Single coil duplexers

One more comment about crossed diodes the series 

Kisman and R. L. Armstrong, Coupling scheme and probe damper or pulsed nuclear magnetic resonance single coil probe, Rev. Sci. Instrum. 45, 1159-1163 (1974). 

Hoult and R. E. Richards, An ultra high frequency receiver protection scheme, J. Magn. Resonance 22, 561-563 (1976). 

Stokes, Tuned limiter for receiver amplifier in a fast-recovery pulsed NMR spectrometer, Rev. Sci. Instrum. 49, 1011-2 (1978). 

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The most common single coil duplexers are designed with cables of appropriate lengths such that the receiver is at the wrong electrical distance from the transmitter to see the transmitter signal while the probe is at the right distances from both the transmitter and the receiver (Lowe and Tarr, 1968). 

The way to get around this problem is to construct a circuit out of lumped components which takes the place of a quarter wave cable. There are many such circuits in the literature and, in fact, many of the single coil duplexers which are not bridges and which do not use quarter wave cables probably utilize such a circuit, intentionally or otherwise. See, for example, the articles by Clark and McNeil (1973) and McLachlan (1980) as well as the widely quoted older works by Gray, et al. (1966) and McKay and Woessner (1966). As far as we know, however, there has been only... 

In section V.A.l we described the general function of duplexers in single coil NMR. We now give a survey of the more common duplexer schemes. We start, however, with two... 

Mel nikov, S.M., Sergeyev, V.G. and Yoshikawa, K. (1997c) Cooperation between salt induced coil-globule transition in single duplex DNA complexed with cationic surfactant and sphere-rod transition of surfactant micelles. Prog. Colloid. Polym. Sci., 106, 209-214. 

Yoshikawa, K., Kidoaki, S., Takahashi, M., Vasilevskaya, V.V. and Khokhlov, A.R. (1996b) Marked discretness on the coil-globule transition of single duplex DNA. Ber. Bunsen-Ges. Phys. Chem., 100, 876-880. 

Although it is usual for DNA to rely on helical twists to coil it around there are other mechanisms by which the direction of the duplex can be radically altered. The simplest occurs when one or more bases insert into an otherwise complementary pair of strands. Assuming that all the other bases pair up, this leaves a small loop, or bulge, where the unpaired bases reside. A more extensive internal loop forms when both strands are mismatched in the same region. Hairpins are formed when a self-complementary single strand of DNA folds back on itself to form a duplex. A loop due to unpaired bases remains at the point where the strand reverses. 

Takahashi M, Yoshikawa K, Vasilevskaya VV, Khokhlov AR. Discrete coil-globule transition of single duplex DNAs induced by polyamines. J. Phys. Chem. 1997 101 9396-9401. 

Fig.2. Reactions catalysed by type II topoisome-rases. In each case the single line represents duplex DNA. Transient breakage occurs in both strands of the double helix. A resolution of a knotted circular duplex. B relaxation of super-coiling of a circular duplex. C one loop of a circular duplex, showing generation of supercoiling.

The stability of a DNA duplex depends on, besides the chemical and physical structure of the DNA itself, many environmental factors such as solvent, temperature, and salt composition and concentration. For example, when a stable DNA duplex is heated to higher temperatures, the double helix structure will be gradually denatured into two coiled single strands. When cooled to room temperature, the two single strands can reassociate to again form the duplex, a process called DNA renaturation (sometimes also called DNA hybridization or annealing). These processes are reversible, as illustrated in the following equilibrium ... 

Double helices (duplexes) are self-assembled foldamers that can equilibrate across two levels of organization (Figure 9.1). The first level is between a random coil and a folded state akin to a-helices and the second level involves the generation of intertwined donble helices from singles akin to the structure of double-stranded DNA. 

Abstract The effect of low-molecular salt, sodium bromide (NaBr), on the conformational behavior of T4DNA globules, compacted with cetyl-trimethylammonium bromide (CTAB), was studied in aqueous buffer solution. The conformational dynamics of individual single duplex T4DNA molecules was visualized directly with the use of fluorescence microscopy (FM), whereas viscometry was used for monitoring of the surfactant structure in the aqueous media. We have found that DNA globules, compacted with CTAB, are unfolded into the elongated coil state... 

Key words Single duplex DNA -cationic surfactant - fluorescence microscopy - coil-globule transition -sphere-rod transition of micelles... 

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