Choose your leaving group: selective photodeprotection in a mixture of pHP-caged compounds by VIPER excitation

Photocages are light-triggerable molecular moieties that can locally release a pre-determined leaving group (LG). Finding a suitable photocage for a particular application may be challenging, as the choice may be limited by for instance the optical or physicochemical properties of the system. Using more than one photocage to release different LGs in a reaction mixture may even be more difficult. In this work an experimental strategy is presented that allows us to hand-pick the release of different LGs, and to do so in any order. This is achieved by using isotopologue photocage-LG mixtures in combination with ultrafast VIbrationally Promoted Electronic Resonance (VIPER) excitation. The latter provides the required molecular selectivity simply by tuning the wavenumber of the used IR pulses to the resonance of a specific photocage isotopologue, as is demonstrated here for the para-hydroxyphenacyl (pHP) photocage. For spectroscopic convenience, we use isotopologues of the infrared (IR) spectroscopic marker -SCN as different LGs. Especially for applications where fast LG release is required, pHP is found to be an excellent candidate, as free LG formation is observed to occur with a 10 ps lifetime. The devised strategy may open up new complex uncaging applications, where multiple LGs can be formed locally on a short time scale and in any sequence.

Evidence for Quantum Chemical Effects in Receptor-Ligand Binding Between Integrin and Collagen Fragments - A Computational Investigation With an Impact on Tissue Repair, Neurooncolgy and Glycobiology

The collagen-integrin interactions are mediated by the doubly charged Mg2+ cation. In nature this cation seems to have the optimal binding strength to stabilize this complex. It is essential that the binding is not too weak so that the complex becomes unstable, however, it is also of importance that the ligand-receptor binding is still labile enough so that the ligand can separate from the receptor in a suited environment. In the case of crystal growing for experimentally useful integrin-collagen fragment complexes it turned out that Co2+ cations are ideal mediators to form stable complexes for such experiments. Although, one can argue that Co2+ is in this context an artificial cation, however, it is now of special interest to test the impact of this cation in cell-culture experiments focusing on integrin-ligand interactions. In order to examine, in particular, the role cobalt ions we have studied a Co2+ based model system using quantum chemical calculations. Thereby, we have shown that hybrid and long-range corrected functional, which are approximations provide already a sufficient level of accuracy. It is of interest to study a potential impact of cations on the binding of collagen-fragments including collagens from various species because different integrins have numerous biological functions (e.g. Integrin - NCAM (Neural cell adhesion molecule) interactions) and are triggered by intact and degraded collagen fragments. Since integrin-carbohydrate interactions play a key role when bio-medical problems such as tumor cell adhesion and virus-host cell infections have to be addressed on a sub-molecular level it is essential to understand the interactions with heavy-metal ions also at the sub-atomic level. Our findings open new routes, especially, in the fields of tissue repair and neuro-oncology for example for cell-culture experiments with different ions. Since Co2+ ions seem to bind stronger to integrin than Mg2+ ions it should be feasible to exchange these cations in suited tumor tissues although different cations are present in other metalloproteins which are active in such tissues. Various staining methods can be applied to document the interactions of integrins with carbohydrate chains and other target structures. Thereby, it is possible to study a potential impact of these interactions on biological functions. It was therefore necessary to figure out first which histological-glycobiological experimental settings of tumor cells are suited for our purpose. Since the interactions of several metalloproteins (integrin, ADAM12) with polysialic acid and the HNK-1 epitope play a crucial role in tumor tissues selected staining methods are proper tools to obtain essential information about the impact of the metal ions under study.

Infrared pre-excitation grants isotopomer-specific photochemistry

Species-selective photochemistry is often hampered by overlapping UV-Vis spectra. We overcome this long-standing problem by combined vibrational and electronic excitation as demonstrated by isotopomer selection. The influence of various factors on selectivity is discussed.

Controlling Photochemistry via Isotopomers and IR Pre-excitation

It is a photochemist's dream to be able to photoinduce a reaction of a specific molecular species in an ensemble of similar but not identical ones. The problem is that similar molecules often exhibit nearly identical UV-Vis absorption spectra, making them difficult or impossible to distinguish or to select spectroscopically. The ultrafast VIPER (VIbrationally Promoted Electronic Resonance) pulse sequence allows to pick a single species for electronic excitation based on its infrared spectrum. The latter usually shows more features, allowing the discrimination between species than the UV-Vis spectrum. Here, we show that it is possible to induce and monitor species-selective photochemistry even for molecules with virtually identical UV-Vis spectra, which is the case for isotopomers. Next to isotope-selective photochemistry in solution, applications to orthogonal photo-uncaging and species-selective spectroscopy and photochemistry in mixtures are within reach.