  The structural properties of L-alanine amino acid in aqueous solution and in
crystalline phase have been studied by means of density-functional
electronic-structure and molecular dynamics simulations. The solvated
zwitterionic structure of L-alanine (+NH3-C2H4-COO-) was systematically
compared to the structure of its zwitterionic crystalline analogue acquired
from both computer simulations and experiments. It turns out that the
structural properties of an alanine molecule in aqueous solution can differ
significantly from those in crystalline phase, these differences being mainly
attributed to hydrogen bonding interactions. In particular, we found that the
largest difference between the two alanine forms can be seen for the
orientation and bond lengths of the carboxylate (COO-) group: in aqueous
solution the C-O bond lengths appear to strongly correlate with the number of
water molecules which form hydrogen bonds with the COO- group. Furthermore, the
hydrogen bond lengths are shorter and the hydrogen bond angles are larger for
L-alanine in water as compared to crystal. Overall, our findings strongly
suggest that the generally accepted approach of extending the structural
information acquired from crystallographic data to a L-alanine molecule in
aqueous solution should be used with caution.
