| 21. |
- Aili, Daniel, 1977-, et al.
(författare)
-
Folding Induced Assembly of Polypeptide Decorated Gold Nanoparticles
- 2008
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:17, s. 5780-5788
-
Tidskriftsartikel (refereegranskat)abstract
- Reversible assembly of gold nanoparticles controlled by the homodimerization and folding of an immobilized de novo designed synthetic polypeptide is described. In solution at neutral pH, the polypeptide folds into a helix-loop-helix four-helix bundle in the presence of zinc ions. When immobilized on gold nanoparticles, the addition of zinc ions induces dimerization and folding between peptide monomers located on separate particles, resulting in rapid particle aggregation. The particles can be completely redispersed by removal of the zinc ions from the peptide upon addition of EDTA. Calcium ions, which do not induce folding in solution, have no effect on the stability of the peptide decorated particles. The contribution from folding on particle assembly was further determined utilizing a reference peptide with the same primary sequence but containing both D and L amino acids. Particles functionalized with the reference peptide do not aggregate, as the peptides are unable to fold. The two peptides, linked to the nanoparticle surface via a cysteine residue located in the loop region, form submonolayers on planar gold with comparable properties regarding surface density, orientation, and ability to interact with zinc ions. These results demonstrate that nanoparticle assembly can be induced, controlled, and to some extent tuned, by exploiting specific molecular interactions involved in polypeptide folding.
|
|
| 22. |
- Alam, Rauful, et al.
(författare)
-
Stereoselective intermolecular allylic C-H trifluoroacetoxylation of functionalized alkenes
- 2012
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:21, s. 8778-8781
-
Tidskriftsartikel (refereegranskat)abstract
- Pd-catalyzed allylic C-H trifluoroacetoxylation of substituted alkenes was performed using PhI(OCOCF3)(2) as the oxidant and acyloxy source. Trifluoroacetoxylation of monosubstituted cyclopentenes and cyclohexenes proceeds with excellent regio- and diastereoselectivity. Studies with one of the possible (eta(3)-allyl)Pd(II) intermediates suggest that the reaction proceeds via stereoselective formation of Pd(IV) intermediates and subsequent stereo- and regioselective reductive elimination of the product.
|
|
| 23. |
- Alam, Rauful, et al.
(författare)
-
Synthesis of Adjacent Quaternary Stereocenters by Catalytic Asymmetric Allylboration
- 2015
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 137:35, s. 11262-11265
-
Tidskriftsartikel (refereegranskat)abstract
- Allylboration of ketones with gamma-disubstituted allylboronic acids is performed in the presence of chiral BINOL derivatives. The reaction is suitable for single-step creation of adjacent quaternary stereocenters with high selectivity. We show that, with an appropriate choice of the chiral catalyst and the stereoisomeric prenyl substrate, full control of the stereo- and enantioselectivity is possible in the reaction.
|
|
| 24. |
- Albinsson, Bo, 1963
(författare)
-
Dual fluorescence from N-6,N-6-dimethyladenosine
- 1997
-
Ingår i: Journal of the American Chemical Society. - 1520-5126 .- 0002-7863. ; 119:27, s. 6369-6375
-
Tidskriftsartikel (refereegranskat)abstract
- The adenosine derivative N-6,N-6-dimethyladenosine (DMA) shows dual fluorescence in solvents of different polarity. In addition to the ''normal'' fluorescence at 330 nm, another band is observed at 500 nm. The long wavelength emission dominates in aprotic solvents but is dynamically quenched by protic solvents. Steady-state and lifetime measurements show that the emissions originate from two excited state species; the short wavelength emission is from the directly populated excited state which irreversibly isomerizes into the species responsible for the long wavelength emission. It is conceivable to assign the long wavelength emitting species to a twisted intramolecular charge transfer state (TICT). The fluorescence quantum yield of the short wavelength emission is approximately 4 x 10(-4) at room temperature and increases by three orders of magnitude when the temperature is lowered to 80 K in accordance with the behavior of normal nucleic acid bases. In contrast, the long wavelength fluorescence quantum yield is almost temperature independent. The different photophysical processes for DMA are summarized into a kinetic scheme where the temperature quenching of the short wavelength fluorescence is exclusively through isomerization into the long wavelength emitting species. Direct internal conversion to the ground state, commonly believed to be the dominant process for nonradiative deactivation of the DNA bases, makes a negligible contribution for DMA.
|
|
| 25. |
- Albinsson, Bo, 1963, et al.
