Protein powder diffraction at cryocooled conditions

Abstract

Keywords: crystal structure determination X-ray powder data, polymorphic structures, structural correlation Aspartame (L-aspartyl-L-phenylalanine methyl ester) is a dipeptide sweetener, ∼200 times sweeter than sucrose. Because of its dietary and pharmaceutical usefulness, various characteristics of aspartame, including the model of its receptor site and its conformation in aqueous solution, have been studied extensively [1,2]. Aspartame is known to grow in different pseudo-polymorphic forms, IA, IB, IIA and IIB, each containing a different amount of water and all having a needle-like morphology. Although a lot of information on aspartame can be found in literature, no clear and complete picture can be obtained of the dehydration process at the molecular level. One object of our work was to obtain the ″missing″ crystal structure of the anhydrate form and the study of its relation to the other forms. The employed technique was X-ray Powder Diffraction (XRPD), using a Bruker AXS D8 ADVANCE X-ray Powder Diffractometer in transmission capillary geometry. The crystal structure of the aspartame anhydrate was solved using the DASH software [3]. The final Rietveld refinement was performed with the Topas software [4]. The anhydrate crystallizes in the monoclinic system with space group P2 1 and cell parameters: a=19.4103(11) Å, b=4.9608(3) Å, c=15.6565(9) Å, β=94.875(2)°, V=1502.14(15) Å 3 (Fig. 1). Comparison of the structures of the hydrates and the anhydrate reveals remarkable similarity between the structures of IA and IB on the one hand and between IIA and IIB on the other hand. [2] Leung S., Padden B., Munson E., Grant D., J. Pharm. Sciences, 1998, 87, 508