This research is about the surprising flexibility of water molecules that makes water the medium of choice for biological systems. The study examines the 50-year-old question of how many water molecules share a proton, a crucial issue in the transportation of charge in biological processes.
Models predict a proton to be strongly bound to one water molecule (Eigen model) or shared between two water molecules (Zundel model) in a manner that depends on how many water molecules are available. With 21 molecules, it was thought that the water could form a "nanocage" structure that holds the Eigen form of the proton in the center. This report confirms the formation of a dodecahedral (20-sided) cage, but the data displayed no trace of the Eigen species.
To determine how a precisely determined number of water molecules interconnect to form these cages, the scientists first weighed the cluster (after the proton was added), and then monitored changes in the infrared absorption that occurred upon addition of each new water molecule.
"The idea was brought to my attention by John Fenn, Yale Professor Emeritus of Chemical Engineering and Nobel Prize winner in Chemistry '02," said Mark Johnson, professor of chemistry and head of the Yale research team. "Fenn suggested that we might be able to crack this important problem with current technology. We collaborated with groups at Pitt and Georgia using experimental techniques developed in my lab and analyzed the results using Pitt's super computers."
"Water is tricky because sometimes it is just a solvent. Like with coffee. All water does for me is hold the caffeine there, evenly distributed throughout the solution" said Johnson. "For many other things, particularly in biology, water is actively participating in chemical change. It is the medium for shuttling protons. Individual water molecules become part of a network or wire that guides the flow of protons."
In nature, proton transport is unlike other things that move through water while retaining their molecular identity. Protons move much more quickly by trading water partners down the chain, like the executive desk toy with five steel balls. The proton that comes out at the end is not the same one that went in. One example where this mechanism is being currently entertained is in photosynthesis, where the conversion of light energy to useful energy by charge separation may be mediated by water molecules. Molecules once thought to be innocent bystanders may turn out to be the main players!
The research was funded by the National Science Foundation and the Department of Energy.
Citation: Sciencexpress/ www.sciencexpress.org / 29 April 2004 / 10.1126/science.1096466