chemistry question #1622



Sandra Lenzi, a 44 year old female from Morris Plains, NJ asks on October 15, 2003,

Q:

Why is the helical content higher in gelatin with higher molecular weight. In other words, it seems the triple helix increase with the increase in molecular weight. Why? Also, gelatin is a good foam stabilizer. Why? How does gelatin do that? I am wiriting a paper about gelatin for my masters and your help would be very very valuable. By the way, this site is wonderful. I told my 11 years old niece about the site and she loved. Actually it is great to any age. It is a fantastic initiative.

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the answer

Vance Williams answered on October 27, 2003, A:

For a discussion of the factors that effect helix formation in gelatin-based gels, I recommend "All Gelatin Networks" by Christine Joly-Duhamal et al., in Langmuir, 2002, 7208-7217. This paper provides a nice overview in the context of a recent study that was trying to elucidate the structure-property relationships in these systems.

The explanation for why high-molecular weight gelatin tends to have a greater percentage of triple helices can be traced back to the Zimm-Bragg model of coil-helix transitions (J. Chem. Phys., 1959, 31, 526-35). Briefly, this statistical mechanical model describes the tendency of a polymer chain to undergo a helix-coil transition using two parameters: (a) the equilibrium constant that determines whether an existing helix will propagate and (b) the likelihood of a new helix being initiated. Since helix propagation is much more favorable than helix initiation, long chains (that tend to have some helix content) will have higher percentages of helical structure.

I am less sure about the second aspect of the question: i.e. why gelatin also stabilizes foams, but I can make an educated guess. Foams are examples of aerogels: in other words a physical network that is "swelled" by a gas such as air. Gelatin usually is encountered in the context of hydrogels, i.e. a physical network that is swelled by a liquid, such as water. The two phenomena are closely related, and many materials that form networks will form both hydrogels and aerogels. For a more in depth discussion of gels, check out any introductory polymer textbook.

Suresh Babu, M.Pharm student at UICT, Mumbai, India answered on June 15, 2004, A:

Gelatin is a protein and proteins stabilize foam better than ordinary surfactants such as a foam created by soap or detergent. Once a foam has been formed, there are three processes which affect its long-term stability near equilibrium.
Drainage: the draining of liquid from foam.
Disproportionation: the change in foam bubble size distribution caused by gas diffusion from small to large bubbles.
Coalescence: the fusion of foam bubbles.
A surfactant foam drains faster than a protein foam. This difference is also related to the possibility of the protein forming a stagnant surface layer. Furthermore, proteins are irreversibly adsorbed at the interface, whereas surfactants are not. Therefore, proteins slow down the disproportionation process more than surfactants can.

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