A-helical form

The first structure, flavodoxin (Figure 4.14a), has one such position, between strands 1 and 3. The connection from strand 1 goes to the right and that from strand 3 to the left. In the schematic diagram in Figure 4.14a we can see that the corresponding a helices are on opposite sides of the p sheet. The loops from these two p strands, 1 and 3, to their respective a helices form the major part of the binding cleft for the coenzyme FMN (flavin mononucleotide). 

In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. 

FIGURE 10.35 The amino acid sequences of several amphipathic peptide antibiotics, a-Helices formed from these peptides cluster polar residues on one face of the helix, with nonpolar residues at other positions. 

Fig. 18. The dimeric boronic acid anhydride 68 can potentially exist in a face-to-face or a helical form. X-ray crystallography proves the helical conformation...

Flory, in 1956, predicted that solutions of rodlike polymers could also exhibit LC behavior. The initial synthetic polymers found to exhibit LC behavior were concentrated solutions of poly(gamma-benzyl glutamate) and poly(gamma-methyl glutamate). These polymers exist in a helical form that can be oriented in one direction into ordered groupings, giving materials with anisotropic properties. 

On the other hand, the dilute solution of poly(p-benzyl-L-aspartate) in chloroform or dichloromethane gives a weak but existant CD band centered at 252 nm. A large positive CD band at 222 nm means that poly(p-benzyl-L-aspartate) (PBLA) exists in the left-handed a-helical form. Poly(y-p-nitrobenzyl-L-glutamate) also shows a definite negative CD band around 275 nm, even if the concentration of the polymer... 

Cotton effects which result from the random coil and a-helical forms of polylysine (37). The ordinate is the reduced mean residue rotation, which is defined as... 

Fig. 9. Piezoelectric strain constant of uniaxially drawn poly(y-methyl L-glutamate) film (a-helical form) plotted against the angle 6 between draw-axis and stress direction. Draw-ratio = 2. Drawn after Fukada, Date, and Hirai [Nature 211, 1079 (1966)] by permission of Macmillan (Journals) Ltd.

Figure 12-12 Formation of the oxyanion hole following cleavage of trypsinogen between Lys 15 and He 16. (A) Stereoscopic view. (B) Schematic representation. The newly created terminal -NH3+ of He 16 forms a hydrogen-bonded ion pair with the carboxylate of Asp 194. This breaks the hydrogen bond between Asp 194 and His 40 in trypsinogen, inducing the peptide segment 192-194 to shift from an extended conformation to a helical form in which the NH groups of Gly 193 and Ser 195 form the oxyanion hole. Notice that the positions and interactions of Asp 102, His 57, and Ser 195, the catalytic triad, are very little changed. From Birktoft et al.270...

A) The absorbance spectrum of poly-y-methyl-L-glutamate in the a-helical form, with an assignment of its three constituent transitions. (B) The far-UV CD spectrum of the same compound, deconvoluted on the assumption that the shapes of the absorbance and dichroic bands are directly related (broken lines). The solid line is the observed spectrum and the filled circles the sum of the three bands. In general, the sign of a CD band may be either positive or negative, and its intensity does not necessarily follow that of the absorbance band. (From Holzwarth and Doty, 1965.)... 

Fig. 5.5 A model of conformational autocatalysis for the oligopeptide of Fig.5.4. The lowest-energy a-helical form (two-helix bundle, left-hand side) interacts with the frozen metastable (3 form (four-member (3 barrel, center). After a long Monte Carlo run (symbolized by an arrow), which consists of a million of intermediate conformations of various type, both molecules form a strong complex of two (3 type metastable forms right). During this run the a conformation unfolds and then refolds to the (3 conformation. Thus, the (3 metastable form plays a role of a catalyst in the transformation of the a native structure into the (3 form (autocatalysis)...

The great challenge in this area of research, then, is to be able to form a chiral stack of the planar organic donors and their ions in a crystalline form, and much of the problem arises from the inability of chemists to prepare compounds which will stack in a helical form in the crystal. In Sect. 5 of this... 

Very recently the viscous polymer processing route (see Section 3.6.7) has been exploited by D.H. Pearce and co-workers [21] to fabricate a helical form of the bimorph, the Helimorph , illustrated in Fig. 6.30. Both axial and rotational actuations are developed which have potential for a variety of applications in the mechanical, optical and acoustical fields. 

Poly-L-glutamate in 100 percent a-helical form shows a trough in the ORD spectrum with [a]233 = -15,000° in the random-coil form, [a]233 = -1,000° (the subscript 233 refers to the wavelength of light used). Calculate the proportion of a helix in a protein for which [a]233 = -7,160°, assuming the presence of a-helical and disordered regions only. 

If the a hydrogen is substituted as, for example, in a-amino isobutyric acid (Aib), then the conformational energy (< >,t j) map is very restricted, and the preferred form of Aib peptides is computed to be the 310 rather than the a-helical form.150 This prediction has been verified by NMR and infrared spectroscopic measurements on solutions of oligomers of Aib.151 The stability of the 310-helix for short poly(Aib-L-alanine) polypeptides and the increased stabilization of the a-helical form with a lengthening of the chain have been demonstrated recently.152... 

---