758 research outputs found
Time resolved structural dynamics of butadiyne-linked porphyrin dimers
In this work the timescales and mechanisms associated with the structural dynamics of butadiyne-linked porphyrin dimers are investigated through time resolved narrowband pump / broadband probe transient absorption spectroscopy. Our results confirm previous findings that the broadening is partly due to a distribution of structures with different (dihedral) angular conformations. Comparison of measurements with excitations on the red and blue sides of the Q-band unravel the ground and excited state conformational re-equilibration timescales. Further comparison to a planarized dimer, through addition of a ligand, provide conclusive evidence for the twisting motion performed by the porphyrin dimer in solution
An alternate proton acceptor for excited-state proton transfer in green fluorescent protein: Rewiring GFP
The neutral form of the chromophore in wild-type green fluorescent protein (wtGFP) undergoes excited-state proton transfer (ESPT) upon excitation, resulting in characteristic green (508 nm) fluorescence. This ESPT reaction involves a proton relay from the phenol hydroxyl of the chromophore to the ionized side chain of E222, and results in formation of the anionic chromophore in a protein environment optimized for the neutral species (the I* state). Reorientation or replacement of E222, as occurs in the S65T and E222Q GFP mutants, disables the ESPT reaction and results in loss of green emission following excitation of the neutral chromophore. Previously, it has been shown that the introduction of a second mutation (H148D) into S65T GFP allows the recovery of green emission, implying that ESPT is again possible. A similar recovery of green fluorescence is also observed for the E222Q/H148D mutant, suggesting that D148 is the proton acceptor for the ESPT reaction in both double mutants. The mechanism of fluorescence emission following excitation of the neutral chromophore in S65T/H148D and E222Q/H148D has been explored through the use of steady state and ultrafast time-resolved fluorescence and vibrational spectroscopy. The data are contrasted with those of the single mutant S65T GFP. Time-resolved fluorescence studies indicate very rapid (<1 ps) formation of I* in the double mutants, followed by vibrational cooling on the picosecond time scale. The time-resolved IR difference spectra are markedly different to those of wtGFP or its anionic mutants. In particular, no spectral signatures are apparent in the picosecond IR difference spectra that would correspond to alteration in the ionization state of D148, leading to the proposal that a low-barrier hydrogen bond (LBHB) is present between the phenol hydroxyl of the chromophore and the side chain of D148, with different potential energy surfaces for the ground and excited states. This model is consistent with recent high-resolution structural data in which the distance between the donor and acceptor oxygen atoms is =2.4 Ã…. Importantly, these studies indicate that the hydrogen-bond network in wtGFP can be replaced by a single residue, an observation which, when fully explored, will add to our understanding of the various requirements for proton-transfer reactions within proteins
Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter
We report on the Hubble Space Telescope program to observe periodic comet
9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were
to study the generation and evolution of the coma resulting from the impact and
to obtain wide-band images of the visual outburst generated by the impact. Two
observing campaigns utilizing a total of 17 HST orbits were carried out: the
first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a
new, short-lived fan in the sunward direction on June 14. The principal
campaign began two days before impact and was followed by contiguous orbits
through impact plus several hours and then snapshots one, seven, and twelve
days later. All of the observations were made using the Advanced Camera for
Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a
spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact.
Baseline images of the comet, made prior to impact, photometrically resolved
the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement
with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following
the impact, the HRC images illustrate the temporal and spatial evolution of the
ejecta cloud and allow for a determination of its expansion velocity
distribution. One day after impact the ejecta cloud had passed out of the
field-of-view of the HRC.Comment: 15 pages, 14 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
Vibronic interactions in the visible and near-infrared spectra of C60− anions
Electron-phonon coupling is an important factor in understanding many properties of the C60 fullerides. However, there has been little success in quantifying the strength of the vibronic coupling in C60 ions, with considerable disagreement between experimental and theoretical results. We will show that neglect of quadratic coupling in previous models for C60- ions results in a significant overestimate of the linear coupling constants. Including quadratic coupling allows a coherent interpretation to be made of earlier experimental and theoretical results which at first sight are incompatible
The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy
We present spectral and spatial information for major volatile species in
Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT
2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62
AU), following the comet's perihelion passage on UT 2010 July 04. The total
production rate for water on July 26 was (1.90 +/- 0.12) x 10^28 molecules s-1,
and abundances of six trace gases (relative to water) were: CH3OH (1.58% +/-
0.23), C2H6 (0.39% +/- 0.04), NH3 (0.83% +/- 0.20), and HCN (0.13% +/- 0.02). A
detailed analysis of intensities for water emission lines provided a rotational
temperature of 35 +/- 3 K. The mean OPR is consistent with nuclear spin
populations in statistical equilibrium (OPR = 3.01 +/- 0.18), and the (1-sigma)
lower bound corresponds to a spin temperature > 38 K. Our measurements were
contemporaneous with a jet-like feature observed at optical wavelengths. The
spatial profiles of four primary volatiles display strong enhancements in the
