Room Temperature Matrix Isolation
Developing a picture of the interaction of water with
other molecules is greatly assisted by investigation of the
target molecule in an environment with limited water.
Classically, a limited water environment is generated with
matrix isolation: Water and the target are codeposited along
with a matrix (often a rare gas such as Ar) at very low
temperature. With little water in the expansion, the most
likely environment contains a small number of water
molecules in addition to the target. The other methodology
also uses an expansion, but probes the clusters on the fly.
The latter method has the advantage of allowing greater
mobility for molecules in the cluster. Nonetheless, the
cluster is generally quite cold. Generating small clusters
at near room temperature requires an alternate strategy. Of
course, biological systems as well as many environmental
systems exist near room temperature.
We have thus developed an alternate matrix isolation method.
Water is only slightly soluble in carbon tetrachloride, so
dispersing a target molecule and water in carbon
tetrachloride allows one to investigate these small clusters
near room temperature. This strategy has been successfully
applied to investigate the interaction between water and
nitric acid, hydrochloric acid, salts, and ammonia. This
methodology is currently being extended to biological
molecules: alanine and tryptophan – alanine is pictured
above with three water molecules. Modeling results indicate
that three water molecules is the minimum number needed to
form a nonstrained bridge between the amine and carboxylic
acid groups. Vibrational excitation of this cluster is
expected to result in formation of the zwitterion. Since
ionic and neutral molecules play very different roles in
biological systems, the results of this work are expected to
be of fundamental interest.
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