Laser temperature-jump

This report has been written in order to demonstrate the nature of spin-state transitions and to review the studies of dynamical properties of spin transition compounds, both in solution and in the solid state. Spin-state transitions are usually rapid and thus relaxation methods for the microsecond and nanosecond range have been applied. The first application of relaxation techniques to the spin equilibrium of an iron(II) complex involved Raman laser temperature-jump measurements in 1973 [28]. The more accurate ultrasonic relaxation method was first applied in 1978 [29]. These studies dealt exclusively with the spin-state dynamics in solution and were recently reviewed by Beattie [30]. A recent addition to the study of spin-state transitions both in solution and the... 

The observation of a single set of resonances in the NMR spectra of [Fe(HB(pz)3)2], spectra that are clearly obtained for a mixture of the high-spin and low-spin forms of the complex, indicates that the equilibrium between the two states is rapid on the NMR time scale [27]. Subsequent solution studies by Beattie et al. [52, 53] using both a laser temperature-jump technique and an ultrasonic relaxation technique have established that the spin-state lifetime for [Fe(HB(pz)3)2] is 3.2xl0 8 s. These studies also established... 

The effect of the DNA sequence dependence on the binding dynamics of 5 and 6 with ct-DNA (42% GC content) and ml-DNA (72% GC content) was investigated using laser temperature jump experiments.118 Only one relaxation process was observed for both guests, but the presence of the leveling off effect at high DNA concentration was dependent on the guest and the type of DNA. No values for the rate constants were reported in this study. 

Raman earth halide clusters, 46 2-3 Raman laser temperature-jump technique, 32 17-18... 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

Fig. 2. Schematic diagram of the Raman laser temperature-jump experiment.

The Raman laser temperature-jump technique has been used in studies of a variety of spin-equilibrium processes. It was used in the first experiment to measure the relaxation time of an octahedral spin-equilibrium complex in solution (14). Its applications include investigations of cobalt(II), iron(II), iron(III), and nickel(II) equilibria. 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

Methods T, Raman laser temperature-jump U, ultrasonic relaxation P, photoperturbation. 

Consideration of the thermodynamics of a representative reaction coordinate reveals a number of interesting aspects of the equilibrium (Fig. 5). Because the complex is in spin equilibrium, AG° x 0. Only complexes which fulfill this condition can be studied by the Raman laser temperature-jump or ultrasonic relaxation methods, because these methods require perturbation of an equilibrium with appreciable concentrations of both species present. The photoperturbation technique does not suffer from this limitation and can be used to examine complexes with a larger driving force, i.e., AG° 0. In such cases, however, AG° is difficult to measure and will generally be unknown. 

The spin-equilibrium dynamics of iron(III) complexes in solution have been examined with the techniques of Raman laser temperature-jump, ultrasonic relaxation, and photoperturbation. The complexes investigated, the relaxation times observed, and one of the derived rate constants are presented in Table IV. Many of the relaxation times are quite short, and some of the original temperature-jump results (45) were found to be inconsistent with more accurate ultrasonic experiments (20) and later photoperturbation experiments (102). It has not been possible to repeat some of these laser temperature-jump observations. Instead, the expected absorbance changes and isosbestic points were found to occur within the heating rise time of the laser pulse, consistent with the ultrasonic and photoperturbation experiments (20). Consequently, none of the original Raman laser temperature-jump results is included in Table IV. 

A number of observations indicate interconversion rates for some iron(III) complexes too fast to measure with existing techniques. Relaxation times less than the 30-nsec limit set by the heating rise time of the laser temperature-jump technique were observed for [Fe(benzac2trien)]+, [Fe(Salmeen)2]+, and [Fe(Me2dtc)3] (45, 128). From ultrasonic observations a limit of less than 1 nsec was placed on the relaxation time for the first of these compounds,... 

This prediction is confirmed by observation of a very rapid relaxation of the spin equilibrium in Co(terpy)22+ in solution. A relaxation time of less than 15 nsec was observed in a Raman laser temperature-jump experiment (14). This is consistent with the absence of any relaxation of the small excess sound absorption found in ultrasonic experiments. An upper limit of 0.2 nsec for the relaxation time in water at 298 K can be calculated from the magnitude of the excess absorption, which is... 

Dewey, T. G., and Turner, D. H. (1980). Laser temperature jump study of solvent effects on poly (adenylic acid) stacking. Biochemistry 19, 1681-1685. 

Laser Temperature Jump for the Study of Early Events in Protein Folding... 

The laser temperature jump instrument can effectively be used to initiate and observe the fast events in protein/peptide folding and unfolding as well as those events that extend out to several milliseconds. In the present study, the unfolding of a helical peptide was determined to occur within tens of nanoseconds, supporting the need for nanosecond or faster initiation techniques. Promising results obtained by the laser temperature jump method will continue to stimulate the development of additional monitoring techniques such as UV absorption and circular dichroism. 

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