Date of Award

1-12-2006

Degree Type

Closed Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Gary Hastings - Chair

Second Advisor

Richard Miller

Third Advisor

William Nelson

Fourth Advisor

Mark Stockman

Fifth Advisor

Giovanni Gadda

Sixth Advisor

Brian Thoms

Abstract

This thesis describes an investigation of the molecular mechanism underlying solar conversion processes that occur in Type I photosynthetic reaction centers, in which P700 plays a central role. Static Fourier transform infrared (FTIR) difference spectroscopy (DS) was used to probe the electronic and structural organization of P700 and P700+. In combination with isotope labeling and site directed mutagenesis we have investigated how protein interactions such as histidine ligation and hydrogen bonding modulate this organization. Comparison of (P700+-P700) FTIR difference spectra (DS) obtained using wild type and mutant PS I led us to suggest that the 131 keto carbonyl group of PA is essentially free from hydrogen bonding in the ground state. Upon cation formation, this hydrogen bonding becomes stronger, probably because of a cation induced reorientation of the hydroxyl group of a nearby threonine residue. We also tentatively suggested that a difference band at 1639(-)/1660(+) cm-1 in (P700+-P700) FTIR DS might be due to a C=C mode of the imidazole side chain of the ligating histidine residues. Most of this thesis is geared towards investigating the validity of this interpretation. (P700+-P700) FTIR DS obtained using mutant PS I particles in which hydrogen bonding to P700 is altered can be reconciled within the context of our new interpretation. (P700+-P700) FTIR DS obtained using uniformly 2H, 15N, and 13C labeled PS I particles also support our new interpretation, and indicate that the difference band at 1639(-)/ 1660(+) cm-1 cannot be associated with a strongly hydrogen bonded keto carbonyl group of PA. To investigate if the imidazole side-chain of ligating histidine residues could contribute to bands in (P700+-P700) FTIR DS vibrational mode frequencies and intensities for several protonation forms of 4-methylimidazole were calculated. The calculations suggest that the 1639(-)/1660(+) cm-1 band in (P700+-P700) FTIR DS may not be due to a C=C mode of the imidazole side chain of the ligating histidine residues. Thus we have produced data that suggests neither of the proposed interpretations alone can adequately explain the origin of the 1639(-)/1660(+) cm-1 difference band in (P700+-P700) FTIR DS. The origin of the 1639(-)/1660(+) cm-1 difference band in (P700+-P700) FTIR DS is therefore still an open question.

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