Laser-induced proton pulse

Gutman M 1986 Application of the laser-induced proton pulse for measuring the protonation rate constants of specific sites on proteins and membranes Methods Enzymol. 127 522-38... 

During these studies, it became apparent that proton transfer is an extremely sharp instrument for gauging the water in the immediate environment surrounding the site of dissociation. It turned out that the general biological solvent, the water, acquires different properties at the site where biochemical reaction takes place—the surface of the enzyme. These local properties of the water can be measured through the technique of the laser-induced proton pulse, free of perturbation caused by the huge mass of the bulk water. 

In this chapter, I shall describe the basic methodology of the laser-induced proton pulse. Starting with the inidal event of a synchronous proton dissociation, going through the reaction of a proton with other solutes in a true solution, and ending with the complex multiphasic system of protons, macromolecules, and interfaces associated with the real life of biochemical reaction. In each level of complexity, I shall point out the pertinent information available for interpretation and the mode of mathematical and physical analysis. In some cases, I shall also reflect the conclusions on current hypotheses of biochemical proton transfer. 

The laser-induced proton pulse is a young, high-resolution method recently introduced to biochemistry. It is a system capable of measuring the diffusion-controlled reaction of a proton with its environment, solvent, and solutes. The information derived from these measurements is divided according to the time scale of the event The primary reaction of proton dissociation, recorded in the nano- and subnanosecond time frames, and slower (microsecond) diffusion-controlled reaction of a proton with other solutes. 

We shall describe the various steps in the evolution of the methods and knowledge of proton diffusion on the membrane surface. The methods used for our studies all emanate from one basic technique—the laser-induced proton pulse (I). The common step of the various forms of this method is a pulse excitation of aromatic alcohols (OH), such as naphthols, sulfono naphthols, or pyranine (8-hydroxypyrene- 1,3,6-trisulfonate), to their first excited electronic singlet state (OH ). In this state the compound is very acidic and the hydroxyl proton dissociates in subnanosecond dynamics. 

Gutman, Menachem, The pH Jump Probing of Macromolecules and Solutions by a Laser-Induced, Ultrashort Proton Pulse—Theory and... 

Gutman, M., The pH jump probing of macromolecules and solutions by a laser-induced, ultrashort proton pulse-theory and applications in biochemistry. Methods Biochem. Anal., 1984, 30, 1-103. 

From conductivity studies, it is known that the cycloalkane holes rapidly react with various solutes, typically by electron or proton transfer [7-19]. These scavenging reactions establish the identity of the high-mobility cations as the solvent holes Rapid generation of aromatic radical cations (A +) in reactions of the holes with aromatic solutes (A) was observed using pulse radiolysis - transient absorption spectroscopy [4,5,6,20,23-25] and, more recently, using pulse-probe laser-induced dc conductivity [26]. Rapid decay of the conductivity and transient absorbance signals from the cycloalkane holes was also observed [4-25]. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

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