temperature). In this state the protein chains can refold themselves. As the temperature increases further, the protein eventually reaches its melting temperature (Tm) (a second point on the stability curve where free energy change ΔG is equal to zero). At this point the protein molecules disappear into amorphous state and the protein chains cannot refold themselves.

This paper (below link) may provide you with satisfactory answer

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2374017/

Melting temperature has an exact definition ΔGfolding(Tm) = 0 - in other words, the temperature at which the free energy of the unfolded and folded states is equal and half of the population is unfolded and the other half is folded. Similar definitions exist for proteins with folding intermediates.

I don't know about a "denaturation temperature" , I have only seen it used as a synonym for Tm. My guess is that hey are referring to the temperature dependence of the protein activity.The two curves are different because of the temperature dependence of the reaction itself and the possibility that some protein are not two state folding systems (the protein can partially unfold and lose activity).

As the temperature increases, protein will become less viscous. At this point the protein molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form and as a results, a peak appeared in the DSC signal (First peak- Denaturaion temperature). In this state the protein chains can refold themselves. As the temperature increases further, the protein eventually reaches its melting temperature (Tm) (a second point on the stability curve where free energy change ΔG is equal to zero). At this point the protein molecules disappear into amorphous state and the protein chains cannot refold themselves.

Dear friends,

thanks for your answers. At last i got the answer.

Just wanted to add a point to the excellent answers above. Protein denaturation is considered irreversible, whereas protein melting is reversible.

The denaturation by definition is any change generated in the three-dimensional structure of a protein. This change may be irreversible in some cases (boiled egg) or be reversible as the case of RNase in 8.0 M urea (when is dialyzed)