You are facing two sizable obstacles with your research proposal to use DSC to study freezing/melting of bacterial cells:

1.  The materials you are trying to study are highly plasticized by the presence of water, which will make it difficult to get a clear measure of their actual Tg while present in the cell environment.  As you cycle through the temperature range, these items are likely to change, with hydration, plasticization, etc. declining with increasing temperature.

2.  A variety of components of cells substantially alter the native freezing/melting point of water, which will again alter the inflection points that you will observe.  Soluble species will likely depress the freezing point, while elevating the boiling point, of water. 

However, in both cases, you can systematically measure the influence of at least the major components, which should give you the necessary reference points to assess the properties of the mixtures that represent real cells. 

I would start with a sample of cells in their native environment:

a.  run a few DSC scans from -100C to +200 C to see what transitions emerge.  These will be destructive runs, so start with an abundance of material so multiple runs can be made.  You're looking for repeatability here.  A half dozen runs should suffice for this exploration.  You should be able to see transitions that relate to both the Tg's of various materials in the cell mixture, and for water in this particular environment. 

b.  run TGA for the same materials (different sample, of course) to see what changes in structure are occurring during the temperature scan, such as dehydration and degradation.  You should also be able to see water loss somewhere above 100C.

c.  A complicating factor is the so-called cloud point that is commonly observed with non-ionic surfactants somewhere below the normal boiling point of water.  This is believed to be due to dehydration behavior that occurs as the energetics of the water disrupts the bonding with nonionic regions of the materials in the cells.  it is best viewed optically, because the sample becomes cloudy as it drops from "solution", or at least from the normal high degree of hydration. 

You may be exploring some new ground here, so don't shy away from it.  A reasonable effort on a literature search will help you to avoid reinventing wheels.  But after that, you will likely find a wide variety of holes in previous work, since this is such a multidisciplinary subject.  When that occurs, you have specialists in one field missing common knowledge in adjacent fields.  But putting together a team of advisors that cover the relevant fields, they should be able to help you with this problem.

Fields that seem relevant to me: 

a.  thermal analysis of polymers, particularly polar polymers that are highly hydrated (not necessarily solubilized);

b.  plasticization behavior of polar polymers by water;

c.  systems that change significantly while being observed.  I believe that thermal analysis results will demonstrate that bothersome behavior in your systems.

d.  cellular biology is needed, because of the cellular changes that will likely occur during your destructive testing.  If you're not careful, that could overshadow what you think you're looking for.

e.  at all times, be careful to repeat your experiments exactly, and then run variations of your experiments to see how your responses fare.  You need to carefully consider randomness in your studies, and exclude that possibility before any results can be seriously considered.

I'll stop for now, and wish you luck.  This discussion could go on for ever.  You are better off taking it in small bites, and getting feedback at each step to make sure that you're on firm ground before proceeding. 

Regards,

Lou Floyd

Biological materials are not my area of expertise, so I phrase this as a question.

Aren't cell membranes by and large ordered structures(sort of liquid crystalline)?  Does the concept of a glass transition apply to these materials?   Might you be looking more for an order/disorder transition, ie a peak, rather than a step change in the heat capacity?  Just curious.  Definitely an interesting project.

Joe Deitzel

Hi  - I wondered if you could purify your membranes before attempting the DSC?  - I think the issue will be that the other bacterial material ( including the water content) will 'cloud' the results. Since the nature of the pure membranous material will be to be mobile at room temperature - my advice would be to take the pure membrane material and run DSC from sub-zero to above RT  - I think it  unlikely that the Tg of the material will be higher than this. Be prepared to try different scan rates  - DSC is not always the best instrument for Tg measurement sometimes better results are obtained with a DMTA. good luck

Yes Joe, membranes are liquid crystalline at RT, and become more gel-like below their phase transition temperature. We are trying to find this transition temperature through DSC.

I agree, with Carole.  Once you go through the order/disorder transition you essentially have an isotropic fluid.  Here is my understanding of things in the case of LC oligomers and polymers.  Upon cooling from the isotropic liquid state, the mesogens will order themselves into successively more stable phases, ie, Nematic to smectic to3D ordered crystals, for example.  If there is no pathway to a more ordered state, the LC phase will simply freeze, forming what is often referred to as a liquid crystalline glass.  

Now, I suppose that in the case of cell membranes, these frozen LC domains could act as "crosslinks"  in a network giving you a gel. 

Assuming I have painted the picture correctly, here is your challenge.  The relative change in heat capacity for the LC glass is very likely going to be much much smaller compared to the heat capacity change of water in the same temperature range, so you are going to have a really really hard time seeing anything with a DSC. Since you are talking about a LYOTROPIC liquid crystalline phase, getting rid of the water really is not an option either.

To deal with this you might consider a couple approaches that look at molecular relaxations,rather than heat capacity measurement. One option is a DMTA type approach like Carole suggested.   This could be done one something like a TA instruments Rheometer.  Another approach is to use something like Solid state NMR to follow changes in T1 relaxation times as a function of temperature.

Finally,  you might consider talking to the Neutron scattering group at NIST.  I think they have the capability to make rheological measurements and scattering measurements at the same time, which could give you information regarding both gelation and LC ordering. 

I hope this is helpful.   I have made a bunch of assumptions regarding your material system, and I apologize if I am off-base.   It is definitely an interesting problem, and I wish you luck!

Regards,

Joe Deitzel

Haha thanks, Joe and Carole. I needed another perspective on the DSC and this made a lot of sense. Thanks again.

Regards,

Aditya

Try looking at these references that report experiments very similar to what you propose.

J. Lee, G. Kaletunç. Appl. Environ. Microbiol. 68, 5379 (2002).

46. J. Lee, G. Kaletunç. J. Appl. Microbiol. 93,178 (2002).

47. G. Kaletunç, J. Lee, H. Alpas, F. Bozoglu. Appl. Environ. Microbiol. 70, 1116 (2004).

48. J. R. Lepock. Methods 35, 117 (2005).

49. H. T. T. Nguyen, J. E. L. Corry, C. A. Miles. Appl. Environ. Microbiol. 72, 908 (2006).

50. I. Milek, M. Črnigoj, N. P. Ulrih, G. Kaletunç. Can. J. Microbiol. 53, 1038 (2007).