Heidelberg University

Semiconducting single-walled carbon nanotubes (SWNTs) combine solution-processability, large carrier mobilities, narrow emission linewidths and environmental stability for optoelectronic devices with light-emission in the near-infrared (800-1800 nm, e.g., for optical data communication and bio-imaging) when sorted by (n,m) species. The recent availability of highly pure, monochiral semiconducting SWNTs as bulk materials allows us to employ and further tailor their charge transport and light emission properties and thus enables their application in practical devices. Two new emissive species - charged excitons (trions) and bright sp3-defects - were recently discovered in SWNTs and have fundamentally changed our notions about SWNT luminescence. Both show red-shifted, narrow and enhanced emission. However, very little is yet known about their photophysical properties and especially their interactions with each other and their environment (e.g., in devices). Their emissive properties could potentially be tailored by external magnetic fields, dielectric environment and additional functional groups. Strong light-matter coupling in suitable optical cavities could be applied to create trion-polaritons in SWNTs as new low-mass charge carriers in polaritonic devices. Trions and emissive sp3-defects are not limited to SWNTs and hence these concepts could be transferred and applied to other low-dimensional semiconductors. The goals of this project are to - understand and use trions and trion-polaritons for light emission and polaritonic charge transport, - understand and tune the interactions of sp3-defects with charges and trions in SWNTs, - modify and apply sp3-defects for enhanced light emission from SWNTs in optoelectronic devices, - explore trions in new low-dimensional materials (e.g., graphene nanoribbons and novel monolayered semiconductors).

Background: RNA is an important regulator in biology. Recently we discovered the ubiquitous redox coenzyme nicotinamide adenine dinucleotide (NAD) to be attached to bacterial RNAs in a cap-like manner, and to modulate the functions of these RNAs, and others identified NAD-RNAs in eukaryotes. Enzymes were discovered that synthesize or break down NAD-RNAs. Scientific problems: NAD is just one of many coenzymes and metabolic intermediates that carry a nucleotide moiety which is not involved in the catalyzed reaction. Yet, it has been conserved through evolution, a fact that suggests high functional relevance. Hypothesis: The nucleotide moiety in coenzymes and metabolites is there for a reason: It enables cells to incorporate these compounds into specific RNAs. Linking reactive organic moieties to RNA may provide a biological strategy to localize these RNAs to enzymes, receptors, membranes or compartments, to sense environmental parameters, or to modulate the turnover or function of the RNAs or their targets. Objectives & research program: The aim of this project is to establish the scope and biological significance of coenzyme-linked RNAs in biology. We will expand the NAD captureSeq protocol to include reduced, phosphorylated, deamidated, and depyridinated NAD-RNAs. We will develop new CoenzymeSeq methods to identify cellular RNAs modified with coenzyme A, flavin, thiamine, and N-acetylglucosamine. We will apply these protocols to RNAs isolated from different organisms to explore the occurrence, abundance, and structural variety of such RNAs. For selected modified RNAs, we will unravel the biological significance and biosynthesis. Impact: This research challenges established textbook wisdom. It will provide fundamentally new links between gene regulation and metabolism in present-day biology and uncover a new layer of epitrancriptomic information. In addition, this project will impact our basic views on the evolution of metabolism and enzymatic catalysis.

The microtubule cytoskeleton is a dynamic array of cylindrical polymers that play important roles in various cellular processes including chromosome segregation and ciliogenesis. Hundreds of accessory factors – grouped under the term microtubule-associated proteins (MAPs) – orchestrate this network by governing the interaction profile and growth behaviour of microtubules. Due to this critical function, the precise abundance and distribution of MAPs within cells is of high importance and their deregulation is associated with various human diseases, such as ciliopathies and cancer. Surprisingly little is known about mechanisms involved in the regulation of MAPs and so far only a few of them have been identified as targets of the major proteolytic control machinery in cells, i.e. ubiquitin-proteasome system. In this work, I will further explore how MAPs are regulated by ubiquitin-dependent degradation by focussing on a multi-subunit ubiquitin E3 ligase called SCFFbxw5 that seems to play an important role in microtubule control according to preceding experiments of the host lab. Hence, I will investigate in detail two MAPs that were identified as substrates of SCFFbxw5 by the host lab in order to unravel the functional, temporal, and spatial characteristics of their ubiquitylation by SCFFbxw5. Since these MAPs are known to fulfil crucial functions in ciliogenesis and chromosome segregation, this work will promote our knowledge about how ubiquitin-dependent degradation contributes to these processes and may open up novel therapeutic strategies with the associated diseases.