jet direction, which favors release from a localized vent on the nucleus. The
measured IR continuum is much more sharply peaked and is consistent with a
dominant contribution from the nucleus itself. The peak intensities for H2O,
CH3OH, and C2H6 are offset by ~200 km in the jet direction, suggesting the
possible existence of a distributed source, such as the release of icy grains
that subsequently sublimed in the coma. On UT September 18, no obvious emission
lines were present in our spectra, nevertheless we obtained a 3-sigma upper
limit Q(H2O) < 2.86 x 10^27 molecules s-1
Plausible home stars of the interstellar object 'Oumuamua found in Gaia DR2
The first detected interstellar object 'Oumuamua that passed within 0.25au of
the Sun on 2017 September 9 was presumably ejected from a stellar system. We
use its newly determined non-Keplerian trajectory together with the
reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify past
close encounters. Such an "encounter" could reveal the home system from which
'Oumuamua was ejected. The closest encounter, at 0.60pc (0.53-0.67pc, 90%
confidence interval), was with the M2.5 dwarf HIP 3757 at a relative velocity
of 24.7km/s, 1Myr ago. A more distant encounter (1.6pc) but with a lower
encounter (ejection) velocity of 10.7km/s was with the G5 dwarf HD 292249,
3.8Myr ago. Two more stars have encounter distances and velocities intermediate
to these. The encounter parameters are similar across six different
non-gravitational trajectories for 'Oumuamua. Ejection of 'Oumuamua by
scattering from a giant planet in one of the systems is plausible, but requires
a rather unlikely configuration to achieve the high velocities found. A binary
star system is more likely to produce the observed velocities. None of the four
home candidates have published exoplanets or are known to be binaries. Given
that the 7 million stars in Gaia DR2 with 6D phase space information is just a
small fraction of all stars for which we can eventually reconstruct orbits, it
is a priori unlikely that our current search would find 'Oumuamua's home star
system. As 'Oumuamua is expected to pass within 1pc of about 20 stars and brown
dwarfs every Myr, the plausibility of a home system depends also on an
appropriate (low) encounter velocity.Comment: Accepted to The Astronomical Journa
WISE/NEOWISE Preliminary Analysis and Highlights of the 67P/Churyumov-Gerasimenko Near Nucleus Environs
On January 18-19 and June 28-29 of 2010, the Wide-field Infrared Survey
Explorer (WISE) spacecraft imaged the Rosetta mission target, comet
67P/Churyumov-Gerasimenko. We present a preliminary analysis of the images,
which provide a characterization of the dust environment at heliocentric
distances similar to those planned for the initial spacecraft encounter, but on
the outbound leg of its orbit rather than the inbound. Broad-band photometry
yields low levels of CO2 production at a comet heliocentric distance of 3.32 AU
and no detectable production at 4.18 AU. We find that at these heliocentric
distances, large dust grains with mean grain diameters on the order of a
millimeter or greater dominate the coma and evolve to populate the tail. This
is further supported by broad-band photometry centered on the nucleus, which
yield an estimated differential dust particle size distribution with a power
law relation that is considerably shallower than average. We set a 3-sigma
upper limit constraint on the albedo of the large-grain dust at <= 0.12. Our
best estimate of the nucleus radius (1.82 +/- 0.20 km) and albedo (0.04 +/-
0.01) are in agreement with measurements previously reported in the literature
Cometary activity in 2060 Chiron at minimum brightness
We present two-colour CCD imaging of 2060 Chiron obtained between 1996 and
1998 with the 2.15 m telescope at CASLEO (San Juan, Argentina). These
post-perihelion observations show that Chiron was then near its historical
brightness minima, however a coma was clearly detected. The dynamical state of
the coma is studied by means of azimuthally averaged surface brightness
profiles, which show the signatures of radiation pressure on the dust grain
distribution. Aperture photometry shows an achromatic dimming with an amplitude
\~ 0.09 mag in approximately one hour. If due to rotation of the nucleus, this
rather high amplitude is used to derive a new value for the nuclear magnitude,
m_0 ~ 6.80 mag.Comment: Accepted for publication in Planetary and Space Scienc
Temporal and Spatial Aspects of Gas Release During the 2010 Apparition of Comet 103P/Hartley-2
We report measurements of eight primary volatiles (H2O, HCN, CH4, C2H6,
CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) in comet
103P/Hartley-2 using high dispersion infrared spectroscopy. We quantified the
long- and short-term behavior of volatile release over a three-month interval
that encompassed the comet's close approach to Earth, its perihelion passage,
and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission.
We present production rates for individual species, their mixing ratios
relative to water, and their spatial distributions in the coma on multiple
dates. The production rates for water, ethane, HCN, and methanol vary in a
manner consistent with independent measures of nucleus rotation, but mixing
ratios for HCN, C2H6, & CH3OH are independent of rotational phase. Our results
demonstrate that the ensemble average composition of gas released from the
nucleus is well defined, and relatively constant over the three-month interval
(September 18 through December 17). If individual vents vary in composition,
enough diverse vents must be active simultaneously to approximate (in sum) the
bulk composition of the nucleus. The released primary volatiles exhibit diverse
spatial properties which favor the presence of separate polar and apolar ice
phases in the nucleus, establish dust and gas release from icy clumps (and
also, directly from the nucleus), and provide insights into the driver for the
cyanogen (CN) polar jet. The spatial distributions of C2H6 & HCN along the
near-polar jet (UT 19.5 October) and nearly orthogonal to it (UT 22.5 October)
are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water
was 2.85 \pm 0.20; the lower bound (2.65) defines Tspin > 32 K. These values
are consistent with results returned from ISO in 1997.Comment: 18 pages, 3 figures, to be published in: Astrophysical Journal
Letter
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