Non-equivalence of a and b subunits within human hemoglobin in conformational relaxation and molecular oxygen rebinding

В работе изучены неэквивалентность субъединиц а и b в гемоглобине человека, конформационная релаксация и повторное связывание молекулярного кислорода

Monitoring the CO and O2 rebinding to human hemoglobin and its subunits with time-resolved visible and mid-infrared spectroscopy

Human hemoglobin (Hb) is a tetramer consisting of two α and two β subunits. Each subunit contains one identical ferrous heme group which can reversibly bind one ligand molecule (CO or O2). Up to now, there has been no definitive agreement on how ligand-induced conformational changes influence individual ligand binding properties of the α and β subunits in the different conformational forms of tetrameric Hb

Direct observation by time-resolved infrared spectroscopy of the bright and the dark excited states of the [Ru(phen)2(dppz)]2+ light-switch compound in solution and when bound to DNA

The [Ru(phen)2(dppz)]2+ complex (1) is non-emissive in water but is highly luminescent in organic solvents or when bound to DNA, making it a useful probe for DNA binding. To date, a complete mechanistic explanation for this “light-switch” effect is still lacking. With this in mind we have undertaken an ultrafast time resolved infrared (TRIR) study of 1 and directly observe marker bands between 1280–1450 cm-1, which characterise both the emissive “bright” and the non-emissive “dark” excited states of the complex, in CD3CN and D2O respectively. These characteristic spectral features are present in the [Ru(dppz)3]2+ solvent light-switch complex but absent in [Ru(phen)3]2+, which is luminescent in both solvents. DFT calculations show that the vibrational modes responsible for these characteristic bands are predominantly localised on the dppz ligand. Moreover, they reveal that certain vibrational modes of the “dark” excited state couple with vibrational modes of two coordinating water molecules, and through these to the bulk solvent, thus providing a new insight into the mechanism of the light-switch effect. We also demonstrate that the marker bands for the “bright” state are observed for both L- and D enantiomers of 1 when bound to DNA and that photo-excitation of the complex induces perturbation of the guanine and cytosine carbonyl bands. This perturbation is shown to be stronger for the L enantiomer, demonstrating the different binding site properties of the two enantiomers and the ability of this technique to determine the identity and nature of the binding site of such intercalators

Time-resolved vibrational spectroscopic study of molecular nanoaggregate photocatalysts

The controlled aggregation of organic chromophores into supramolecular structures offers a way to control and tune photocatalytic activity.</jats:p

Time-resolved Raman spectroscopy of polaron formation in a polymer photocatalyst

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H2, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR3) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H2 production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons

On the shape of the power spectra of intensity fluctuations of a laser beam crossing a vortex flame

In this work, we used the data of a laboratory experiment on registration of fluctuations in the intensity of a laser beam passing through a vortex flame (model of a fire tornado) to calculate their power spectra. The nature of the decay of the power spectra in the high-frequency region was studied. The dominance of the two-scale decay of such spectra over the single-scale decay is established. The analysis carried out makes it possible to compare the results with the well-known Kolmogorov-Obukhov theory for estimating the turbulence parameters of the free atmosphere and to assume the presence of an inertial interval in the vortex flame

Solvent-Controlled Excited State Relaxation Path of 4‑Acetyl-4′-(dimethylamino)biphenyl

Stationary and picosecond time-resolved fluorescence (TRF) and absorption spectra were compared in different aprotic solvents at various temperatures for 4-acetyl-4′-(dimethylamino)­biphenyl (ADAB). A large value of the excited state dipole moment, 18–25 D, was estimated from the plot of solvatochromic shift. TRF spectra of ADAB recorded as a function of solvent polarity and temperature show unusual temporal evolution (shift and decay) of the fluorescence bands. In some cases, the dynamic Stokes shifts occur on a time scale much shorter than expected on the basis of literature data on solvent relaxation. In order to investigate variations in the energy of electronic transitions, oscillator strengths, and dipole moments upon changing the molecular geometry, quantum chemical modeling (DFT, TD-DFT, CIS) was performed for ADAB and its ground-state pretwisted derivative, 4-acetyl-4′-dimethylamino-2,2′-dimethylbiphenyl (ADAB-Me). Combination of spectroscopic data and computational results leads to the model of excited state relaxation which involves dynamic solvent-dependent interaction between two close-lying ¹nπ* and ¹ππ* excited electronic states

Structural dynamics around a hydrogen bond: Investigating the effect of hydrogen bond strengths on the excited state dynamics of carboxylic acid dimers

The photochemical dynamics of the acetic acid and trifluoro-acetic acid dimers in hexane are studied using time-resolved infrared absorption spectroscopy and ab initio electronic structure calculations. The different hydrogen bond strengths of the two systems lead to changes in the character of the accessed excited states and in the timescales of the initial structural rearrangement that define the early time dynamics following UV excitation. The much stronger hydrogen bonding in the acetic acid dimer stabilizes the system against dissociation. Ground state recovery is mediated by a structural buckling around the hydrogen bond itself with no evidence for excited state proton transfer processes that are usually considered to drive ultrafast relaxation processes in hydrogen bonded systems. The buckling of the ring leads to relaxation through two conical intersections and the eventual reformation of the electronic and vibrational ground states on a few picosecond timescale. In trifluoro-acetic acid, the weaker hydrogen bonding interaction means that the dimer dissociates under similar irradiation conditions. The surrounding solvent cage restricts the full separation of the monomer components, meaning that the dimer is reformed and returns to the ground state structure via a similar buckled structure but over a much longer, ∼100 ps, timescale