Pulse Saturation recovery

When tr is sufficiently long, cw EPR spectra become insensitive to nitroxide motion, and the spectra are indistinguishable from powder patterns. Thus, one must rely on spectroscopic methods beyond cw EPR for the extraction of motional processes. Historically, the motional regime between 1(K) nsec and 1 psec has presented a blind spot to EPR spectroscopy, and reliable methods for extraction of t did not exist. More recently, though, this problem has been solved. The technique combines the methods of pulsed saturation recovery EPR (SR-EPR) and pulsed saturation recovery electron-electron double resonance (SR-ELDOR). Spectroscopic saturation occurs when the applied radiation is sufficiently intense... 

Altenbach C, Eroncisz W, Hyde JS, Hubbell WL (1989) Conformation of spin-labeled melittin at membrane surfaces investigated by pulse saturation recovery and continuous wave power saturation electron-paramagnetic resonance. Biophys J 56 1183-1191... 

Yin JJ, Pasenkiewiczgiemla M, Hyde JS (1987) Lateral diffusion of lipids in membranes by pulse saturation recovery electron-spin-resonance. Proc Natl Acad Sci USA 84 964—968... 

The structural analysis on KcsA was performed based on the mobility of each spin labeled side chain in the protein segments under investigation. It is worth recognizing in Pig. 4b that most of the CW RT spectra show multiple spectral components, characterized by different mobility (a few examples are highlighted by arrows). This is a very general property of the R1 side chain in proteins. The components reflect the anisotropy of the spin label reorientational motion, but their appearance could also have other causes. They could arise from a slow equilibrium between two different protein conformations or the presence of asymmetric sites in the protein. The molecular interpretation of different spectral components is cumbersome. Multifrequency EPR [17], temperature analysis of the CW spectra [27], pulse saturation recovery techniques [28], or high pressure EPR [29] can help unravel the possible origins of the spectral components. In the case of KcsA, the spin labels motional information was quantitatively extracted from the inverse central line width (A//q, mobility parameter) and was corroborated by the measure of the accessibility of the spin labeled side chains towards lipids (O2... 

A number of pulse-sequence methods are available for measurement of Ti values, and those most commonly used are the methods of saturation recovery (s.r.F.t.),69,70 progressive saturation (p.s.F.t.),71 inversion recovery (i.r.F.t.),72 and the Freeman-Hill modification of in-... 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

Fig. 2.28. Motion of the magnetization vector during a saturation-recovery experiment the first 90J pulse rotates the magnetization vector M0 to the x y plane (a -> b). where the resultant transverse magnetization is dispersed by a field gradient pulse (homo-spoil) after r s (d), a second 90 pulse monitors the partially relaxed magnetization Mt (d - f), and the resultant FID signal is Fourier transformed to the NMR signal with amplitude Ax. (Reproduced by permission of the copyright owner from E. Breitmaier and G. Bauer ...

Spin-Lattice Relaxation. In order to determine whether each resonance line comprises a single component, we first measured the spin-lattice relaxation time Tic by the pulse sequence developed by Torchia [53] or by the standard saturation-recovery pulse sequence. The Tic values thus obtained were 2560,263 and 1.7 s for resonance line I and 0.37 s for line II. As reported by several investigators, the line at 33 ppm is associated with three different Tic values [ 17,54,55]. This means that this line is contributed to by three components with different molecular mobilities. However, since each component was represented by a single Lorentzian line shape at 33 ppm, they are all assignable to methylene groups in the orthorhombic crystalline form or in the trans-trans conformation. The component with a Tic of s can be assigned to methylene groups with a some-... 

There are several ways of determining T, the most reliable method probably being saturation-recovery, in which two 90° pulses are applied separated by a time x of the order of Ti. The signal height is then proportional to Mz(x), given by... 

The inversion and saturation recovery pulse sequences are used for measurement of the T relaxation time and for partial suppression of signals in samples with distributions of T relaxation times. These pulse sequences can be employed for contrast enhancement in imaging (cf. Section 7.2.1). 

Fig. 2.2.8 Pulse sequences for measurement of Ti relaxation times by (a) saturation recovery and (b) inversion recovery. The effect of the pulse sequences is illustrated in terms of the vector model of the nuclear magnetization and by graphs showing the evolution of the longitudinal magnetization M, as a function of ti.

Fig. 7.1.3 [Blii2] NMR-timescale of molecular motion and filter transfer functions of pulse sequences which can be utilized for selecting magnetization according to the timescale of molecular motion. The concept of transfer functions provides an approximative description of the filters. A more detailed description needs to take into account magnetic-field dependences and spectral densities of motion. The transfer functions shown for the saturation recovery and the stimulated-echo filter apply in the fast motion regime.

Fig. 7.2.1 Pulse sequences for T and related magnetization filters, typical evolution curves of filtered magnetization components, and schematic filter transfer functions applicable in the slow motion regime. Note that the axes of correlation times start at Tc = Wo (a) Saturation recovery filter, (b) Inversion recovery filter, (c) Stimulated echo filter.

The echo maxima are weighted by a function of both T and T2. Similarly, the stimulated echo (Fig, 7.2.1(c)) can be used as a combination filter to introduce T and T2 weights. The echo time (tf2 in Fig. 7.2.19(c)) determines the T2 weight and the mixing time between the second and the third pulses (tn in Fig. 7.2.19(c)) the T weight. Note that the filter transfer functions for T) contrast by saturation recovery and the stimulated echo are inverted (cf. Fig. 7.2.1 (a) and (c)), so that both combination filters introduce different contrasts (cf. eqn (7.2.3)). 

Fig. 7.2.22 [Gutl] Pulse sequences for combined determination of T and T2 in inhomogeneous Bo fields, (a) Steady-state inversion recovery filter [Sezl). (b) Steady-state saturation recovery filter [Gutl]. (c) Train of echoes measured by sequence (b) which shows the magnetization build-up with T] and the decay with Ti of unfilled, cross-linked SBR.

CW experiments (sometimes called stationary or steady state ) are ones in which either no modulations are used, or they are so low in frequency that no spectral complications ensue. (This is only approximately the case if 100 kHz field modulation is employed. This frequency gives rise to modulation sidebands and, under saturating conditions, rapid passage effects.) Time-domain ESR involves monitoring the spin system response as a function of time. Pulse ESR can be divided into two broad categories the response of spin systems to sequences of microwave pulses (spin echo) and the response of spin systems to step changes in resonance conditions (saturation recovery). 

Pulse or time domain ESR can be divided into two categories the transient response of spin systems to abrupt or step changes in resonant condition and the transient response to sequence of pulses [20]. The step response, as in saturation recovery, is used commonly to measure T, and the pulse response, as in 90-180° spin echo, to measure Tj. 

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