Hydrogen bonding continued acids

Fig. 16. An unusual interrupted helix from subtilisin (residues 62-86), in which the helical hydrogen bonds continue to a final tum that is formed by a separate piece of main chain. Such interrupted helices (broken on one side of the double helix) are apparently a fundamental feature of nucleic acid structure as illustrated by tRNA, but are exceedingly rare in protein structure.

Proteins are biopolymers formed by one or more continuous chains of covalently linked amino acids. Hydrogen bonds between non-adjacent amino acids stabilize the so-called elements of secondary structure, a-helices and / —sheets. A number of secondary structure elements then assemble to form a compact unit with a specific fold, a so-called domain. Experience has shown that a number of folds seem to be preferred, maybe because they are especially suited to perform biological protein function. A complete protein may consist of one or more domains. 

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

Ostrowska and Narebska noted an infrared continuous absorption in hydrated acid form Nafion 120 membranes that began at 3400 cm and extended toward low wavenumbers. This feature was not present in dry membranes and, based on the work of Zundel et al., was proposed to be due to the existence of Hs02 and Hg04 groups, in which there are easily polarizable hydrogen bonds. This paper by Ostrowska and Narebska is also useful, as it contains a number of band assignments for Nafion. 

Cyclic peptides with an even number of amino acids and alternating d- and l-configuration have been shown to form nanotubes by a continuous array of hydrogen bonds above and below the ring planes (see Section 6.8.6.1). 

---