Structure and Properties of Molecular Crystals Based on Short Chain Peptides
Peptides are diverse in their supramolecular assembly pathways, the property that makes proteins the building block of life. This thesis explored the properties of one of these assemblies, layered structures, to gain a better understanding of the formation and flexibility of peptide structures, and the adaptability of peptide molecules. To carry out this research, short chain peptide crystals were analyzed using both single crystal and powder X-ray diffraction as well as TGA-FTIR and GC-MS experiments. A series of dipeptides with the first residue containing leucine and the second residue containing one of six R groups were selected for analysis. During this research the crystal structures of 15 new cocrystals, 10 inclusion compounds and 7 other crystals were determined. Of these, 75% were found to exhibit disorder, with five containing whole molecule disorder and three structures exhibiting twinning, adding to the complexity of refinement. The investigation of dipeptide structures was carried out by combining established measurements and schemes from the literature (torsion angle analysis, interlayer distance, cavity volume, peptide molecular distance, interchain spacing, and layer classifications) with newly formulated ones (nitrogen carbon distance, body-end hydrogen bond analysis, layer distortion angles and cavity space per angstrom). By analyzing the newly determined structures and 19 structures from the literature, trends in peptide materials were discovered and the flexibility present in the peptide framework was demonstrated. Two series containing homopeptides were also analyzed, homoglycine oligomers of different length and inclusion compounds of trileucine. In the investigation of these two series, six new crystal structures were determined, and the new analysis method was implemented and expanded upon (peptide plane area). From these results dipeptide structures were shown to contain a unique flexibility, while other oligomer lengths behaved similarly to one another. The thermal properties of bulk homoglycines and homoleucines were also analyzed. The thermal stability in both series increased as a function of chain length, while the presence of the leucine R group induced a reduction in the thermal stability of the peptide material. The leucine R group also reduced the prevalence of side reactions during pyrolysis.