I will use this space to share some reviews on (or brief summaries of) interesting papers and/or seminars (not necessarily directly related to my research!)

This paper by Karsili et al. explores the possible ultrafast internal conversion routes in oxybenzone, caffeic acid and ferulic acid in an ab initio (theoretical) study (using Molpro 2010.1 and a variety of computational methods and basis sets). The interest in these molecules comes from the fact that they are used in commercial sunscreens, and hence they are supposed to provide photoprotection without photodegrading (should be photostable). The processes by which these molecules deal with UV radiation (effectively, sunlight) determine whether they are effective sunscreens. Moreover, these studies will help determine if the relaxation processes undergone by these sunscreen agents release free radicals (harmful species which ultimately cause DNA damage), which has been one of the concerns that has recently caused sunscreen controversy.
• Caffeic Acid (CA) and Ferulic Acid (FA)
The calculations showed that for both of these molecules (which differ only in a OH group in CA that is replaced by a OCH3 group in FA), the E isomer is more stable than the Z isomer, reflecting greater steric hindrance in the latter. Moreover, the most stable geometries are the ones with an intramolecular H-bonding.
The internal conversion pathways calculated for these molecules are O-H/O-CH3 bond extension, ring centred out-of-plane deformations and E/Z photoisomerism. If O-H bond extension (and eventually fission) in CA is a competitive relaxation pathway (which remains to be determined), this suggests that CA might release free radicals upon UV radiation absorption, rendering it not as a good sunscreen as FA, where the replacement of the O-H group by a O-CH3 reduce the chance for radical formation upon excitation by UV radiation and consequent relaxation.
• Oxybenzone (OB)
The most stable geometric configuration of OB is the enol tautomer where there is an intramolecular H-bonding.
The O-CH3 bond fission that occurs for FA was found to be unlikely to occur in OB. Instead, the internal conversion pathways found for OB were electron-driven hydrogen atom transfer (effectively, keto-enol tautomerism) and oxetane formation. Following H atom transfer, the new O-H bond elongates, and this motion is accompanied by a twist of OB’s molecular frame to such an extent that a 2+2 cycloaddition reaction occurs and OB breaks to yield a stable ground state oxetane. The electron-driven H atom transfer is suggested to be the explanation for OB’s photostability, and it is also hypothesised that this is the dominant internal conversion pathway for OB with UV radiation. Therefore radical formation should not be an issue for oxybenzone.
These conclusions are an excellent starting point for my research, since the molecules to be studied experimentally are exactly the ones studied in this computational work. Since the experiments will be ran in the gas phase, they will have the same problem as these calculations, as mentioned in the paper by the authors – the results relate to isolated molecules and lack “real-life” complexity, for example, solvent interactions. However, running the experiments alongside these results will not only be very useful but also make it possible to address some of the aspects that are left to be confirmed in this paper, such as which is, in fact, the dominant internal conversion pathway in OB.

This paper reports on an experiment that aims at establishing the level of protection that so called “high factor” commercial sunscreens provide. Haywood et al. used fresh Caucasian skin samples to which different amounts of various brands of commercial sunscreens were applied and submitted them to UVA radiation. The quantity of free radicals was then evaluated using Electron Spin Resonance (ESR) spectroscopy.

Before reviewing the results obtained in this experiment, it is interesting to also comment on some of the aspects mentioned in this paper’s introduction. First of all, the fact that UV radiation is carcinogenic is a well-established fact (as it was at the time of the publication of this paper). However, whereas UVB radiation interacts directly with DNA, damaging it and causing both basal and squamous cell carcinomas, UVA radiation interacts with it indirectly, inducing the production of free radicals in the human body (skin) which can then damage DNA and contribute to photoaging. However, the precise wavelengths and mechanisms for this were not known at the time of publication of this paper. Moreover, a link between the use of sunscreens and melanoma had not been recognised, and the level of protection that sunscreens provide against UVA-induced free radicals had not been measured (further literature research is needed in order to establish if these parameters have now been determined).

The results from this experiment showed that so called “high factor” sunscreens only provide 55% protection (for Caucasian skin) against UVA-induced radical formation, and this is at the recommended application of 2 mg/cm2 – lower applications provide less protections, while higher applications will significantly increase radical formation. Moreover, it was found that radical signal is proportional to the amount of UVA radiation the skin is submitted to, strongly suggesting that UVA radiation is indeed closely interlinked with the formation of radicals in the skin.

Regardless of the useful information provided by this study, there are some issues which still need clarification. In particular, the role of UVA radiation in melanoma development is not known, and it cannot be stated that these UVA does cause melanoma or even that it increases the risk of its incidence. Moreover, the role of sunscreens in protecting against skin cancer is also not clear and instead it remains controversial. Further studies with sunscreens are essential to determine this.

The research Prof. Kukura’s presented in this seminar is aimed at mapping chemical change dynamics using ultrafast spectroscopy. More precisely, the topic was a nouveau ultrafast spectroscopic technique with unique temporal and spatial resolution capabilities, Femtosecond Stimulated Raman Spectroscopy (FSRS) which Prof. Kukura and his collaborators have developed (Annu. Rev. Phys. Chem. 2007. 58:461–88).

Ultrafast dynamics are usually studies with some variety of a “Pump-Probe” experiment, where system under study is “pumped” (activated, or excited) by a first laser pulse, followed by a second laser pulse at several different time delays to “probe” the system, effectively following its evolution with time and mapping the dynamics occurring some femtoseconds – picoseconds after excitation. Even though this technique is an excellent source of kinetic information, the structural dynamics (motion of nuclei along reaction coordinate) have to be inferred for all but the simplest cases, since the electronic absorption bands that it pumps and probes are broad, unresolved, and insensitive to molecular structure.

The solution that Prof. Kukura and his collaborators found for this problem was to use the same pump-probe approach but with a form of vibrational spectroscopy instead (Raman), since vibrational bands are dependent on both electronic and molecular structure, and a small displacement of an atom within a complex molecule will yield a detectable vibrational frequency shift. Time domain versions of vibrational spectroscopy techniques were available before, but were limited to picosecond time frames and in some cases poor signal-to-noise ratio. FSRS now offers the possibility to monitor vibrational structural changes with a time resolution even faster than the vibrational periods of the motions under study, and with no major practical issues, being a simple and quick technique to use.

Being able to monitor structural changes rather than electronic features is a major milestone in chemical and biochemical reactions and it is of great relevance to the field of spectroscopy, particularly the researchers interested in following dynamics of complex systems.