Shedding Light on History
Research in the de Vries Group stems from a molecule’s inherent ability to absorb light. Throughout our lives, the compounds and materials that surround us absorb light from the sun. The color that we see is dependent on the colors of light that are capable of being absorbed by the object. This can be best shown by a color wheel. If an object absorbs red light, the object will appear as the color opposite red on the color wheel, green, due to the absence of that absorbed color. This fundamental property of all matter can be manipulated to find unique properties of these compounds and help us explain why certain events in nature occur. Through the use of ultrafast lasers coupled with mass spectrometry, we are able to both mass and wavelength select the compound of interest. Through these efforts, we have explored properties of large compounds that were once difficult to explore in the gas phase.
Gas phase techniques enable the study of isolated molecules, free of interactions. A major thrust is the laser spectroscopy of isolated biomolecular building blocks, and more specifically the excite state dynamics of DNA. These include single DNA bases and amino acids, as well as their clusters with each other and with water molecules. We investigate the general hypothesis that photostability to UV radiation was a primary mechanism in the prebiotic selection of life's building blocks. Notice how the canonical bases all have excited state lifetimes on the picosecond timescale and less, while alternative bases have lifetimes in excess of nanoseconds!
During the Prebiotic era about 4 billion years, there were many compounds that have been theorized to be available that could have potentially formed the backbone for life as we know it. Two of these compounds that could have been available are isocytosine and isoguanine. These molecules have the potential to replace cytosine and guanine as a base pair in DNA due to the fact that they pair similar to the GC pair, yet guanine and cytosine still prevailed as two of the five canonical base pairs. To better understand why GC won in the biological race over isoGisoC, a variety of photochemical studies have been performed to probe many properties of these compounds. Through this effort, it has been found that isocytosine has a similar excited state lifetime to its counterpart, guanine. However, isoguanine has been found to have a much longer lived excited state than cytosine in the tautomeric form that would form Watson-Crick base pairing.
The de Vries lab has helped develop the use of an atomic force microscope (AFM) to sample material at submicron resolution, well below the optical diffraction limit. The sampled material is later analyzed by a combination of high resolution laser spectroscopy and mass spectrometry. Other analytical methods are currently being explored.
Atomic force microscopy enables much higher spatial resolution but lacks the chemical information that light based techniques can provide. Our new combination of laser MS with the AFM-TD sampling now makes it possible to perform spectroscopic analysis with submicron spatial resolution. We have shown that this approach allows R2PI analysis on small samples such as painting cross-sections typically found in cultural heritage.
A field of interest that is prominent within the art and art history community is the determination of the photostability of different pigments. As paintings grow older, the original color deteriorates causing fading and in some cases a complete change in color from the original work. This can be seen in the Van Gogh painting below. Due to the photostability of red pigment in the walls, the current walls now appears to be blue. In the de Vries group, we measure the excited state lifetimes of a variety of organic pigments to provide insight as to why some colors fade faster than others. In collaboration with the Getty Conservation Institute, we identify organic paint pigments within microscopic layers of artwork. The Microscope Laser Mass Spectrometer allows for unparalleled identification of organic molecules with high spatial resolution.
Additionally, we partner with the Department of Anthropology as well as the MesoAmerican Research Facility to investigate archaeological artifacts in order to identify trace “marker” compounds. These marker compounds include molecules from a variety of sources, including beer, wine and coffee.
The color blue is hard to naturally find on Earth. One natural blue compound is indigo, a pigment that is widely used in art as well as dyeing goods such as blue jeans and other textiles. In addition to its color, indigo also is very photo-stable. Because of this, paintings that use indigo maintain their blue color much longer than the other colors that were used. Although indigo is so widely used, there have been very little studies on the compound since it is not very soluble. As a result, most research involving indigo uses a similar compound, indigo carmine, since it is more soluble. In order to better understand the relaxation mechanisms that indigo undergoes, we performed many REMPI and pump-probe experiments on indigo in the gas phase.
IR-R2PI spectroscopy together with quantum calculations identifies shapes of small peptides, isolated in the gas phase.