Helical structures pyrrole

Although most of the available literature supports the planar trans arrangement for electrochemically prepared PPy, one study suggests an alternative conformation. Davidson and coworkers154 found clear evidence of a helical structure produced from an all cis coupling of pyrrole rings. These workers used dodecyl sulfate as dopant, but also argue that all published literature on XRD of PPy is also consistent with the helical structure. Furthermore, the helical structure is identical to that proposed for poly(3-alkylthiophenes) as described further in Chapter 6. 

The first reported circular dichroism (CD) study of chiral PPy s was by Delabouglise and Gamier105 on the polymers 4 (R = CH2OH, CHMc2, and Ph), in which a series of amino acids are covalently bound at the 3-position of each pyrrole ring. They exhibit a CD band at ca. 470 nm associated with the absorption band at 460 nm. This optical activity is believed to be induced by the presence of the chiral amino acid substituents, leading to the adoption of a one-handed helical structure by the PPy chains. 

In these examples it was not possible to visualise any chiral structure with a microscope, but when PANI was prepared using poly(acrylic acid) as an in situ template, helical microwires were visualised [65]. In an even more general sense, helical fibres of PANI, poly(ethylenedioxythiophene) (PEDOT), and poly(pyrrole) were prepared using synthetic lipids as templates [66,67]. The synthetic lipid molecules used are shown in Fig. 6 along with some of the helical fibres of PEDOT that are formed when the sulphonate salt is used to shape the fibres during the polymerisation. The procedure involves growing the fibres by electrochemical polymerisation onto an ITO electrode with the lipid molecules in the electrolyte. 

Therefore, we investigated a molecular interaction of cardiolipin with cytochrome c. As shown in Figure 2, cytochrome c consists of 5 helices and inter-helical loops which harbor a heme c prosthetic group by covalent thioether-bonds through cysteine-14 and -17 residues. Ferric ion, centered in the pyrrole ring, is axially liganded by histidine-18 and methionine-80 residue. The lower half of the protein consisting of flexible random coils is a rather soft structure and has a space between the heme c plate and P-loops inside the small basic protein. 

Cytochrome ci shows no significant sequence homology to other cytochromes or even to to cytochrome f its counterpart in the b complex however, its structure is related to mitochondrial cytochrome c and to cytochrome C2, the prototypes of Ambler s class I cytochromes [20]. In Ambler s class I cytochromes, the heme is surrounded by five helices (al - a5) three of die five helices (al, a3, a5) are conserved in cytochrome c. Helix a2 of class I cytochromes is absent diis results in die exposure of the heme propionates at the protein surface. Thus, in contrast to cytochrome c, cytochrome C has two electron transfer padiways one through the heme propionates at die back face (where cytochrome ci receives electrons from the Rieske protein), and die other at the front face through die exposed pyrrole C comer of the heme (where electrons are passed on to cytochrome c). 

Various PFs [61-63] and their pyrrole analogues [64] functionalized with chiral side chains exhibit NLO character and it is not clear how the chirality, helicity and NLO character are related. Because PF2/6 has become a model test system in the study of helical PFs, fully understanding its intramolecular structure can serve as a reference point for further comparisons. The molecular level heterogeneity intrinsic in the single chain structure of PF2/6 insures a high level of frustration and thus it represents a model polymer for paracrystals and paracrystallinity [24]. 

---