Application of surface-enhanced resonance Raman spectroscopy to chlorophyll and chlorophyll derivatives

Abstract

Chlorophylls and chlorophyll derivatives are fundamental components of green plants and are directly involved in light trapping and electron transfer during photosynthesis. In Chapters 1-3 surface-enhanced resonance Raman spectroscopy (SERRS) has been employed to obtain Raman vibrational spectra for chlorophyll a, pheophytin a, and chlorophyll b adsorbed on a roughened silver electrode and excited from within the Soret band to the lowest energy transition in the red at 77K. Cooling to liquid nitrogen temperature was found to improve the spectral quality by minimizing sample heating and photooxidation. Most significantly, the fluorescence accompanying red excitation was quenched at the roughened Ag surface at low temperature and this resulted in richly detailed SERRS spectra. Distinctive SERRS spectra were obtained for each excitation wavelength. Selective excitation within the various electronic transitions can thus be utilized to verify assignments of the vibrational modes of the chlorophylls and pheophytin a and to monitor their interactions and photochemical behavior in biomimetic systems and in vivo. Metal electrodes are preferable for SERRS studies of photosynthetic pigments because the electrogenerated species can be created and monitored with minimal sample manipulation and chemical interference. Although the roughened silver electrode provides intense Raman spectra for the neutral vi photosynthetic pigments, its oxidation potential is too low for SERRS studies of electron transfer reactions involving chlorophyll a in vitro and/or in vivo within the light harvesting and reaction center complexes of green plants. Roughened gold provides surface enhancement with red excitation. While this is a narrower spectral enhancement range than for silver, Au can provide information on the RR spectrum coupled to the lowest tt-tt* transitions, about which little is known because of fluorescence problems. In addition, Au has a broader electrochemical range that is well suited for all the photosynthetic pigments. Thus, in Chapter 4 a roughened gold electrode was investigated as a SERRS substrate for chloropyll a, pheophytin a, and chlorophyll b with red excitation at 77K. Distinct spectra of each of the photosynthetic pigments were obtained from the roughened Au electrode demonstrating that it is a viable SERRS substrate. Also, Au provides a means to monitor electron transfer in photosynthetic systems by excitation within the lowest electronic transition.