Photochemical Transformations of Proteinogenic and Non-Proteinogenic Amino Acids

The photochemistry of N-activated enantiomerically pure α-amino acids is described with emphasis on chemo-, regio-, stereo-, and spin selectivity. An especially valuable chromophore is the phthalimido group. The first excited singlet states are short-lived and deactivated (chemically) via homolytic CH cleavage or (physically) via electron-transfer steps. The first excited triplet states are chemically deactivated via electron-transfer reactions and subsequent deprotonation/coupling steps. A wide variety of product types were synthesized, and potential target molecules were available by tuning the reaction conditions. Also remote groups can be activated by means of electron-transfer steps, which represents an attractive new synthetic protocol for macrocyclization

Spin dephasing and photoinduced spin diffusion in high-mobility 110-grown GaAs-AlGaAs two-dimensional electron systems

We have studied spin dephasing and spin diffusion in a high-mobility two-dimensional electron system, embedded in a GaAs/AlGaAs quantum well grown in the [110] direction, by a two-beam Hanle experiment. For very low excitation density, we observe spin lifetimes of more than 16 ns, which rapidly decrease as the pump intensity is increased. Two mechanisms contribute to this decrease: the optical excitation produces holes, which lead to a decay of electron spin via the Bir-Aranov-Pikus mechanism and recombination with spin-polarized electrons. By scanning the distance between the pump and probe beams, we observe the diffusion of spin-polarized electrons over more than 20 microns. For high pump intensity, the spin polarization in a distance of several microns from the pump beam is larger than at the pump spot, due to the reduced influence of photogenerated holes.Comment: 4 pages, 3 figure

Spin-dependent diastereoselectivity in the photocycloaddition of aldehydes to 2,2-dimethyl-2,3-dihydrofuran

The simple (non-induced) diastereoselectivity of the photocycloaddition of aliphatic as well as aromatic aldehydes to 2,2-dimethyl-2,3-dihydrofuran (1) was analyzed as a function of the substrate concentration (spin mapping) and rationalized in terms of optimal spin-orbit coupling controlled triplet biradical geometries

Cyclotron effect on coherent spin precession of two-dimensional electrons

We investigate the spin dynamics of high-mobility two-dimensional electrons in GaAs/AlGaAs quantum wells grown along the $[001]$ and $[110]$ directions by time-resolved Faraday rotation at low temperatures. In measurements on the $(001)$-grown structures without external magnetic fields, we observe coherent oscillations of the electron spin polarization about the effective spin-orbit field. In non-quantizing magnetic fields applied normal to the sample plane, the cyclotron motion of the electrons rotates the effective spin-orbit field. This rotation leads to fast oscillations in the spin polarization about a non-zero value and a strong increase in the spin dephasing time in our experiments. These two effects are absent in the $(110)$-grown structure due to the different symmetry of its effective spin-orbit field. The measurements are in excellent agreement with our theoretical model.Comment: 4 pages, 3 figure

Anisotropic spin relaxation revealed by resonant spin amplification in (110) GaAs quantum wells

We have studied spin dephasing in a high-mobility two-dimensional electron system (2DES), confined in a GaAs/AlGaAs quantum well grown in the [110] direction, using the resonant spin amplification (RSA) technique. From the characteristic shape of the RSA spectra, we are able to extract the spin dephasing times (SDT) for electron spins aligned along the growth direction or within the sample plane, as well as the $g$ factor. We observe a strong anisotropy in the spin dephasing times. While the in-plane SDT remains almost constant as the temperature is varied between 4 K and 50 K, the out-of-plane SDT shows a dramatic increase at a temperature of about 25 K and reaches values of about 100 ns. The SDTs at 4 K can be further increased by additional, weak above-barrier illumination. The origin of this unexpected behavior is discussed, the SDT enhancement is attributed to the redistribution of charge carriers between the electron gas and remote donors.Comment: 6 pages, 3 figure

The two-pore channel TPCN2 mediates NAADP-dependent Ca2+-release from lysosomal stores

Second messenger-induced Ca2+-release from intracellular stores plays a key role in a multitude of physiological processes. In addition to 1,4,5-inositol trisphosphate (IP3), Ca2+, and cyclic ADP ribose (cADPR) that trigger Ca2+-release from the endoplasmatic reticulum (ER), nicotinic acid adenine dinucleotide phosphate (NAADP) has been identified as a cellular metabolite that mediates Ca2+-release from lysosomal stores. While NAADP-induced Ca2+-release has been found in many tissues and cell types, the molecular identity of the channel(s) conferring this release remained elusive so far. Here, we show that TPCN2, a novel member of the two-pore cation channel family, displays the basic properties of native NAADP-dependent Ca2+-release channels. TPCN2 transcripts are widely expressed in the body and encode a lysosomal protein forming homomers. TPCN2 mediates intracellular Ca2+-release after activation with low-nanomolar concentrations of NAADP while it is desensitized by micromolar concentrations of this second messenger and is insensitive to the NAADP analog nicotinamide adenine dinucleotide phosphate (NADP). Furthermore, TPCN2-mediated Ca2+-release is almost completely abolished when the capacity of lysosomes for storing Ca2+ is pharmacologically blocked. By contrast, TPCN2-specific Ca2+-release is unaffected by emptying ER-based Ca2+ stores. In conclusion, these findings indicate that TPCN2 is a major component of the long-sought lysosomal NAADP-dependent Ca2+-release channel

An Image-Free Opto-Mechanical System for Creating Virtual Environments and Imaging Neuronal Activity in Freely Moving Caenorhabditis elegans

Non-invasive recording in untethered animals is arguably the ultimate step in the analysis of neuronal function, but such recordings remain elusive. To address this problem, we devised a system that tracks neuron-sized fluorescent targets in real time. The system can be used to create virtual environments by optogenetic activation of sensory neurons, or to image activity in identified neurons at high magnification. By recording activity in neurons of freely moving C. elegans, we tested the long-standing hypothesis that forward and reverse locomotion are generated by distinct neuronal circuits. Surprisingly, we found motor neurons that are active during both types of locomotion, suggesting a new model of locomotion control in C. elegans. These results emphasize the importance of recording neuronal activity in freely moving animals and significantly expand the potential of imaging techniques by providing a mean to stabilize fluorescent targets

Total synthesis of the Amaryllidaceae alkaloid clivonine

Two syntheses of the Amaryllidaceae alkaloid clivonine (1) are described. Both employ previously reported 7-arylhydrindane 6 as an intermediate but differ in the method employed for subsequent introduction of what becomes the ring-B lactone carbonyl carbon (C7). The synthesis featuring a Bischler–Napieralski reaction for this transformation constitutes the first asymmetric synthesis of natural (+)-clivonine. Crystal structures for compounds (±)-13, (±)-16, (−)-20 and (±)-28 are also reported