Spectroscopy as a Window into History: Photochemistry as It Pertains to Archeology, Pigments, and Early Earth Nucleobases

Spectroscopy is a broad tool set that can be applied to further the understanding of many interdisciplinary questions. The technique of jet-cooling, laser-desorption, resonance-enhanced-multi-photon-ionization, mass spectrometry (LD-JC-REMPI-MS) was used to study the photodynamics of pigments and alternative nucleobases as well as quantify residues in archeological pottery. LD-JC-REMPI-MS as a cold gas phase technique offers the benefits of simplifying absorption spectra to individual vibronic transitions unique to individual tautomeric forms of a molecule. As a form of action spectroscopy, mass spectrometry is the detector, while the intermediate REMPI step allows for ps level detection of lifetimes and relaxation mechanisms. The earliest interdisciplinary question of historical interest our technique has been applied to is the understanding of the early earth conditions that lead to the selection of canonical nucleobases over alternative forms. The same reactions responsible for the canonical nucleobases would’ve synthesized many other molecules similar in structure. Recent research has shown that many of these alternative molecules could have successfully functioned in DNA or RNA. Thus it is not biological pressures that selected the canonical nucleobases. What then was the selection method that led to only the 5 nucleobases inclusion in DNA and RNA over these many alternative forms? Photochemistry, specifically distinct differences in excited state lifetimes, are the answer. The second question in the historical timeline that our technique has been applied to is the identification of cacao residues in Maya pottery. Cacao is an important ritualistic beverage made from the combination of hot water and the dried, roasted, ground seeds of the Theobroma cacao plant. Many archeological records describe how cacao was ceremonially important through its use in marriage and funerary ceremonies as well as its association with fine vessels from elite Maya burial caches. Without evidence, archaeologists have assumed because of the ritualistic importance of cacao that it must be a substance controlled by the elite population. To question this assumption, our study has focused on ordinary vessel fragments from the Mayan site of El Pilar located in modern day Belize. In this study LD-JC-REMPI-MS is used as the analytical technique to detect theophylline as an indicator of cacao. Finally, the last window of history our technique was applied to is an understanding of pigments. Conservationists value highly the study of pigments as an understanding of photochemistry can predict fading. Pigments with faster excited state lifetimes are much less likely to undergo fading as the fast decay prevents the molecules from undergoing damaging reactions from absorption of UV light. As pigments similar in structure can have vastly different decay pathways, each pigment must be studied individually to determine the lifetime and thus its lightfastness. Previous studies have shown that excited state proton or hydrogen transfer is often one of the fast decay pathways that prevents fading of pigments. Both curcumin and indigo studied in this dissertation are dominated by an excited state proton/hydrogen transfer. Both are studied in the deuterated form to further study the role of excited state proton/hydrogen transfer as a decay pathway, as well as contribute a broader understanding of both pigments