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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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