A fast recoiling silk-like elastomer facilitates nanosecond nematocyst discharge.

BACKGROUND: The discharge of the Cnidarian stinging organelle, the nematocyst, is one of the fastest processes in biology and involves volume changes of the highly pressurised (150 bar) capsule of up to 50%. Hitherto, the molecular basis for the unusual biomechanical properties of nematocysts has been elusive, as their structure was mainly defined as a stress-resistant collagenous matrix. RESULTS: Here, we characterise Cnidoin, a novel elastic protein identified as a structural component of Hydra nematocysts. Cnidoin is expressed in nematocytes of all types and immunostainings revealed incorporation into capsule walls and tubules concomitant with minicollagens. Similar to spider silk proteins, to which it is related at sequence level, Cnidoin possesses high elasticity and fast coiling propensity as predicted by molecular dynamics simulations and quantified by force spectroscopy. Recombinant Cnidoin showed a high tendency for spontaneous aggregation to bundles of fibrillar structures. CONCLUSIONS: Cnidoin represents the molecular factor involved in kinetic energy storage and release during the ultra-fast nematocyst discharge. Furthermore, it implies an early evolutionary origin of protein elastomers in basal metazoans.

Ring-closure and isomerization capabilities of spiropyran-derived merocyanine isomers.

We report the photochemistry of two ring-open isomers, namely TTC and TTT, of a bidirectional photoswitchable spiropyran, 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro BIPS). Both isomers are capable of ring closure after excitation with visible fs laser pulses, as disclosed by pump-wavelength-dependent transient absorption experiments in the visible spectral range. The main isomer TTC has its maximum absorption at 560 nm, whereas the minor isomer TTT is red-shifted (600 nm). The excited-state lifetimes differ strongly (τ ≈ 900 ps for TTT and τ ≈ 95 ps for TTC), nevertheless the quantum efficiencies for ring closure (40% for TTC and 35% for TTT) and isomerization (1-2% for TTC and 1-2% for TTT) are comparable. With regard to the bidirectional photoswitching capabilities, 6,8-dinitro BIPS is the first molecular switch based on a 6π-electrocyclic reaction where both ring-open isomers are capable of ring closure.

Coherent two-dimensional ultraviolet spectroscopy in fully noncollinear geometry.

We introduce fully noncollinear coherent two-dimensional (2D) spectroscopy in the UV domain with an all-reflective and miniaturized setup design. Phase stability is achieved via pairwise beam manipulation, and the concept can be transferred to all wavelength regimes. Here we present results from an implementation that has been optimized for wavelengths between 250 and 375 nm. Interferometric measurements prove phase stability over several hours. We obtained 2D spectra of the nonpolar UV chromophore p-terphenyl in ethanol, excited with 50 fs pulses at 287 nm.

Ultrafast bidirectional photoswitching of a spiropyran.

We report on bidirectional photochemical switching of 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open merocyanine form. This is studied by femtosecond three-color pump-repump-probe experiments. Both ring opening and ring closure are photoinduced. Completion of an entire cycle, consisting of opening and subsequent closure, can be achieved within 40 ps. A much shorter time (<6 ps) is needed for the converse cycle, consisting of initial ring closure and subsequent ring opening. Furthermore, we perform pump-probe experiments with ultraviolet/visible pump and visible/mid-infrared probe pulses for an unambiguous spectroscopic identification of the open and closed molecular forms. Following visible excitation of the ring-open molecules, ultrafast ring closure is observed directly in the mid-infrared. The quantum efficiencies for ring opening and ring closure starting from the respective equilibirum states are determined to be approximately 9% and 40%. These results show that 6,8-dinitro-BIPS is an ultrafast bidirectional molecular switch exhibiting a high quantum efficiency.

Ultrafast photoconversion of the green fluorescent protein studied by accumulative femtosecond spectroscopy.

The irreversible photoconversion of T203V green fluorescent protein (GFP) via decarboxylation is studied under femtosecond excitation using an accumulative product detection method that allows us to measure small conversion efficiencies of down to DeltaOD = 10(-7) absorbance change per pulse. Power studies with 800- and 400-nm pulse excitation reveal that excitation to higher states of the neutral form of the GFP chromophore induces photoconversion very efficiently. The singly excited neutral chromophore is a resonant intermediate of the two-step excitation process that leads to efficient photoconversion. We determine the dynamics of this two-step process by separating the excitation step of the neutral chromophore from the further excitation step to the reactive state in a time-resolved two-color experiment. The dynamics show that a further excitation to the very reactive higher excited state is only possible from the initially excited neutral chromophore and not from the fluorescent intermediate state. For applications of GFP in two-photon fluorescence microscopy, the found photochemical behavior implies that the high intensity conditions used in microscopy can lead to photoconversion easily and care has to be taken to avoid unwanted photoconversion.

Inherently phase-stable coherent two-dimensional spectroscopy using only conventional optics.

We introduce an inherently phase-stable setup for coherent two-dimensional femtosecond spectroscopy in noncollinear box geometry using only conventional beam splitters, mirrors, and delay stages. Avoiding diffractive optics, pulse shapers, and active phase-locking loops, our spectroscopy setup is simple, robust, and works for ultrabroad bandwidths in all spectral regimes (infrared, visible, and ultraviolet).

Generation of polarization-shaped ultraviolet femtosecond pulses.

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.

Product accumulation for ultrasensitive femtochemistry.

We present a novel experimental method for studying photochemical reactions that involve permanent products. The accumulation of photoproducts facilitates the measurement of extremely small product yields. A calibration of the setup accounts for diffusion effects, and the experimental results can be expressed in terms of single-pulse photochemical efficiencies. A demonstration experiment on indocyanine green (ICG) is presented. The general method is suited both for femtosecond spectroscopy and quantum control experiments.

Transformations to diagonal bases in closed-loop quantum learning control experiments.

This paper discusses transformations between bases used in closed-loop learning control experiments. The goal is to transform to a basis in which the number of control parameters is minimized and in which the parameters act independently. We demonstrate a simple procedure for testing whether a unitary linear transformation (i.e., a rotation amongst the control variables) is sufficient to reduce the search problem to a set of globally independent variables. This concept is demonstrated with closed-loop molecular fragmentation experiments utilizing shaped, ultrafast laser pulses.

Gaining mechanistic insight from closed loop learning control: the importance of basis in searching the phase space.

This paper discusses different routes to gaining insight from closed loop learning control experiments. We focus on the role of the basis in which pulse shapes are encoded and the algorithmic search is performed. We demonstrate that a physically motivated, nonlinear basis change can reduce the dimensionality of the phase space to one or two degrees of freedom. The dependence of the control goal on the most important degrees of freedom can then be mapped out in detail, leading toward a better understanding of the control mechanism. We discuss simulations and experiments in selective molecular fragmentation using shaped ultrafast laser pulses.