(författare)
-
ELECTRONIC-TRANSITION MOMENT DIRECTIONS AND IDENTIFICATION OF LOW-ENERGY N-PI-ASTERISK STATES IN WEAKLY PERTURBED PURINE CHROMOPHORES
- 1993
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 115:1, s. 223-231
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of UV linear dichroism on purine and three methyl derivatives partially oriented in poly(vinyl alcohol) matrix gave direct evidence for the assignment of the first singlet npi* state. Intensity distributions and moment directions for the first three pi --> pi* transitions were also determined. The pi --> pi* transitions in purine were found to be polarized at (angles, relative to the pseudo-symmetry long axis, counted positive in the N7 direction): -31-degrees +/- 5-degrees (II at 265 nm), +38-degrees +/- 5-degrees (III at 244 nm), and +36-degrees +/- 10-degrees (IV at 214 nm). The transition energies and moment directions were not markedly perturbed by methyl substitution at the sixth, seventh, or ninth position. Therefore, these methyl substituents could be used as orientational perturbers to resolve a sign ambiguity problem regarding transition moment directions. The orientation were determined by infrared dichroic measurements using both in-plane and out-of-plane polarized vibrational transitions. In addition, the phosphorescence spectra were studied, including phosphorescence anisotropy, phosphorescence lifetimes, and quantum yields, for the purines in an organic glass at 80 K. Based on these measurements, the lowest triplet state is concluded to have effectively pipi* character, and its emission allowedness appears to originate from spin-orbit interactions primarily with singlet sigmapi* states but also with singlet pipi* states via vibronic mixing. The phosphorescence emission spectra of purine and 6-methylpurine are complex, compared to 7-methylpurine and 9-methylpurine, with emission wavelength-dependent lifetimes and excitation spectra. This is ascribed to a prototropic tautomeric equilibrium between the 7H and 9H forms of purine and 6-methylpurine, a ground-state heterogeneity that we believe has caused confusion in earlier studies and, e.g., led to an assignment of the phosphorescence origin of purine.
|
|
| 26. |
|
|
| 27. |
- Aleksich, Mariya, et al.
(författare)
-
XFEL Microcrystallography of Self-Assembling Silver n-Alkanethiolates
- 2023
-
Ingår i: Journal of the American Chemical Society. - 0002-7863. ; 145:31, s. 17042-17055
-
Tidskriftsartikel (refereegranskat)abstract
- New synthetic hybrid materials and their increasing complexity have placed growing demands on crystal growth for single-crystal X-ray diffraction analysis. Unfortunately, not all chemical systems are conducive to the isolation of single crystals for traditional characterization. Here, small-molecule serial femtosecond crystallography (smSFX) at atomic resolution (0.833 Å) is employed to characterize microcrystalline silver n-alkanethiolates with various alkyl chain lengths at X-ray free electron laser facilities, resolving long-standing controversies regarding the atomic connectivity and odd-even effects of layer stacking. smSFX provides high-quality crystal structures directly from the powder of the true unknowns, a capability that is particularly useful for systems having notoriously small or defective crystals. We present crystal structures of silver n-butanethiolate (C4), silver n-hexanethiolate (C6), and silver n-nonanethiolate (C9). We show that an odd-even effect originates from the orientation of the terminal methyl group and its role in packing efficiency. We also propose a secondary odd-even effect involving multiple mosaic blocks in the crystals containing even-numbered chains, identified by selected-area electron diffraction measurements. We conclude with a discussion of the merits of the synthetic preparation for the preparation of microdiffraction specimens and compare the long-range order in these crystals to that of self-assembled monolayers.
|
|
| 28. |
- Allardice, Jesse R., et al.
(författare)
-
Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission
- 2019
-
Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 141:32, s. 12907-12915
-
Tidskriftsartikel (refereegranskat)abstract
- Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 +/- 5%) before quantitative harvesting of the triplet excitons (95 +/- 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.
|
|
| 29. |
- Allgöwer, Friederike, et al.
(författare)
-
Molecular Principles of Redox-Coupled Protonation Dynamics in Photosystem II
- 2022
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:16, s. 7171-7180
-
Tidskriftsartikel (refereegranskat)abstract
- Photosystem II (PSII) catalyzes light-driven water oxidization, releasing O2 into the atmosphere and transferring the electrons for the synthesis of biomass. However, despite decades of structural and functional studies, the water oxidation mechanism of PSII has remained puzzling and a major challenge for modern chemical research. Here, we show that PSII catalyzes redox-triggered proton transfer between its oxygen-evolving Mn4O5Ca cluster and a nearby cluster of conserved buried ion-pairs, which are connected to the bulk solvent via a proton pathway. By using multi-scale quantum and classical simulations, we find that oxidation of a redox-active Tyrz (Tyr161) lowers the reaction barrier for the water-mediated proton transfer from a Ca2+-bound water molecule (W3) to Asp61 via conformational changes in a nearby ion-pair (Asp61/Lys317). Deprotonation of this W3 substrate water triggers its migration toward Mn1 to a position identified in recent X-ray free-electron laser (XFEL) experiments [Ibrahim et al. Proc. Natl. Acad. Sci. USA 2020, 117, 12,624–12,635]. Further oxidation of the Mn4O5Ca cluster lowers the proton transfer barrier through the water ligand sphere of the Mn4O5Ca cluster to Asp61 via a similar ion-pair dissociation process, while the resulting Mn-bound oxo/oxyl species leads to O2 formation by a radical coupling mechanism. The proposed redox-coupled protonation mechanism shows a striking resemblance to functional motifs in other enzymes involved in biological energy conversion, with an interplay between hydration changes, ion-pair dynamics, and electric fields that modulate the catalytic barriers.
|
|
| 30. |
|
|
