Characterization of Asx Turn Types and Their Connate Relationship with β-Turns

Models of asparagine-containing dipeptides specifically designed to favor intrinsic folding into an Asx turn were characterized both theoretically, by using quantum chemistry, and experimentally, by using laser spectroscopy in the gas phase. Both approaches provided evidence for the spontaneous folding of both the Asn-Ala and Asn-Gly dipeptide models into the most stable Asx turn, a conformation stabilized by a C10 H-bond that was very similar to a type II' β-turn. In parallel, analysis of Asx turns implicating asparagine in crystallized protein structures in the Protein Data Bank revealed a sequence-dependent behavior. In Asn-Ala sequences, the Asx turn was found in conjunction with a type I β-turn for which the first of the four defining residues was Asn. The observation that the Asx turn in these structures is mostly of type II' (i. e., its most stable innate structure) suggests that this motif might foster the formation and/or enhance the stability of the backbone β-turn. In contrast, the Asx turns observed in Asn-Gly sequences extensively adopted a type II Asx-turn structure, thus suggesting that their formation should be ascribed to other factors, such as hydration. The fact that the Asx turn in a Asn-Gly sequence is also often found in combination with a hydrated β-bulge supports the premise that a Asn-Gly sequence might efficiently promote the formation of the β-bulge secondary structure.