Time-resolved Mid-infrared Spectroscopy Research Articles

Influence of Förster-type energy transfer on the vibrational relaxation of anionic hydration shells

We study the influence of Förster energy transfer on the vibrational relaxation dynamics of anionic hydration shells by performing time-resolved mid-infrared spectroscopy on the OH-stretch vibration of water molecules in aqueous solutions of sodium iodide. We observe that the Förster energy transfer leads to a pronounced acceleration of the vibrational relaxation. We describe the observed dynamics with a model in which we include the Förster vibrational energy transfer between the different hydroxyl groups in solution. With this model we can quantitatively describe the experimental data over a wide range of isotopic compositions and salt concentrations. Our results show that resonant energy transfer is an efficient mechanism assisting in the vibrational relaxation of anionic hydration shells.

Structure and dynamics of water in nonionic reverse micelles: A combined time-resolved infrared and small angle x-ray scattering study

We study the structure and reorientation dynamics of nanometer-sized water droplets inside nonionic reverse micelles (water/Igepal-CO-520/cyclohexane) with time-resolved mid-infrared pump-probe spectroscopy and small angle x-ray scattering. In the time-resolved experiments, we probe the vibrational and orientational dynamics of the O-D bonds of dilute HDO:H(2)O mixtures in Igepal reverse micelles as a function of temperature and micelle size. We find that even small micelles contain a large fraction of water that reorients at the same rate as water in the bulk, which indicates that the polyethylene oxide chains of the surfactant do not penetrate into the water volume. We also observe that the confinement affects the reorientation dynamics of only the first hydration layer. From the temperature dependent surface-water dynamics, we estimate an activation enthalpy for reorientation of 45 ± 9 kJ mol(-1) (11 ± 2 kcal mol(-1)), which is close to the activation energy of the reorientation of water molecules in ice.

Charge and Structural Dynamics in Photoinduced Phase Transition of (EDO-TTF)2PF6 Examined by Picosecond Time-Resolved Vibrational Spectroscopy

Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF)2PF6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83–1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300–1700 cm–1, where charge- and structure-sensitive C═C stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation.

Ultrafast Protein Conformational Alterations in Bacteriorhodopsin and Its Locked Analogue BR5.12

Bacteriorhodopsin, reconstituted with a sterically "locked" retinal chromophore, BR5.12, has frequently been used to elucidate elementary photoinduced processes in the native pigment bacteriorhodopsin. In this work, the vibrational response of BR5.12 to photoexcitation is investigated by means of femtosecond time-resolved mid-infrared and UV-vis spectroscopy. The electronically excited state of BR5.12 decays with a time constant of 18 ps. Neither in the UV-vis nor in the mid-IR spectral region are indications found for chromophore photoproducts, besides the full recovery of the electronic ground state. However, vibrational bands are observed at around 1660 and 1550 cm(-1) in the protein amide I and amide II band regions, respectively. They are formed within a few picoseconds or even instantaneously. Thus, they appear faster than the S1 decay and persist for at least 130 ps, i.e., for much longer than the S1 lifetime. These findings strongly suggest that the observed bands must be assigned to protein vibrations and that they are not caused by a photoinduced temperature rise. Thus, for the first time, ultrafast protein vibrational changes are detected in BR5.12, that are not associated with isomerization. Possibly they can be related to the enhanced chemical reactivity of photoactivated BR5.12 reported in the literature. In wild-type bacteriorhodopsin, bands with very similar spectral and kinetic characteristics are observed, suggesting that they might originate from a similar mechanism which is not isomerization. A plausible mechanism is a polarization induced protein conformational change, as discussed in the literature.

Formation of the Early Photoproduct Lumi-R of Cyanobacterial Phytochrome Cph1 Observed by Ultrafast Mid-Infrared Spectroscopy

The photoreactions of the Pr ground state of cyanobacterial phytochrome Cph1 from Synechocystis PCC 6803 have been investigated by picosecond time-resolved mid-infrared spectroscopy at ambient temperature. With femtosecond excitation of the Pr state at 640 nm, the photoisomerized Lumi-R product state is generated with kinetics and associated difference spectra indicative of vibrational cooling with tau(1) = 3 ps time constant and excited state decay with tau(1) = 3 ps, tau(2) = 14 ps, and tau(3) = 134 ps time constants. The Lumi-R state is characterized by downshifted absorption of three C=C modes assigned to C(15)=C(16), C(4)=C(6), and a delocalized C=C mode, in addition to the downshifted C(19)=O mode. The Lumi-R minus Pr difference spectrum is indicative of global restructuring of the chromophore on the ultrafast timescale, which is discussed in light of C(15) Z/E photoisomerization in addition to changes near C(5), which could be low bond order torsional angle changes.

The Yellow Excitonic Series of Cu2O Revisited by Lyman Spectroscopy

We report on the observation of the yellow exciton Lyman series up to the fourth term in Cu2O by time-resolved mid-infrared spectroscopy. The dependence of the oscillator strength on the principal quantum number n can be well reproduced using the hydrogenic model including an AC dielectric constant, and precise information on the electronic structure of the 1s exciton state can be obtained. A Bohr radius a_{1s}=7.9 A and a 1s-2p transition dipole moment \mu_{1s-2p}= 4.2 eA were found.

Watching a protein as it functions with 150-ps time-resolved x-ray crystallography.

We report picosecond time-resolved x-ray diffraction from the myoglobin (Mb) mutant in which Leu29 is replaced by Phe (L29Fmutant). The frame-by-frame structural evolution, resolved to 1.8 angstroms, allows one to literally "watch" the protein as it executes its function. Time-resolved mid-infrared spectroscopy of flash-photolyzed L29F MbCO revealed a short-lived CO intermediate whose 140-ps lifetime is shorter than that found in wild-type protein by a factor of 1000. The electron density maps of the protein unveil transient conformational changes far more dramatic than the structural differences between the carboxy and deoxy states and depict the correlated side-chain motion responsible for rapidly sweeping CO away from its primary docking site.

Time-resolved mid-infrared spectroscopy: methods and biological applications

Recent developments in time-resolved infrared spectroscopy have paved the way to probe transient intermediates with a high degree of functional group specificity on timescales as short as femtoseconds. This capability has been exploited in studies of biophysical phenomena ranging from protein folding/unfolding to ligand migration in proteins.