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A crystallographic titration of the dipeptide L-isoleucyl-L-isoleucine

Acta Cryst. (2004). B60, 569–577 [doi:10.1107/S0108768104014569]

[structure of L-isoleucyl-L-isoleucine] Structure of L-isoleucyl-L-isoleucine as an anion in the ammonium salt, the first of this kind for an amino acid or a peptide. Crystals were prepared by slow evaporation from a NH3(I) solution.
The dipeptide L-isoleucyl-L-isoleucine has been crystallized in four different protonation states including as a cation, a zwitterion and an anion as well as a dimeric cation where two peptide molecules, linked by a strong COO···H···OOC hydrogen bond, share an overall positive charge of +1. The paper on this unprecedented series of crystal structures highlights differences in hydrogen bonding capabilities, conformational properties and molecular packing arrangement. The packing is particularly interesting for the cationic form (as a nitrate), which forms a nanotubular structure. Channels running parallel to a trigonal axis have a 4.0 Å van der Waals’ diameter.

C. H. Görbitz