The geminate ligand recombination reactions of photolyzed carbonmonoxyhemoglobin were studied in a nanosecond double-excitation-pulse time-resolved absorption experiment. The second laser pulse, delayed by intervals as long as 400 ns after the first, provided a measure of the geminate kinetics by rephotolyzing ligands that have recombined during the delay time. The peak-to-trough magnitude of the Soret band photolysis difference spectrum measured as a function of the delay between excitation pulses showed that the room temperature kinetics of geminate recombination in adult human hemoglobin are best described by two exponential processes, with lifetimes of 36 and 162 ns. The relative amounts of bimolecular recombination to T- and R-state hemoglobins and the temperature dependence of the submicrosecond kinetics between 283 and 323 K are also consistent with biexponential kinetics for geminate recombination. These results are discussed in terms of two models: geminate recombination kinetics modulated by concurrent protein relaxation and heterogeneous kinetics arising from alpha and beta chain differences.
Pumping picosecond optical parametric oscillators by a pulsed Nd:YAG laser mode locked using a nonlinear mirror Appl.Nanosecond time-resolved optical rotatory dispersion ͑TRORD͒ techniques are coupled with laser temperature-jump ͑T-jump͒ triggering in an instrument that measures ultrafast protein folding-unfolding dynamics with high specificity to secondary structure. Far-ultraviolet ͑UV͒ ORD can be measured with this instrument over a wide wavelength range at times as early as 35 ns after a 3 ns laser T-jump pulse. The fundamental of a Nd:YAG laser is passed through a D 2 -filled Raman shifter to generate a pulse of 1.5 m infrared ͑IR͒ light that is efficiently absorbed by water. The resulting T-jump is stable for at least 1 ms before decaying back to the starting temperature with a time constant of ϳ30 ms. The ability to measure entire TRORD band shapes during this temporal window makes it possible to distinguish between changes in the signal due to a genuine unfolding or refolding process and changes due to artifacts. The technique, applicable to a wide variety of proteins, is demonstrated here in submillisecond unfolding studies of RNAse A and cytochrome c.
Several years ago a time-resolved circular dichroism technique for the far ultraviolet spectral region with submicrosecond (10−7 s) time resolution was developed using a xenon flash lamp probe source for measurements of circular dichroism (CD) signals. Recent improvements in Ti:sapphire lasers, providing the ability to frequency-convert the fundamental outputs to produce second, third, and fourth harmonic pulses, allow single wavelength measurements of CD with nanosecond (10−9 s) time resolution over a broad spectral region (205–910 nm). This provides a powerful technique to study fast biophysical phenomena such as protein folding processes. In this article, the methodology and preliminary application of this new technique are presented.
The effect of chuck temperature adjustment on critical dimension uniformity was studied for the shallow trench isolation etch process by introducing a temperature gradient in a multi-temperature-zone electrostatic chuck. It is shown that the initial radial critical dimension non-uniformity can be improved by a gradual temperature adjustment of the electrostatic chuck and results in the target specification values of uniformity, 3σ ≤ 1.5 nm, for a critical dimension of about 35 nm. Both temperature and RF sensor wafers were used to analyze the impact of an electrostatic chuck temperature gradient on process uniformity by utilizing their unique in situ spatial and temporal mapping capabilities. Thus, the across-wafer thermal sensitivity of the critical dimension was estimated for dense structures: a temperature change of 1 °C leads to a critical dimension change of ∼0.7 nm. The RF sensor wafer was also shown to have a clear response of RF current uniformity to the electrostatic chuck temperature gradient that suggests there could be other phenomena affecting critical dimension uniformity besides temperature itself. The pure temperature contribution to critical dimension change was found to be less than 0.3 nm/°C for the temperature range studied. Finally, a possible mechanism of critical dimension tuning is discussed and an assessment of each separate etch step’s sensitivity to the electrostatic chuck temperature gradient is performed.
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