Title

The Rational Design of Coiled-Coil Peptides towards Understanding Protein-Crystal Interactions and Amorphous-to-Crystalline Transitions

Date of Award

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Michael Ogawa

Second Advisor

George Bullerjahn (Committee Member)

Third Advisor

Marshall Wilson (Committee Member)

Fourth Advisor

John Farver (Committee Member)

Abstract

This dissertation reflects efforts to the combine biomineralization research and rational peptide design using the coiled-coil peptide motif and calcium hydrogen phosphate dihydrate (CaHPO4 x 2H2O), commonly known as brushite. Coiled-coils were designed to alter the complete growth pathway of brushite beginning with an amorphous precursor and resulting in a modified crystalline state. Both the designed chemical character and the secondary structure of the coiled-coil peptides were found to be important factors in controlling the growth pathway of brushite and the morphology of the final crystalline state. The impact of peptide secondary structure on the growth of brushite was studied by comparing the effects of a well structured coiled-coil peptide (AQQ5E) to a structurally disordered analog with nearly identical chemical properties (RCA5E) on the final crystal morphology. Typically, brushite crystals formed in the absence of growth modifying agents display {100}, {010}, {10-2} and {10-1} crystal faces. However, in the presence of AQQ5E the growth of the {10-2} faces were selectively inhibited in the final crystal product. Conversely, crystals grown in the presence of RCA5E adopted a wide variety of morphologies without a preferred means of crystal modification. Computational analysis demonstrated how the two peptides may interact with the different crystal faces of brushite and provided insight for explaining this behavior. In another study, a series of coiled-coil peptides with similar secondary structure yet increasing acidic amino acid content were used to determine the impact of designed peptides on amorphous-to-crystalline transition of brushite leading up to the final crystalline state. In the absence of peptides, the amorphous-to-crystalline transition of brushite occurred rapidly with the final crystalline state being achieved within several hours. However, the addition of acidic peptides prolonged this process over several days and allowed for the study and characterization of a novel amorphous-crystalline, hybrid phase of brushite adopting a ribbon-like morphology. Studying the formation and stability of the intermediate brushite phase in the presence of structurally similar yet chemically distinctive peptides suggested that both secondary structure and chemical composition impact the manner in which peptides interact with amorphous and crystalline materials.