45214-G7
Diffusion and Adsorption of Single Dendrimers for Gas Hydrate Prevention
My Type-G grant was awarded for the two-year period of September 1, 06-August 31, 08. The major goal of this research was to understand the molecular level interaction between dendrimer and hydrocarbon for the prevention of gas hydrate formation in the oil pipeline, a practical problem which costs the petroleum industry one million dollars a day. My graduate student Karthikeyan Pasupathy and undergraduate student Michelle Reid have participated in this research during this past year. My laboratory has also formed active research collaborations with Prof Monica Lamm's group at Iowa State University, and with Prof Hong Luo’s laboratory in Genetics and Biochemistry here at Clemson University. For this project we have examined the molecular interactions between dendrimer and hydrocarbon using combined techniques of spectrophotometry, total-internal-reflection fluorescence microscopy, and atomistic simulations. We have found the interactions between dendrimer poly(amidoamine) or PAMAM and hydrocarbon squalane are temperature insensitive but are dendrimer-generation and solvent-pH dependent. The binding of squalane to PAMAM is stronger for lower dendrimer generations, possibly due to the more open interiors of the latter for interaction. Regarding the pH effect, at a high pH value of 10, the PAMAM dendrimer is neutral and assumes a small radius of gyration. Such a flexible structure allows for the uptake of squalane hydrocarbon via hydrophobic interaction. At neutral pH, the end groups of the PAMAM become cationic. As a result, the radius of gyration of the PAMAM is increased and the interiors of the dendrimer cater more water molecules to discourage squalane binding. At low pH where all the PAMAM amines are protonated, the dendrimer molecule is swelled and its branches become more rigid due to mutual repulsion. Consequently, the interiors of the dendrimer allocate more water to further suppress the attachment of the hydrocarbon. The interactions between PAMAM and squalane have also been found to be reversible at physiological pH, where weak inter- and intramolecular forces may succumb to Brownian motion. We have further determined the approaching, binding, and dissociation constants as 0.5 s, 7.5 s and 0.5 s, respectively. Since the binding affinity is subject to the total number of squalane molecules in contact with one PAMAM molecule, or vice versa, we believe the time constants, especially the binding time constant T2, will further increase with the pH of the solvent. Due to technical limitations our experimental designs and simulations conditions are less hydrophobic than the practical environment of an oil pipeline. However, we believe that our study of dendrimer-hydrocarbon interaction at the molecular level is a major step toward unraveling the mechanisms of gas hydrate prevention. We comprehend that high pH values are favorable for the encapsulation of hydrocarbon or gas molecules by dendrimers or hyper-branched polymers. Such conditions will facilitate the removal of gas hydrate blockages in the oil pipeline. In the past year we have explored using the fundamental knowledge of dendrimer-hydrocarbon interaction for the application of dendrimer gene delivery. We have studied the feasibility of using PAMAM dendrimers as a new carrier for gene delivery into plants with intact cell walls. We first formed supramolecular complexes of PAMAM and green fluorescence protein (GFP)-encoding gene via electrostatic interactions. We then observed these complexes penetrating through the cell walls of turfgrass callus and expressing GFP genes within the cells. This novel method of dendrimer gene delivery offers a number of advantages such as low cost and noninvasiveness, and could dramatically enhance the procedures for crop improvement using transgenic technology. Overall our project has made significant progress in advancing our knowledge on the interaction of dendrimers and hydrocarbons, especially at the single-molecule level. This knowledge will benefit not only polymer science but also the applications of using dendrimers for preventing gas hydrate formation in the oil pipelines and for gene delivery. Within two years this grant has enabled the training of two graduate students and four undergraduate students from Clemson and other institutions, and enabled collaborations which will benefit my research for the years to come. We have published one refereed journal paper in Biotechnology Journal and have one manuscript in review with Soft Matter. We have also presented our results at the NSTI 2007 conference in San Jose and the MRS Chongqing meeting in 2008. I sincerely thank the PRF fund for this crucial support at the early stage of my career and hope this grant will benefit my future research and funding opportunities in the areas of polymer science and applications.
