Proton: decay, 226 lifetime

The simplest GUT model [10], namely the SU(5) unification and its super-symmetric extension, is almost certainly ruled out from many reasons lack of proton decay at the predicted lifetime level, inability to produce neutrino masses, and so on. On the other hand, the next target, SO(IO) models, and their SUSY extensions in particular, are very promising. It has just the needed component of the right handed Majorana lepton for realization of the seesaw mechanism. The models also accommodates B - L violation which is needed for baryogenesis, as is explained later. 

In the darkest Communist times, a colleague of mine came to my office. Conspiratorially, very excitedly, he whispered The proton decays He just read in a government newspaper that the lifetime of protons turned out to be finite. When asked about the lifetime, he gave an astronomical number, something like 10 years or so. I said Why do you look so excited then and why all this conspiracy He answered The Soviet Union is built of protons, and therefore is bound to decay as well ... 

The new protonated species display increased quantum yield values and longer decay lifetimes. This condition can be reversed by treating the protonated Re-(i) complex with a base such as triethylamine. 

This agrees quite well with the rate constants for intramolecular proton transfer in 2,4-bis(dimethyl-amino )-6-(2-hydroxy-5-methylphenyl)-5-triazine which had been measured by Shizuka et al. ( l6) using laser picosecond spectroscopy. The fluorescence decay constant t of (TIN) was found to be 60 20 ps. Because of the weak intensity all fluorescence lifetimes refer to the pure substance in crystalline form at room temperature. 

Since tRNA is more varied structurally than DNA, ethidium could reside in pockets as well as intercalate into double-strand regions. The fluorescence decay provides information about the type, or types, of binding sites occupied by ethidium. It is currently believed that the excited state of ethidium is quenched by proton transfer to the solvent0 86-1 and that its lifetime is reduced with increasing solvent exposure. If ethidium occupies two or more kinds of sites with different degrees of exposure to solvent, then its fluorescence decay is expected to be multiexponential. 

This chain of events involved the so-called weak interaction, a puny and slow force compared with the strong and electromagnetic interactions. The weak interaction governs the conversion of protons into neutrons and vice versa, with creation of a neutrino (antineutrino). It thus determines the lifetime of free neutrons, which naturally decay into protons. In fact, neutrons have a life expectancy of around 10 minutes. However, before they disappear, they may have the opportunity to combine with protons, one which they readily accept. In that case, nuclear physics makes its appearance in the Universe. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

The dynamic behavior of the chromoprotein is much more clearly multiexponential. Two pronounced decay components were identified with 5-ps (25 % wt.) and 60-ps (25% wt.) lifetimes. The strong weight of these exponentials is not in favor of a comparison to the weak 13-ps decay of the free pigment. It is more likely that they actually reveal a new and specific deactivation channel in the protein complex. It has, in fact, been speculated that light irradiation of the living organism induces an acidification of the intracellular medium [11], so proton transfer from the chromophore to the associated protein was first proposed as the initial phototransduction step. More recent experiments on the isolated pigment in the presence of electron acceptors [12] proved that the optical excitation can induce an electron transfer from the chromophore to an acceptor site possibly situated inside the associated... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

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