The aqueous and crystalline forms of L-alanine zwitterion - Physics Biological PhysicsReport as inadecuate

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Abstract: The structural properties of L-alanine amino acid in aqueous solution and incrystalline phase have been studied by means of density-functionalelectronic-structure and molecular dynamics simulations. The solvatedzwitterionic structure of L-alanine +NH3-C2H4-COO- was systematicallycompared to the structure of its zwitterionic crystalline analogue acquiredfrom both computer simulations and experiments. It turns out that thestructural properties of an alanine molecule in aqueous solution can differsignificantly from those in crystalline phase, these differences being mainlyattributed to hydrogen bonding interactions. In particular, we found that thelargest difference between the two alanine forms can be seen for theorientation and bond lengths of the carboxylate COO- group: in aqueoussolution the C-O bond lengths appear to strongly correlate with the number ofwater molecules which form hydrogen bonds with the COO- group. Furthermore, thehydrogen bond lengths are shorter and the hydrogen bond angles are larger forL-alanine in water as compared to crystal. Overall, our findings stronglysuggest that the generally accepted approach of extending the structuralinformation acquired from crystallographic data to a L-alanine molecule inaqueous solution should be used with caution.

Author: Ivan Degtyarenko, Karl J. Jalkanen, Andrey A. Gurtovenko, Risto M. Nieminen


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