GdmCl is just a stronger denaturant than urea. It should normally give higher and lower values for m and for Cm, respectively, but identical deltaG° values. Since m values in both denaturants can be predicted/estimated from the size of the protein (Myers et al., 1995) and delatG° should not change, yes it is possible - in principle - to predict Cm in one denaturant, knowing its value in the other.

Molar concentrations of chaotropic salts need to be corrected to denaturing concentration (D) for equivalence as they have variable activity (eg Guanidine isothiocyanate > GuHCl > urea > NaI) which can be expressed in nominal terms as molar according to the equation : D= C[Salt]/(C + [Salt]). The value C is empirically determined and for GuHCl this is 7.5, for urea this is 25.3. For a detailed explanation see Parker MJ et al, 1996, Biochemistry and Parker MJ, Spencer, J and Clarke AR, 1995 JMB.

GndHCl is charged while urea is neutral. This has an effect on the stability of a protein above and beyond its effect on water structure and its competition for H-bonds. The net charge of the protein and it's pH stability will relate to Gnd unfolding but not so much to urea unfolding. That is, if the protein is highly charged, with stabilizing salt bridges, then it will be more destabilized by GndHCl than by the same amount of urea.

GdnHCl is a better denaturant than urea but to have a general correlation co-efficient is almost impossible as every protein has unique folding. Though such co-relation may be found for a special protein fold, but what purpose it may serve apart from giving a tentative idea that GdnHCl or urea may be a better denaturant! If you are looking for even better denaturant than Gdn HCl then use GdnSCN, it is better and more effective.

As a rule of thumb. at the same concentration, urea has approx. only half the denaturating effect that GdnHCl has.

In GndHCl solution can I still determine concentration of protein with nano drop? (like I do with Urea)

As per my experience, there is no such specific correlation between denaturing ability of urea and GdnCl or any other denaturant. It depends on what kind of interactions are stabilizing your protein of interest and the nature of denaturant. If your protein is a membrane protein, than it would be difficult to be denatured by SDS, as membrane protein love the membrane like environment of SDS solution. I am working on one hyperthermophilic protein, and it exhibits no denaturation upto 8M Urea. However, it gets completely denatured at 3.25M GdnCl.  Here the ionic nature of GdnCl comes into play which destabilizes salt bridges of hyperthermophilic protein (which are more prominent). 

I think I am late in providing the answers. However, I give a try.

People have already said that urea and GdnHCl are two different kinds of denaturants. One is polar but non-ionic (urea) and another ionic in nature. Since, proteins are stabilized by a range of non-covalent interactions, difference in chemical nature of both is very important.

Now coming to first question. It is still not completely understood why both denaturants behave as denaturants. Literature argues that urea unfolds protein by providing hydrogen bonding to the unfolded state (urea is very similar to peptide backbone and hence, breaking of intrachain hydrogen bond can be stabilized by urea-protein hydrogen bond). However, such thing cannot be said for GdnHCl. Again, literature suggest that GdnHCl modulates the water dynamics as well as it can preferentially interact with side chains in proteins and unfolds protein. REFF (England, J.L., and Haran, G. (2011). Role of solvation effects in protein denaturation: from thermodynamics to single molecules and back. Annual review of physical chemistry 62, 257-277).

Coming to your other questions, Myers et al#. studied 22 proteins and found that urea Cm value is generally 2.25 ± 0.38 times the GdnHCl Cm values. REFF ( Myers, J. K.; Pace, C. N.; Scholtz, J. M. Denaturant m-Values

and Heat Capacity Changes: Relation to Changes in Accessible Surface Areas of Protein Unfolding. Protein Sci. 1995, 4, 2138−2148). Hence, one can theoretically know from the urea denaturation curve what would be the GdnHCl Cm values.

Hope I answered your questions

Hey Marcin,

I too work on these two denaturants and they both have different effects on the protein denaturation. And as a matter of fact you can determine the protein concentration in both these solutions using the Nanodrop.

I prefer 6M Gdn.HCl and 8M urea.