Loss Of Signal Amplitude Research Articles

Large coaxial germanium detectors-correction for ballistic deficit and trapping losses

The use of a ballistic deficit corrector circuit that calculates and applies a correction on a pulse-by-pulse basis has been shown to improve the energy resolution of spectrometers using large-diameter coaxial detectors at high energies and high counting rates. The potential importance of this circuit in correcting for the loss in signal amplitude due to majority carrier trapping has also been recognized. This work has led to further approaches to improving performance. Experimental results are described and demonstrate the complex nature of the problem of simultaneously correcting for both types of loss. The proposed design of a signal processor that should achieve good simultaneous correction for both types of loss is discussed. >

Selective excitation in electron spin-echo modulation experiments

Electron spin-echo spectroscopy (1-6) has become a powerful method for the study of hyperfine and quadrupole interactions in inhomogeneously broadened systems. Both twoand three-pulse experiments have been used to measure the echo modulation envelope as a function of pulse separation. Modulation effects are caused by interactions which do not change sign under a 7r pulse and necessarily involve a violation of the high-field assumption for nuclear resonance under the particular measurement condition (1, 2). It is now widely accepted that weak selective microwave pulses cause the echo envelope modulation to disappear (4, 5). Reduction of modulation depth due to partial excitation has recently been analyzed in detail by Barkhuijsen et al. (7). On the other hand we have recently found that weak extended-time excitation reproduces the entire echo envelope modulation in a single experiment, provided that the weak irradiation is followed by a short and strong refocusing pulse (8). In this communication we give further experimental evidence for the fact that a weak selective excitation pulse followed by a strong refocusing pulse produces the same envelope modulation as obtained with two strong pulses. We also present an explanation for this finding. The experimental results are summarized in Fig. 1. Figure la gives the echo envelope modulation for a Cu(II)-aquo complex at 4.2 K using short microwave pulses both for excitation (10 ns) and refocusing (20 ns) with nominal flip angles of (a/2) and K, respectively. The Fourier transform of this pattern consists of two peaks at 14 and 28 MHz corresponding to the proton Zeeman frequency and its second harmonic. When the short excitation pulse is replaced by a long (210 ns) weak pulse, no change in the modulation pattern is observed (Fig. lb). However, a loss of signal amplitude occurs caused by the reduced bandwidth of excitation, not visible in Fig. lb due to normalized amplitudes. In Fig. lc, the short refocusing x pulse is also replaced by a long (2 10 ns) and weak pulse. In contrast to the first two experiments the echo modulation pattern disappears completely. By increasing the amplitude of the two pulses part of the echo modulation may be recalled (Fig. Id). The pattern observed, however, does not fully correspond to

Transient state kinetic analysis of the ATP-induced dissociation of the dynein-microtubule complex.

The kinetics of ATP-induced dissociation of dynein from the dynein-microtubule complex has been investigated by stopped flow light scattering methods. The addition of ATP to the dynein-microtubule complex induced a large, rapid decrease in light scattering followed by a smaller and much slower decrease. The fast light scattering change was shown to be a measure of the ATP-induced dissociation of dynein from the dynein-microtubule complex and was distinguished from microtubule disassembly by several criteria. (i) The fast reaction occurred over a period of milliseconds and the rate was a function of the ATP concentration, whereas, the slow reaction occurred over a period of several seconds and was independent of ATP concentration; (ii) the amplitude of the fast reaction was directly proportional to the amount of dynein bound to the microtubule lattice; and (iii) only the slow phase was inhibited by the addition of the microtubule-stabilizing drug, taxol. The rate of ATP-induced dissociation of dynein from the microtubule increased linearly with increasing ATP concentration to give an apparent second order rate constant for ATP binding equal to k1 = 4.7 X 10(6) M-1 s-1 according to the following pathway: (formula; see text) where M X D represents the dynein-microtubule complex and D represents dynein. The loss of signal amplitude at high ATP concentration provided a minimum estimate for the rate of dissociation of the ternary complex (M X D X ATP) equal to kd greater than 1000 s-1. Thus, the dynein-microtubule system is similar to actomyosin in that ATP induces an extremely rapid dissociation of dynein from the microtubule.

Intermediate states of subfragment 1 and actosubfragment 1 ATPase: reevaluation of the mechanism.

The kinetics of the increase in protein fluorescence following the addition of ATP to subfragment-1 (SF-1) and acto-SF-1 have been reinvestigated. The concentration dependence of the rate obtained with SF-1 did not fit a hyperbola and at high ATP concentration, approximately 40% of the signal amplitude was lost due to a fast phase at the beginning of the transient (20 degrees C). At lower temperature (less than or equal to 10 degrees C) the fluorescence transient was biphasic, with a fast phase observed at high ATP concentration. These results indicate that there are two steps in the SF-1 pathway in which there is a change in protein fluorescence. Measurements of ATP binding and hydrolysis by chemical quench-flow methods indicate that the rate of ATP binding is correlated with the fast fluorescence step and hydrolysis is correlated with the slow fluorescence change. The SF-1 mechanism can thus be described as: (formula: see text) where M represents SF-1 and states of enhanced fluorescence are given by M (16%) and M (36% enhancement, relative to SF-1). Step 1 is a rapid equilibrium with K1 approximately 10(3) M-1. Tight binding of ATP occurs in step 2 and the loss of signal amplitude requires k2 greater than or approximately 1500--2000 s-1. The maximum observed fluorescence rate defines the rate of hydrolysis, k3 + k-3 = 125 s-1 (20 degrees C, 0.1 M KCl, pH 7.0). The steps in the mechanism correspond to the Bagshaw--Trentham scheme, with the important difference that the assignment of rate constant is altered. Formation of the acto-SF-1 complex gave a fluorescence enhancement of approximately 14% relative to SF-1. Dissociation of acto-SF-1 by ATP produced a 20--22% enhancement in fluorescence. There was no detectable fluorescence change during dissociation as evidenced by a lag in the fluorescence transient which corresponded to the kinetics of dissociation. The fluorescence change occurred at the same maximum rate as for SF-1 but there was no loss in signal amplitude at high ATP concentration. The kinetics of the fluorescence change corresponded to the rate of ATP hydrolysis, whereas tight ATP binding occurred at a much faster rate in approximate agreement with the rate of dissociation. Thus the fluorescence change in the acto-SF-1 pathway corresponds to step 3 in the SF-1 mechanism. The complete scheme can be described as follows: (formula: see text) where AM represents acto-SF-1. The tight binding step in the SF-1 pathway (k2) is sufficiently fast so that a similar step (k2') in the acto-SF-1 pathway could precede dissociation but the AM-ATP intermediate has not been detected. Following hydrolysis on the free SF-1, actin recombines with M.ADP.Pi or possibly with a second SF-1 product intermediate as proposed by Chock et al. (1976) and the fluorescence returns to the original AM level with product release.