Helical parameters

Some oligonucleotides adopt an A-form helical stmcture (Fig. 2a) (5). The average stmctural parameters have been found consistent with the fiber diffraction model, but, as for B-form DNA, considerable variation is apparent among iadividual base pairs. 

Helically wrapped fins are fabricated such that the fm height can be between about 3/8 to 3/4 of the tube diameter, but limited because of fabrication requirements to a maximum of about 2.54 cm (1.0 in.) in height. Fin spacings vary between about 275 and 450 fins per meter of tube length, while fin thicknesses range from 0.025 to 0.075 cm. For particular cases these parameters may be varied further. 

It is clear that a large number of parameters influence the formation of the sheets in a tubule and that their relative importance is still unknown, as is also the cause of the occurrence of the defects responsible for the eventual polygonization of the sheets. Although the model presented here highlights the necessity of including, as one of the parameters, the uncertainty 8r or dd on the separation of successive cylindrical sheets, it is impossible to predict with absolute certainty the final characteristics of any of these sheets, symmetric or not, on the basis of the characteristics of the previous one. Nevertheless, a number of features of their structure, such as the presence or absence of helicity, and the presence of groups of sheets with nearly the same angle of pitch, can be explained and quantified. 

Table 2. Structural parameters, cohesive energies per atom, and spring constant for helices C4(,0 and C,4(, here / and r, are outer and inner diameter of a helix, respeetively...

Preparation methods for PCNTs have been reviewed in the context of parameters which may lead to large-scale MWCNT synthesis free of by-products. It is noteworthy that the formation of aligned CNTs is currently an active area of research in conjunction with PCNT preparation. The use of SWCNTs and/or MWCNTs in electronic devices are being developed. As yet it has not proved possible to produce CNTs with diameters and helicities to order. The formation of SWCNTs by the PCNT process has not yet been reported and it is of interest to examine whether this process can be used to prepare them. 

DNA duplex of 400 bp, L is 40 (assuming 10 bp per turn in B-DNA). The linking number for relaxed DNA is usually taken as the reference parameter and is written as Lq. L can be equated to the twist (T) and writhe (W) of the duplex, where twist is the number of helical turns and writhe is the number of supercoils ... 

X-ray diffraction studies indicate the existence of a novel double-stranded DNA helical conformation in which AZ (the rise per base pair) = 0.32 nm and P (the pitch) = 3.36 nm. What are the other parameters of this novel helix (a) the number of base pairs per turn, (b) Abase pair), and (c) c (the true repeat) ... 

As Fig. 16 shows, the preferential binding of DMSO, DMF and NMF from aqueous solution to (Lys HBr)n at low contents of the organic solvent x increases with its concentration. However, at approximately x3 = 0,2 a maximum is reached and then preferential hydration between x3 = 0,3 and 0,5 occurs. No preferential binding was observed for NMP, EG or 2 PrOH, however increasing hydration occured with x3. Only in 2 PrOH at x3 > 0,3 a-helix formation occured. Furthermore binding parameters for the systems NMP + DMSO, EG + DMSO and DMF + DMSO have been determined. An initial preferential binding of DMSO by (Lys HBr)n, a maximum and a subsequently inversion of the binding parameter was also observed in these mixtures. The order of relative affinity is DMSO > DMF > EG > NMP. In DMF/DMSO-mixtures (Lys HBr) attains an a-helical conformation above 20 vol.- % DMF and in 2-PrOH/water above 70 vol.- % 2 Pr-OH. 

As already mentioned, the enthalpy change A//° involved in an elementary propagation step corresponds to the equilibrium constant S. The parameter a, however, is purely entropically influenced mainly due to the steric restrictions during the formation of a helical nucleus. The determination of a, since it is related to the same power (3n - 2) of s, requires the consideration of the dependence of the thermodynamic parameters on the chain length (Eq. (9 a)). 

Helical Parameters and Conformation Angles in (1 — 4)-linked Polysaccharides... 

The trisulfane molecule exists as two conformers which have been termed as cis- and trans-HzSi. While the trans-form is a helical molecule of C2 symmetry with the motif ++ (or — for the enantiomer), the cfs-form is of Q symmetry with the motif +- (identical to -+). Both forms have been detected by rotational spectroscopy [17, 45, 46]. The motif gives the order of the signs of the torsion angles at the SS bonds. The geometrical parameters [17] are presented in Table 4. The trans-isomer is by only 1 kj mol more stable than the cfs-form but the barrier to internal rotation from tmns to cis is 35 kJ mor [46]. The dipole moments were calculated by ab initio MO theory at the QCISD/TZ+P level as 0.68 D (trans) and 2.02 D (cis) [46]. For geometrical parameters of cis- and trans-trisulfane calculated at the MP2/6-311++G> > level, see [34]. 

While the a-helix of L-a-peptides and the (M)-3i4 helix of the corresponding peptides have opposite polarity and helicity (see Section 2.2.3.1), the inserhon of two CH2 groups in the backbone of L-a-amino acids leave these two hehx parameters unchanged, both the a-helix and the 2.614-hehx of the resulting y" -peptides being right-handed and polarized from N to C terminus. In view of these similarities, the y-peptide hehcal fold might prove useful as a template to elaborate functional mimetics of bioachve a-polypeptides. 

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