Molecular brake

Hsps serve other ftmctions during nonstressful conditions. Hsp90 acts as a molecular brake for the steroid receptor family (Bresnick et al., 1988 Bagchi et al., 1990,1991). The complex of hsp90 and the glucocorticoid receptor prevents DNA binding of the receptor until the ligand for the receptor is present (Cadepond et al.,... 

A chemical signal can also be used, as in the recently described molecular brake".1381 In this case, rather than its shape, it is more the motion dynamic properties of the compound (rotation of a group about a C-C bond) that are modified, by addition of a transition metal. 

A point of interest in molecular machines is the control of the rotational speed. The modulation of the rotation rate around an N-Ar bond was achieved through a redox-mediated molecular brake by Jog, Brown, and Bates [39], This consists of the oxidation of sulfide group close to the rotational axis (compound 29) into a sulfoxide 30 and further into a sulfone 31 (Fig. 8a), so engendering successive covalent constitutional changes in strategic places of the molecule, changes that decrease the rotation. The N-Ar rotational barriers of the sulfone 31 (about 13.6 kcal mol 1) and sulfoxide 30 (13.6 kcal mol-1) are about 5 kcal mol-1 higher than in the sulfide 29 (8.6 kcal mol 1). The rate of rotation is reduced from the sulfide 29 (1.6 x 106 s ) to the sulfoxide 30 (1.3 x 102 s-1), or from the sulfide 29 (1.6 x 106 s ) to the sulfone 31 (1.4 x 102 s-1) by a factor of about 104 s-1. 

Fig. 8 (a) Chemical and stylized representation of the strategy of redox-mediated molecular brake passing from sulhde to sulfoxide and sulfone (b) an example of oxygen-flipped rotary switch (c) its stylized representation (d) X-ray structure of a bisarylanthracene peroxide (H atoms were omitted for clarity) (e) control of the frequency of molecular motions in rotaxanes of which annulus (macrocycle) contains a photoisomerizable dianthrylethane group (see text for details)... 

Jog PV, Brown RE, Bates DK (2003) A redox-mediated molecular brake dynamic NMR study of 2-[2-(methylthio)phenyl]isoindolin-l-one and S-oxidized counterparts. J Org Chem... 

In designing a molecular brake we envisioned the use of a triptycene as a three-toothed gear, and the brake being incorporated as a bipyridine attached to the triptycene through an axle. The brake would be activated by a change of conformation of the bipyridine unit (noncoplanar to coplanar) upon complexation of the bipyridine by a metal ion, thereby obstructing the rotation of the triptycene. Scheme 2 represents the concept in both general and specific terms. 

Scheme 2. Schematic and actual depiction of the concept of a molecular brake...

In summary, we developed the first example of a molecular brake. The brake operates by coordination of a metal at a remote site, which brings about a conformational change that reversibly halts rotation of a molecular-scale gear. 

The H NMR room temperature spectrum of 37b reflects an absence of symmetry, where all three rings of the triptycene are nonequivalent. At 160 °C, but not at lower temperatures, however, some peak broadening of triptycene (but not helicene) resonances is observed, indicating somewhat less retarded rotation. Extrapolation, based on analogy to our earlier work with the molecular brake, suggests a coalescence temperature of about 220 °C, which corresponds to a AG of approximately 25 kcal mol-1, a value that is corroborated by the studies described below and which is reassuringly close to the calculated barrier (AH, not AG ) of 22 kcal mol-1. 

Over the last 10 years, efforts of several groups have led to the synthesis of a variety of molecular devices that exhibit controlled or coordinated rotation. In our research group the ultimate achievement has been the synthesis of a molecular system that functions as a prototype of a motor. This work was possible thanks to the experience gained in preparing a molecular brake and ratchet . 

Scheme 34. Stille coupling of (hetero)aryl halides with (hetero)aryltin reagents Kelly s molecular brake (68, [22]) and nicotelline (69, [124a]) a) Pd(PPh3)4,b) xylene, A...

Fig. 3 In polar aprotic solvent, a salt-binding [2]rotaxane populates multiple axle/wheel orientations. Binding CP does not measurably alter the rotaxane s dynamic properties, whereas binding K " freezes out a single coconformation (molecular brake). (Reprinted with pennission from Ref. [22]. Copyright 2002, Elsevier Science.)...

In 1994, Kelly et al. synthesized a molecule that could operate as a reversible molecular brake (Figure 83), " in which the wheel—represented as a three-toothed gear and constructed as a triptycene—spins rapidly at 30 °C in the absence of Hg + (or other metal ions), while after the addition of metal ions the rotation of the triptycene is blocked and the brake works. 

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