Ultraviolet and infrared spectroscopy of model lignins, plant UV-B sunscreens, and polyglycine in the gas phase.

摘要

The work presented herein focuses on the detailed UV and IR spectroscopic investigation of biologically-relevant molecules in the gas phase. Comprehensive interrogation of the conformational landscapes and UV absorption properties of increasingly complex bio-molecules, presents a formidable challenge as the number of internal degrees of freedom and/or the number of ultraviolet chromophores increases. As such, supersonic expansion cooling coupled with double-resonance spectroscopy, provides the ultimate set of tools for probing the conformational preferences and electronic excited states of molecules isolated from their environment. In this thesis, these tools are applied to probing the intramolecular hydrogen bonding interactions, folding propensities, and electronic excited states of molecules relevant to plant biology and proteins. In addition to experimental interrogation of these molecules, an ongoing theme in this work is the comparison of experimental observations with modern computational methods to assess their accuracy and predictive capabilities.;The primary subject of this thesis is the spectroscopic study of model lignin compounds. As an abundant, flexible heteroaromatic biopolymer, it presents an intriguing arena for UV and conformational evaluation. The approach taken herein is to work from the "bottom-up" with regards to the complexity of lignin structure, beginning with determination of the UV and IR spectroscopic signatures of the simple aromatic subunits (with particular focus on guaiacol and syringol), and working up to the model G-type beta-O-4 and beta-beta dilignol linkages. Further, this work extends this approach into the next "era" of lignin spectroscopy, by concluding with the spectroscopy of the beta-O-4 and beta-beta dilignol linkages complexed with alkali metal cations performed in a 22-pole cold ion trap. The combined set of results provides a library of information, necessary for extension of these techniques to larger, more complex lignin oligomers in ion form, where site-selective fragmentation can be thoroughly investigated.;Another class of molecules discussed in this thesis is the sinapate esters, with a particular interest in sinapoyl malate. Sinapoyl malate is the primary UV-B screening agent in plants, providing the vital protection required to avoid destruction of sensitive plant cells such as those used for photosynthesis. A series of seven molecules were explored, starting with the simplest UV-B chromophore sinapic acid, and following with ester derivatives of increasing complexity up to sinapoyl malate. It was found that the electronic excitation intrinsic to these molecules poises them as prime candidates for their role in UV-B screening in plants.;Finally, the intrinsic conformational preferences of a set of increasingly large homoglycines is presented, unveiling a natural preference for 14/16 mixed helix formation when isolated from their environment. This unique helix was characterized by a distinctive set of hydrogen bonds which alternate in direction, including the large 14- and 16-membered H-bonded rings which stabilize the helix conformation. The stability of this conformation is found to drastically increase with the addition of glycine residues, separating its minimum on the potential energy surface significantly from other prototypical secondary structures.