Photoactivatable rhodamine spiroamides and spirocyclic diazoketones emerged recently as synthetic markers applicable in multicolor super-resolution microscopy. However, their applicability in single molecule localization microscopy (SMLM) is often limited by aggregation, unspecific adhesion, and low reactivity caused by insufficient solubility and precipitation from aqueous solutions. We report here two synthetic modifications increasing the polarity of compact polycyclic and hydrophobic labels decorated with a reactive group: attachment of 3-sulfo-l-alanyl-beta-alanine dipeptide (a "universal hydrophilizer") or allylic hydroxylation in photosensitive rhodamine diazoketones (and spiroamides). The super-resolution images of tubulin and keratin filaments in fixed and living cells exemplify the performance of "blinking" spiroamides derived from N, N, N', N'-tetramethyl rhodamine.

New fluorescent photochromic compounds (1-H and 1-Boc) have been synthesized and characterized in different solvents. The fluorescence emission can be switched "on" and "off" with visible light and UV, respectively, by means of the photochromic reaction. The emission wavelength and efficiency strongly depend on the polarity of the solvent. The compounds show a positive solvatochromic effect in the emission maxima, and their fluorescence quantum yield decreases as the solvent's polarity increases (from cyclohexane to dioxane). In solvents more polar than dioxane the emission is too weak and therefore undetectable, and thus 1-H and 1-Boc behave as "normal" photochromic compounds. The pho-

Selective single and double labelling of the natural ganglioside GM1 enables one to introduce various markers into different parts of the glycosphingolipid molecule without changing the natural skeleton. To that end, N‐Fmoc‐2‐amino‐, N‐Fmoc‐18‐amino‐ and S‐(ethoxythiocarbonyl)‐18‐mercaptostearic acids have been prepared and coupled with the primary amino group in the sphingosine part of lyso‐GM1 and deAc‐deAcyl‐GM1 gangliosides. The products of these coupling reactions – building blocks 16a, 16b, 16c, 26 and 27 – may be used for the synthesis of GM1 derivatives with one or two fluorescent dye moieties or other labels of various polarities. Examples of various labelling strategies, using hydrophilic and lipophilic photostable fluorescent dyes, have been made available. The GM1 derivatives 17a, 22a and 23c labelled with the fluorescent dye ATTO 647N or the doubly labelled derivative 25b can be used as probes in fluorescence correlation spectroscopy (in conventional microscopy or stimulated emission depletion nanoscopy) to study the diffusion of lipid analogues in model or live cell membranes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture is misleading! the light‐induced changes of a newly designed photochromic, fluorescent, and solvatochromic compound. The open isomer (top) is colorless and fluorescent. Its emission changes from blue to yellow‐green with increasing polarity of the solvent (in the picture: cyclohexane, tetrachloromethane, benzene, toluene, xylene, and dioxane, from left to right). The closed isomer (bottom) is blue and nonemissive (for clarity, the picture was taken with some amount of the open isomer remaining). UV and visible (green) light interconverts both isomers. As a result, the dye provides multiple readout signals: color changes, on/off switching of the fluorescence signal, and the color of the emission that can be used to sense changes in the polarity of the microenvironment. Detailed synthesis and properties are reported in the article by M. L. Bossi et al. on p. 2531 f. Irene B?ttcher‐Gajewski (MPI for Biophysical Chemistry) is acknowledged for the photographical content of the cover picture.

DISSERTATION zur Erlangung des mathematisch-naturwissenschaftlichen Doktorgrades "Doctor rerum naturalium" der Georg-August-Universität Göttingen vorgelegt von Sergey Yan aus Leningrad (UdSSR) Göttingen, 2010