Double z-sampling

The structure proposed by Watson and Crick was modeled to fit crystallographic data obtained on a sample of the most common form of DNA called B DNA Other forms include A DNA which is similar to but more compact than B DNA and Z DNA which IS a left handed double helix... 

MAT 731 double-focussing mass spectrometer, a combined EI/FI/FD/FAB ion source and an AMD Intectra direct introduction system, has been previously described in detail (12). The samples were heated linearly from 50 C to 750 C at a rate of 100 K/min. In general, 35 FI mass spectra were recorded in the m/z 50-900 mass range. 

Fig. 12.3. STM with a double-action lever. Various parts are shown in (a) (A) The macor block onto which the x, y piezo bars (horizontal) and the z piezo bar (vertical) are mounted, (B) the microscope base plate, (C) carriage rod, actuated by a linear feedthrough and a lead screw, (D) stop, (E) ball bushing assembly, (F) lever, (G) sample and sample holder, (H) catch, the pivot point for coarse motion, (I) foot, the pivot point for fine motion, and (J) the probe tip assembly, (b) Shows the STM in coarse motion. Using the catch as the pivot point, the sample can be removed away from the tip. (c) Shows the STM in fine motion. Using the foot as the pivot point, the linear motion of the carriage rod is reduced by a large factor. (After Demuth et al., 1986a.)...

Minor variations of the backbone and glycosyl rotations from the fixed values used in the sample computations above produce a variety of theoretically acceptable double helices. As evident from the partial list of structures in Table I, these structures include several 10-fold duplexes similar to the B-DNA models from fiber diffraction studies as well as the larger 13-fgld complex. Despite the large fluctuations in h from 1.7 to 4.3 A, the bases associate at standard separation distances (3 ft < < 4 A and 2.8 X < < 3.0 A) and orientations (A < 30° and < 30°) in all cases. In order to avoid severe steric contacts at small values of h, the bases may tilt up to values of n = 45° with respect to the standard orientation (n = 90°) perpendicular to the helix axis. 

The modulation frequency is typically in the range from 100 Hz to 3 kHz, and thus much lower than the resonance frequencies of the cantilever and the scanner. This enables better control of the forces exerted on the sample. The z-mod-ulation amplitude can be varied between 10 nm and 1 pm to ensure that that the tip is retracted from the surface. Shear forces are reduced permitting investigation of soft samples because of the small duration of the tip-surface contact, between 10 3 and 10 4 s. Pulse force mode SFM has been used to map adhesion of heterogeneous polymers in dependence of temperature and molecular weight as well as map electrostatic double-layer interactions [158-160]. 

In benzyl idene-t/Z-piperi tone, the two pairs of reactive double bonds that are related by a center of inversion are properly oriented for [2 + 2] photocycloaddition with 0 = 0°, 02 = 105.2°, 03 = 86.4°, and d = 3.93 A. However, there is no evidence for the presence of a dimer in the irradiated sample. The lattice energy... 

The effect of z-i is to double 22.5. That gives 45. An enrichment in depth can be assumed Cd content increases by 45 (%) irrespective of the sampling location. 

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