Sequence, heptad configurational

Note Configurational sequences comprising two configurational units are termed diads, those with three such configurational units triads, and so on. In order of increasing sequence lengths they are called tetrads, pentads, hexads, heptads, octads, nonads, decads, undecads, etc. 

Fig. 7.12. The 90-MHz C-NMR spectra of atactic polypropylene in a 20% (w/v) solution of heptane at 67°C (a) CH3, (b) CH2, and (c) CH. Line spectra appearing below the experimental spectra correspond to theoretically calculated resonance positions for (a) heptad, (b) hexad, and (c) pentad configurational sequences. (Reproduced with permission from Ref. [1]. 1988, Academic Press.)...

Spectra of model compounds are also utilized to assign stereochemical sequences in polymer.182 However, model compounds with defined stereochemical configurations are rather difficult to obtain. Zambelli et a/.183 reported that heptad models for polypropylene, 3(5), 5(R), 1 (RS), 9(RS),... 

Each intermediate filament protein has globular head and tail domains joined by a long a-helical domain. The a-helical domain contains many direct repeats of an amino-acid sequence motif called the heptad repeat. Since an a-helix makes just under two turns in seven residues, each position in a seven-residue repeat will slowly wind around the helix. Thus, if two such helices with heptad repeats interact through hydrophobic interactions of side chains, they will slowly wind around each other. This structure is known as an a-helical coiled coil and is stabilized by the presence of hydrophobic residues at positions 1 and 4 of the repeat. The intermediate filament is built from pairs of proteins in the coiled-coil configuration. Two of these pairs bind together, in an antiparallel, staggered, configuration. Further pairs can then add on where the overlap occurs (Fig. 10.4). 

The quaternary carbon (ca. ppm) is likewise split, but more narrowly. The methoxyl carbon, like the methoxyl proton, is insensitive to configuration. The 3 Carbons (52-56 ppm) show tetrad and hexad splitting. Most sensitive is the carbonyl spectrum at ca. ITT Ppm, shown expanded in (b) this shows splitting to the base line for most of the pentad sequences, and clear indications of heptad splitting as well the latter observation indicates resolution of the resonances of structures differing only in the relative orientation of carbonyl groups separated by seven intervening carbon-carbon bonds. 

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