MATRIX-ASSISTED LASER DESORPTION/IONIZATION (MALDI) TARGET MODIFICATION FOR ENHANCED PROTEOMICS ANALYSIS AND PLASMA POLYMER CHARACTERIZATION BY MALDI MASS SPECTROMETRY

Author

LIJUAN PENG, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2010

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

KINSEL, GARY

Abstract

The work described in this dissertation is divided into three sections. In the first section three surface modifications are used to produce MALDI targets having reduced surface-protein binding affinity with a goal of increasing peptide/protein MALDI ion signals and lowering the limits of detection (LODs) for proteins and peptides. The second section discusses a bioselective MALDI target, produced via radio frequency (rf) plasma deposited ethylenediamine (EDA), for on-target separation of complex protein mixtures. The third section develops a new approach for characterization of rf plasma-deposited bulk polymers by using MALDI MS. Previous studies in our group have shown that the analyte signal in a MALDI MS experiment is strongly influenced by the binding interactions between the target surface and the analyte. Specifically, the analyte signal increases with decreasing surface-analyte binding affinity, which has been attributed to more unbound analyte being available for incorporation within the MALDI matrix. In the presented studies MALDI targets are modified with polyethylene glycol (PEG)-like structures via chemical grafting of PEG onto polyurethane (PU) film and rf plasma polymerization of ethylene oxide vinyl ether (EO2) and tetraglyme. It is shown that there are enhancements in the protein MALDI ion signals on these modified targets and that the LOD for target proteins is decreased by a factor of 2-10 in comparison with the conventional stainless steel MALDI target. On-probe affinity capture (OPAC) MALDI MS, developed in our group, has shown that functional group modified MALDI targets can be used to rapidly and selectively isolate target analytes from complex samples. For applications involving analysis of complex peptide/protein mixtures, fractionation of the mixture on the basis of component pI can reduce MALDI ion suppression effects leading to efficient ionization of larger numbers of mixture components. In the present studies a MALDI target is modified by rf plasma deposition of polymerized EDA to yield an OPAC target suitable for capture of proteins with low pI (expected to be negatively charged at neutral pH). In subsequent MALDI MS analyses of both control and biological mixtures after fractionation on the OPAC target it is observed that a significant number of additional peptide/protein ion signals are detected. The results of these studies, along with studies of the effects of the density of the primary amine functionality on the bio-selective MALDI ion signals, are presented. The complex nature of the polymer films resulting from plasma polymerization makes it very difficult to characterize their molecular structures. The presented study is the first to use MALDI MS for characterization of rf plasma-deposited bulk polymers and for investigation of the rf plasma polymerization process. It is shown that the mass spectra of the soluble fraction of allyl alcohol, EO2 and ethylene glycol butyl vinyl ether -plasma polymers contain clear polymer series. Furthermore, it is found that the peaks of the EO2-plasma polymer series shift to higher molecular weight distribution with decreasing plasma duty cycle. In contrast to predictions based on conventional radical polymerization, the mass spectra of all three plasma polymers exhibit the same repeat unit of 44 Da, for which the most likely structure would be -(CH2CH2O)-.