The mechanisms controlling the conversion of spider silk proteins into insoluble fibres, which happens in a fraction of a second and in a defined region of the silk glands, are still unresolved. The N-terminal domain changes conformation and forms a homodimer when pH is lowered from 7 to 6; however, the molecular details still remain to be determined. Here we investigate site-directed mutants of the N-terminal domain from Euprosthenops australis major ampullate spidroin 1 and find that the charged residues D40, R60 and K65 mediate intersubunit electrostatic interactions. Protonation of E79 and E119 is required for structural conversions of the subunits into a dimer conformation, and subsequent protonation of E84 around pH 5.7 leads to the formation of a fully stable dimer. These residues are highly conserved, indicating that the now proposed three-step mechanism prevents premature aggregation of spidroins and enables fast formation of spider silk fibres in general.

Alzheimer disease (AD) affects around 30 million people, and the number is growing as life expectancy increases around the world. Pathologically, the disease is characterized by intracellular tangles formed by the microtubule-associated protein tau, and extracellular fibrillar deposits called amyloid plaques. The fibrils are composed of the amyloid β-peptide (Aβ), a proteolytic product generated from processing of the amyloid precursor protein (APP). A 40 residues long variant (Aβ40) is produced at high levels, (around 80-90% of all Aβ produced), but it is the longer and more hydrophobic 42-residues variant (Aβ42) that is the dominating species in plaques 1 . Most evidence from experiments in vitro and in transgenic mice over expressing Aβ suggest that oligomeric species, and not the fibrils, formed by Aβ are crucial for AD-

PostprintThis is the accepted version of a paper published in Analytical Chemistry. This paper has been peerreviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Chmyrov, V., Spielmann, T., Hevekerl, H., Widengren, J. (2015) Trans-Cis isomerization of lipophilic dyes probing membrane microviscosity in biological membranes and in live cells. Corresponding AuthorJerker Widengren: jerker@biomolphysics.kth.se.ABSTRACT Membrane environment and fluidity can modulate the dynamics and interactions of membrane proteins, and can thereby strongly influence the function of cells and organisms in general. In this work, we demonstrate that trans-cis isomerization of lipophilic dyes is a useful parameter to monitor packaging and fluidity of biomembranes. Fluorescence fluctuations, generated by trans-cis isomerization of the thiocarbocyanine dye Merocyanine 540 (MC540) was first analyzed by Fluorescence Correlation Spectroscopy (FCS) in different alcohol solutions. Similar isomerization kinetics could then also be monitored of MC540 in lipid vesicles, and the influence of lipid polarity, membrane curvature and cholesterol content was investigated. While no influence of membrane curvature and lipid polarity could be observed, a clear decrease in the isomerization rates could be observed with increasing cholesterol contents in the vesicle membranes. Finally, procedures to spatially map photo-induced and thermal isomerization rates on live cells by transient state (TRAST) imaging were established. Based on these procedures, MC540 isomerization was studied on live MCF7 cells, and TRAST images of the cells at different temperatures were found to reliably detect differences in the isomerization parameters. Our studies indicate that trans-cis isomerization is a useful parameter for probing membrane dynamics, and that the TRAST imaging technique can provide spatial maps of photo-induced isomerization as well as both photo-induced and thermal back-isomerization, resolving differences in local membrane micro-viscosity in live cells.

By narrowing the detection bandpass and increasing the signal-to-noise ratio in measuring the time-resolved fluorescence decay spectrum of colloidal CdSe-CdS/ZnS quantum dots (QDs), we show that directly after the photoexcitation, the fluorescence decay spectrum is characterized by a single exponential decay, which represents the energy relaxation of the photogenerated exciton from its initial high-energy state to the ground exciton state. The fluorescence decay spectrum of long decay time is in the form of β/t(2), where β is the radiative recombination time of the ground-state exciton and t is the decay time. Our findings provide us with a direct and quantitative link between fluorescence decay measurement data and fundamental photophysics of QD exciton, thereby leading to a novel way of applying colloidal QDs to study microscopic, physical and chemical processes in many fields including biomedicine.