Quantitative analysis confirms that WT cells have the shortest mean FA lifetimes, Vector and 15F/mPR cells have the longest lifetimes, and 15F and mPR single mutant cells have intermediate lifetimes (Fig. 5B).
FAs in the 15F/mPR cells also appear to persist throughout the 90-minute time-course, while FAs in both the 15F and mPR cells appear to have intermediate lifetimes.
Indeed, the observation that the residence times of SRP on RNCs containing exposed signal sequences between 40-50 acids andds and 60-70 amino acids are two-fold shorter than that on RNCs containing exposed signal sequences between 50-60 acids acimplymply that binding intermediates with average lifetimes shorter than that of the final complex are indeed populated.
In fact, smFRET experiments performed at SRP concentrations greater than physiological might even be desired in some instances (e.g., in experiments in which a functional signal sequence is not exposed) in that it would increase the probability that binding intermediates with average lifetimes on the order of or slightly shorter than the time resolution of the smFRET experiments could be observed.
The appearance rate constant kapp of the desired product is equal to the inverse of the rate-determining intermediate's lifetime τrd, kapp = 1/τrd, which often depends on the solvent as well as on the concentrations of acids and bases including those of the general acids and bases contained in buffers.
Consequently, most of the detected products are intermediates whose lifetimes are highly variable and regulated by factors widely unknown.
We overview different experimental methods that can be used to investigate intermediates with lifetimes in the range from hundreds of picoseconds to hundreds of microseconds.
Describing positron trapping in terms of two-state model with only one kind of such defects, which are described by defect-specific intermediate short-lived lifetime τ 2 = τ d, the defect-free bulk lifetime τ b, trapping rate in defects κ d, and fraction of trapped positrons η can be respectively calculated [12, 13].
If, however, the anionic intermediate has sufficient lifetime, rotation about the new carbon-carbon single bond can precede loss of the negatively charged group, resulting in production of two products of differing molecular geometry that is, products in which the substituents are differently situated with respect to the double bond.
