Complete relaxation

Erom a given structure, the NOE effect can be calculated more realistically by complete relaxation matrix analysis. Instead of considering only the distance between two protons, the complete network of interactions is considered (Eig. 8). Approximately, the... 

Figure 8 Effects of spin diffusion. The NOE between two protons (indicated by the solid line) may be altered by the presence of alternative pathways for the magnetization (dashed lines). The size of the NOE can be calculated for a structure from the experimental mixing time, and the complete relaxation matrix, (Ry), which is a function of all mterproton distances d j and functions describing the motion of the protons, y is the gyromagnetic ratio of the proton, ti is the Planck constant, t is the rotational correlation time, and O) is the Larmor frequency of the proton m the magnetic field. The expression for (Rjj) is an approximation assuming an internally rigid molecule.

Thus, in the series of Ti measurements of 2-octanol (42, Fig. 2.27) for the methyl group at the hydrophobic end of the molecule, the signal intensity passes through zero at Tq = 3.8 s. From this, using equation 10, a spin-lattice relaxation time of Ti = 5.5 s can be calculated. A complete relaxation of this methyl C atom requires about five times longer (more than 30 s) than is shown in the last experiment of the series (Fig. 2.27) Tj itself is the time constant for an exponential increase, in other words, after T/ the difference between the observed signal intensity and its final value is still 1/e of the final amplitude. 

Moseley, H. N. B., Curto, E. V., Krishna, N. R. Complete Relaxation and Conformational Exchange Matrix (CORCEMA) analysis of NOESY spectra of interacting systems two-dimensional transferred NOESY. J. Magn. Reson. Sen B 1995, 108, 243-261. 

As mentioned previously, a single action potential lasting only 2 msec causes a muscle twitch that lasts approximately 100 msec. If the muscle fiber has adequate time to completely relax before it is stimulated by another action potential, the subsequent muscle twitch will be of the same magnitude as the first. However, if the muscle fiber is restimulated before it has completely relaxed, then the tension generated during the second muscle twitch is added to that of the first (see Figure 11.3). In fact, the frequency of nerve impulses to a muscle fiber may be so rapid that there is no time for relaxation in between stimuli. In this case, the muscle fiber attains a state of smooth, sustained maximal contraction referred to as tetanus. 

Figure 11.3 Muscle twitch summation and tetanus. A single action potential (represented by A) generates a muscle twitch. Because duration of the action potential is so short, subsequent action potentials may restimulate the muscle fiber before it has completely relaxed, leading to muscle twitch summation and greater tension development. When the frequency of stimulation becomes so rapid that no relaxation occurs between stimuli, tetanus occurs. Tetanus is a smooth, sustained, maximal contraction.

If the conditions for Forster transfer are not applicable, then the theory must be extended. There is recently experimental evidence that coherent energy transfer participates in photosynthesis [74, 75], In this case, the participating molecules are very close together. The excited state of the donor does not completely relax to the Boltzmann distribution before the energy can be shared with the acceptor, and the transfer can no longer be described by a Forster mechanism. We will not discuss this case. There has been active discussion of coherent transfer and very strong interactions in the literature for a longer time [69], and references can be found in some more recent papers [70-72, 76, 77],... 

The solid-state structures of orthorhombic S864 and monoclinic Se866 are shown in Figure 15. As may be seen from the average E-E-E bond and EE EE torsion angles, this structure is almost completely relaxed and consequently lone pair repulsion and ring strain are lowest in this arrangement. Other S8 structures were assessed by quantum chemical calculations.69... 

Wier Yes, we reported this in 1999 in pressurized arteries. Nifedipine produced a complete relaxation but the synchronous Ca2+ waves continued. 

Most interesting in this light are our initial experiments on mouse bladder showing the absolute dependence of [i adrenergic relaxation in the bladder to the presence of PLB. Isoproterenol or forskolin activation of the A -kinase pathway led to complete relaxation of bladder from wild type mice, with little or no response in the PLB knockout bladder (K. Nobe R. J. Paul, unpublished observations). On the other hand, relaxation via G -kinase pathway activation was identical in the PLB knockout and wild-type bladder. PLB modulation of SR appears to be the dominant pathway for A-kinase mediated relaxation in mouse bladder in contrast to its lesser role in this pathway for the vascular tissues studied. 

As is the case for NMR of liquids, another important consideration for SSNMR spectroscopy is relaxation. There are different types of relaxation present during a SSNMR experiment. The spin-lattice relaxation, Ti, dictates how fast one can repeat scans. This time between experiments must be set greater than five times T1 in order for complete relaxation to occur. Otherwise, the full signal intensity will not be... 

One feature of the Maxwell model is that it allows the complete relaxation of any applied strain, i.e. we do not observe any energy stored in the sample, and all the energy stored in the springs is dissipated in flow. Such a material is termed a viscoelastic fluid or viscoelastic liquid. However, it is feasible for a material to show an apparent yield stress at low shear rates or stresses (Section 6.2). We can think of this as an elastic response at low stresses or strains regardless of the application time (over all practical timescales). We can only obtain such a response by removing one of the dashpots from the viscoelastic model in Figure 4.8. When a... 

An equilibrium model may not be representative of the true situation commonly faced in the laboratory. The relaxation behaviour of the samples becomes progressively longer with increasing volume fraction. It is quite reasonable to suppose that, at high particle concentrations and low electrolyte concentrations, the relaxation times become so long that it is impractical to allow all the stresses and strains to relax from the sample prior to measurement. Stress relaxation studies for a range of particles that show nearly complete relaxation is shown in Figure 5.16.21... 

Alternatively, the much more common situation in trNOE studies involves fast exchange on the chemical shift time scale where the observed resonance shifts are weighted averages of the corresponding shift in the free and bound state [13]. A full account of the complete relaxation matrix and conformational exchange effects for n spins has been performed by London et al. [13], and a similar treatment was later incorporated into the programme CORCEMA [14]. 

The low torsion constant at a = —0.025 is very similar to that observed in a supercoiled pBR322 that was partially relaxed by saturation binding of Escherichia coli single-strand binding (ssb) protein, and which persisted for over a month.(56) It is also similar to that recently inferred from an in vivo assay based on variation in repression efficiency with size of a putative DNA loop.(234) Indeed, it appears that anomalously low torsion constants may be universally encountered in the course of either partial or complete relaxation of supercoiled DNAs, regardless of whether the superhelix density is reduced by action of topoisomerase I, binding of ssb protein, binding of intercalated... 

The quantum mechanical extention of the MM method covers an enormous potential range of applications. Complete relaxation calculations of biphenyl (37) in the Si excited state were carried out for the first time using QCFF (30) and the conformation was con-... 

The basic theoretical formulation for the quantitative analysis of the STD-NMR data is similar to the one we developed for the analysis of transferred NOESY data, and is based on a complete relaxation and conformational exchange matrix (CORCEMA) theory [33]. 

The thermoneuttal temperature is that at which there is no net gain or loss of heat from or to the environment. For humans, this temperature is not easy to ascertain but experiments suggest that the preferred temperature for subjects with light clothing is 23-26 °C. Mental rest (or complete relaxation) is not always an easy state to achieve under experimental conditions. 

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