H-D exchange (or D-exchange as I’ve sometimes referred to it) has been a problem I’ve dealt with in the lab for some time. It is essentially something that I need to minimize but can never actually stop. It is also a process that I know almost nothing about other than it happens, it occurs somewhat instantaneously, but may be mediated by evaporation rates (when dealing with DDW and 99% D2O). Now I’m perusing the internet looking for some information. Come take a walk with me:
- At ScienceOnline 2013, I met a person who pointed me in the direction of a Dr. Richard Zare. She told me he was very knowledgable in the field and so I looked up his papers. He had five papers relevant to exchange reactions, all of which are way above my head. So I’ll start on Wikipedia and work my way up.
- For those unaware, hydrogen-deuterium exchange is a reaction where a covalently bonded hydrogen is replaced with a deuterium atom. In the case of my experiments this happen with water. If I have deuterium depleted water and I keep it in contact with the environment (which has deuterium in it at around 16mM), eventually it will reach equilibrium and the deuterium level (of the DDW) will rise. The mechanism that causes this, I presume, is D-exchange.
- Apparently the reaction is pH dependent. It can be quenched around pH 2.6, but seems to work best at pH 7.0-8.0. That’s really interesting to me. The pH levels that I’m normally working around are optimal for these reactions. Unfortunately there is nothing I can do about that.
- The reaction may also be quite slow. According to Wikipedia, exchange is slow/unlikely intramolecularly, but is quite rapid via exposed surface hydrogen bonds. For the purposes of NMR, in vivo deuterium incorporation would be valuable and is hard to attain by dissolving proteins in D2O.
- The first person to measure H-D exchange was Kaj Ulrik Linderstrøm-Lang, and I would read some of his stuff right now but they are locked down. The mechanism he used to study D-exchange was pretty interesting and involved a density gradient tube. If I read this correctly, KULL filled a long tube with oils of different desities. He could place a drop of water, with a small amount of proteins in the drop, into the oils to determine the density. As reactions occur, the density would change and the drop would move accordingly.
Ok I’ve reached the limit of what I can do tonight without using the UNM network for access to papers. I’ll try to look for more information this weekend. The quest for a basic understanding of H-D exchange continues…