Peptides polypeptides Hydrogen bonding effects

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

In addition to the stereochemical features imposed on a polypeptide chain as a consequence of the insertion of a proline residue, the imino nitrogen atom of the pyrrolidine ring is devoid of a proton and is consequently unable to participate in the formation of inter- or intrachain hydrogen bonds. This means that the regularity of any systematic, hydrogen-bonded peptide structure will be interrupted in proline-containing regions. The effect of such internal breaks on the stability of the a-helix has been considered by Schellman (1955) who demonstrated that usu-... 

Biochemists distinguish four levels of the structural organization of proteins. In primary structure, the amino acid residues are connected by peptide bonds. The secondary structure of polypeptides is stabilized by hydrogen bonds. Prominent examples of secondary structure are a-helices and / -pleated sheets. Tertiary structure is the unique three-dimensional conformation that a protein assumes because of the interactions between amino acid side chains. Several types of interactions stabilize tertiary structure the hydrophobic effect, electrostatic interactions, hydrogen bonds, and certain covalent bonds. Proteins that consists of several separate polypeptide subunits exhibit quaternary structure. Both noncovalent and covalent bonds hold the subunits together. 

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