Helical transitions

P. Bayley, S. Martin, and G. Jones, The conformation of calmodulin A substantial environmentally sensitive helical transition in Ca4-calmodulin with potential mechanistic function, FEES Lett. 238, 61-66 (1988). 

Kobayashi S, Hamasaki N, Suzuki M, Kimura N, Shirai H, Hanabusa K (2002) Preparation of helical transition-metal Oxide tubes using organogelators as structure-directing agents. J Am Chem Soc 124 6550-6551... 

Figure 9 Fraction helical bonds as a function of the temperature T of the discotic compound 1 of Figure 6 in the solvent n-butanol. Symbols indicate experiments done in the range of concentrations of 10 6-10 2 M (van der Schoot et al., 2000). The temperature has been scaled to the helical transition temperature Th and a temperature Tc = Th(AcBTh/ /Jh )exp(//ArBTh) that depends on the frustration energy j and the excess enthalpy of the helical bond h. Remarkably, data taken over four orders in concentration collapse onto the theoretical curve, indicated by the drawn line.

The model outlined in this section seems specific but in actual fact is very versatile. It applies not only to helical transitions in supramolecular polymers but can easily be adapted to model other types of structural reorganization (Bouteiller et al., 2005), chirality amplification (Palmans and Meijer,... 

Figure 10 Assembly diagram of compound 1 of Figure 6 in the solvent n-butanol (adapted from van Gestel, 2004a Weiss and Terech, 2005). Symbols represent results from UV-vis absorption, UV, fluorescence decay, FD, and circular dichroim spectroscopy, CD, and the drawn line the theoretical fits to the data. There are three types of transition I, isodesmic polymerization II, helical transition of long supramolecular polymers and III, nucleated helical assembly of the monomer units.

Figure 11 Comparison of prediction of the self-assembled Ising model with the experimentally determined heat capacity of compound 1 of Figure 5 around the co-operative helical transition point (Brunsveld, 2001 Brunsveld et al., 2000).

Keywords Aluminum, DNA, Helical transitions, Spectroscopy, Alzheimer s disease... 

Fig. 3. a The binding of aluminum nitrate to calf thymus DNA with no helical transitions, b Role of aluminum maltolate in inducing helicity changes in calf thymus DNA... 

From our studies, it can be assumed that Al is likely to play a role in switching off the FMR1 gene by inducing a helical transition from B- to 2-DNA, in addition to the factors involved in the massive expansion of CCG triplet repeats and hypermethylation. This may provide a clue for the discovery of new drugs which convert Z- to B-DNA. It is hypothesized that chelation of metal ions like aluminum, demethylation of the triplet repeats, and acetylation of histones - if achieved - may make it possible to switch on the FMR 1 gene. 

Huston and Marshall (87) used this approach to map the reaction coordinates of the a- to 3io-helical transition in model peptides. 

The details of these sampling procedures that allow one to focus on the aspect of the problem of interest are the subject of a review by Beveridge (133). Application of this approach to determining conformational transitions in model peptides (137,139,140)are exemplified in the work of Elber s group on helix-coil (85, 86,141), the Brooks group on tum-coil (142-146), and Huston and Marshall and Smythe et al. (147,148)on helical transitions in peptides. 

Arakawa T and Coddette D. The Mechanism of Helical Transition of Proteins by Organic Solvents. Arch Biochem Biophys 1985 240 21-32. 

Smythe M L, S E Huston and G R Marshall 1993 Free Energy Profile of a 3io to a-Helical Transition ot an Oligopeptide in Various Solvents. Journal of the American Chemical Society 115 11594-11595. 

The helical transitions from B-DNA to A-DNA and from B-DNA to Z-DNA can be treated as the zipper model. In these transitions, a defines the extra energy AGj° required to form a junction between the two forms. A relatively small AGj° for the nucleation is probably due to the small difference in the bp stacking between A-DNA and B-DNA. The parameter s is associated with the difference in energy between bps in A-DNA versus B-DNA and on environment factors such as temperature, salt and organic solvents, all of which stabilize left-handed Z-DNA. 

Helical transition n. In the transition zone (compression zone) of an extruder screw, the root surface of the screw describing an advancing spiral surface of increasing radius. 

Taper n. (1) In a conical transition section of an extruder screw, the vertex angle of the axial cone defined by the increasing root diameter of the screw. Compare with helical transition. (2) Often used. Syn draft in molds. 

See conical transition, helical transition, and compression ratio. 

Figures Theoretical state diagram under free boundary conditions (FF) for a = 1 (dashed line) and CT = 1.5 X 10 (drawn line). Vertical axis A/i = -M -I- ln(/> horizontal axis -P = Ins. The top line is the helical transition line, where half the bonds are helical and (0)w = The bottom line is the polymerization line, where half the material is in the aggregated state and n = (From J. van Gestel, P. van der Schoot, M.A.J. Michels Langmuir 19 1375, 2003. Copyright 2003 ACS.)...

Figure 14 Master curve for the mean fraction helical bonds d as a function of a dimensionless temperature scale. T is the temperature and the temperature at the helical transition temperature. The symbols give the experimental results obtained from a solution of the C3 discotic molecules shown in Figure 13 [3,71], Different symbols represent data taken at different concentration in the range from 10 to 10 2 M in the solvent n-butanol. The drawn curve is obtained from the theory of [73] that fits the data reasonably well [73].

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