Analysis of the local conformation of proteins with two-dimensional fluorescence techniques

Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides. Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids. In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca²⁺ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone. Received: 10 February 2000 / Published online: 13 September 2000     

Cold atomic beam from a rubidium funnel

We report an experimental demonstration of a continuous, slow and cold beam of rubidium atoms from a two-dimensional magneto-optic trap or atomic funnel. Typically 7.3(7)×10⁸ atoms/s are ejected from the funnel with a variable velocity in the range 2–8 m/s and a temperature of 45–55 μK in the moving frame. This represents the first demonstration of sub-Doppler laser cooling in an atomic beam and temperatures as low as ≈25 μK have been observed. Received: 30 September 1999 / Published online: 5 April 2000     

Optical manipulation of group III atoms

We present details for the laser manipulation of group III atoms, specifically aluminum, gallium, and indium. The practical considerations of accomplishing this manipulation are discussed and alternative schemes are presented for each species. The possibility of using such an optical technique for composition modulation during semiconductor growth for the fabrication of quantum wire and quantum dot structures is also discussed. Received: 29 September 1999 / Published online: 5 April 2000     

GaN blue diode lasers: a spectroscopist’s view

We have characterized the spectroscopic properties of one of the first samples of blue-emitting diode lasers based on GaN. With such a laser diode operated inside a standard extended cavity arrangement we find a mode-hop free tuning range of more than 20 GHz and a linewidth of 10 MHz. Doppler-free spectroscopy on an indium atomic beam reveals the isotope shift between the two major indium isotopes as well as efficient optical pumping. Received: 8 November 1999 / Revised version: 31 January 2000 / Published online: 8 March 2000     

Pulsed holmium laser ablation of tissue phantoms: correlation between bubble formation and acoustic transients

5 Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 μm, duration: 180 μs FWHM), were delivered through 400 and 600 μm diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated using simultaneous flash micro-videography and pressure recording for radiant exposures of 20–382 J/cm². Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of up to 4.2 mm in length were observed in gels with a Young’s modulus of 2.9×10⁵ Pa at a pulse energy of 650 mJ. An increase of Young’s modulus (reduction in water content) led to a monotonic reduction of bubble size. In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased from 100±14 bars to 17±6 bars with increasing Young’s modulus over the studied range. Theoretical analysis suggested a major influence on bubble dynamics of the mass and energy transfer through the bubble boundary. Received: 26 August 1996/Revised version: 10 February 1997     

The effect of DNA phase structure on DNA walks

We have performed several kinds of DNA walks which often are the first steps for further analysis of DNA structure and long range correlations. The DNA walks analysing frequency of G+C versus A+T cannot indicate the coding strand while purine versus pyrimidine DNA walks or two-dimensional (A-T,G-C) DNA walks in some instances can indicate the coding strand but cannot resolve the coding frame. The modified two-dimensional (A-T,G-C) DNA walks respecting the three-nucleotide codon structure show very high correlation in nucleotide composition of DNA coding sequences. They can distinguish between coding and non-coding sequences and indicate the strand and the phase in which DNA is coding. Received: 8 January 1998 / Received in final form and Accepted: 28 January 1998     

From torsional spectra to Hamiltonians and dynamics: effects of coupled bright and dark states of 9-(N-carbazolyl)-anthracene

The torsional dynamics of the 9-(N-carbazolyl)-anthracene (C9A) molecule is investigated by means of time-independent (1) and time-dependent (2) quantum-mechanical simulations in a diabatic representation. The study includes effects of surface crossing of the brightS ₁ state with a dark state. (1) The intensity pattern of theS ₀ S ₁ fluorescence excitation spectrum is used to fit an effective one-dimensional Hamiltonian with a single-minimum potential for the dark state together with diabatic couplings to the double well potential of the bright state. (2) Based on this Hamiltonian, first predictions for a pump-probe scheme are made. In the pump process the molecules are excited to theS ₁ state followed by competing torsions in the bright state and diabatic curve crossings to the dark state, depending on the pump frequency. Assuming the probe process to be an ionization from the bright state, the interfering effects of the dark state on the dynamics in the bright state can be monitored in a directly time-dependent way on a fs-ps time scale.     

Be和B的几个小分子及其离子的电子结构

B和Be化合物的电子结构研究可以为~(10)Be质谱检测中~(10)B干扰的消除提供 理论依据。用B3LYP/6-311 + + G（3df, 3pd）方法和G3方法计算了BF(~1∑~+)， BeF~(~2∑~+)，HBO(~1A')，HBeO(~2∑~+)，HBeF(~1∑~+)，BBeB(~1A_1)，H_2BO (~2B_2)和H_3BO(~1A')等分子及其负离子BF~- (~2∏)，BeF~- (~1∑~+)，HBO~- (~2A')，HBeO~- (~1∑~+)，HBeF~- (~2A')，BBeB~- (~2A_1)，H_2BO~- (~1A_1) ，H_3BO~-(~2A')的电子结构，并利用CASSCF-MRCISD方法计算了BeF(2~∑~+) /BeF~-(~1∑~+)，BF(~1∑~+)/BF~-(~2∏)和BF(~3∏)/BF~-(~4∑~-)的热能曲线。 探讨了这些体系的稳定性。解释了~(10)Be质变测量方法中的有关结果。     

Angle and bond-length dependent C6 coefficients for H2 interacting with H,Li, Be and rare gas atoms

Summary Accurate new C₆ dispersion energy coefficients, and their dependence on the diatom orientation and bond length, are calculated for molecular hydrogen interacting with an atom of H, Li, Be, He, Ne, Ar, Kr or Xe. They are generated from accurateab initio pseudo dipole oscillator strength distributions (DOSD) for H₂, H, He and Be, and reliable semiempirical ones for Li, Ne, Ar, Kr and Xe. Compact power series expansions for the diatom bond-length dependence of these coefficients, suitable for incorporation into representations of full potential energy surfaces for these systems, are determined and assessed.     

炔烃桥和氢桥铍化合物中铍原子的共价

根据作者之一提出的共价的新定义，用DV－X_αSCC和自然健轨道法研究了[MeBe(C≡CMe)NMe_3]_2，[Be(C≡CR)_2]_n，[MeBeH·NMe_3]及(BeH_2)_n等化合物中铍原子的共价．结果表明，在这几种化合物中铍原子的共价都是6．