notes

Written (2004-04-16) by Jonathan Sheehan
Adapted from Pace and Sholtz¹

Purpose

When you really need to know the concentration of your urea or guanidinium HCl solutions, use this method. Especially GdmCl, because it's so hygroscopic, making weighing it out non- trivial.

Equipment/Materials

• You can borrow the refractometer from Laura Mizoue

Measurement

1. Use 200ul of water or buffer to get your baseline
   (should be near 1.333 at room temp.)
2. Use 200ul of your solution to get your unknown
   (clean well, then return to Laura)

Calculate concentrations

1. [GdmCl] = 57.147 (deltaN) + 38.68(deltaN)² - 91.60(deltaN)³
2. [Urea] = 117.66 (deltaN) + 29.753(deltaN)² + 185.56(deltaN)³
3. where deltaN is the difference in refractive index between your solution and water

Note Solubilities (H20, 25C):

• Urea: 10.49 M
• GdmCl: 8.54 M

References

1. from:
   Measuring the conformational stability of a protein
   by C. Nick Pace and J. Martin Scholtz
   in "Protein Structure: A Practical Approach" (Creighton, T. E.) pp. 299-321, IRL Press, Oxford, 1996.