PhD Oral Exam - Janet Hazel Dolman Gaba, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The self-assembly of phenolic surfactant monolayers at interfaces is driven by a complex network of noncovalent interactions. This is exemplified by gallate-based surfactants, which have shown an unusually high degree of organization. The specific interactions driving this behaviour are difficult to determine using experimental methods alone. Therefore, in this work, electronic structure methods are utilized to locate and quantify these interactions.

Studies of gallate analog trimers with varying functional groups, arrangements, and solvation reveal that a π-stacked assembly becomes more energetically favourable over a hydrogen-bonded cyclic assembly similar to that seen in phenol, when either meta hydroxy groups or an ester linker between the headgroup to the tail, is present. This is largely driven by C-H…O interactions between the ester groups, and O-H…O interactions between the meta hydroxy groups, demonstrating the collected effect of many individually-weak interactions.

Monolayer models containing 42 phenolic monomers show that ester linkages reduce variation in ring…ring stacking and shear distances, indicating greater organization. This ordering effect is ascribed to a lower π/π* HOMO-LUMO gap and altered LUMO character, and/or additional noncovalent interactions that enhance ordering. The gallate monolayer models accurately reproduce experimentally-determined unit cell distances.

Finally, to determine the effect on the monolayer of a known peptide binding partner, gallate-proline systems are investigated using 16 methylgallate monomers and either one poly-L-proline pentamer or three L-proline monomers. The O-H…O and C-H…O interactions are also seen to drive gallate-proline binding. Both free proline and PPII induce a displacement of the gallates, through both electronic and steric factors.