Conformational properties of trans Ac-Asn-Pro-Tyr-NHMe and trans Ac-Tyr-Pro-Asn-NHMe in dimethylsulfoxide and in water determined by multinuclear n.m.r. spectroscopy

Vicinal coupling constants between various nuclei provide backbone and side‐chain conformational information for a series of asparagine‐ and tyrosine‐containing peptides in DMSO and in H2O. By enriching Tyr of Ac‐Asn‐Pro‐Tyr‐NHMe with 15N, it has been possible to distinguish between the resonances of the two side‐chain β protons of Tyr. Analysis of the coupling constants in terms of the distributions of side‐chain conformations in these peptides indicates that the addition of Asn to the Pro‐Tyr sequence leads to a less random conformational distribution. When compared to the side‐chain rotamer distribution of Ac‐Asn‐NHMe and Ac‐Tyr‐NHMe, particular Asn and Tyr side‐chain conformations of Ac‐Asn‐Pro‐Tyr‐NHMe are stabilized in dimethylsulfoxide solution. The interaction(s) which stabilize a unique Tyr side‐chain conformation of Ac‐Asn‐Pro‐Tyr‐NHMe in dimethylsulfoxide are not present in Ac‐Ala‐Pro‐Tyr‐NHMe and are unaffected by the addition of Val‐Pro to the C‐terminus of Asn‐Pro‐Tyr. In water, a preferential stabilization of one Asn side‐chain conformation of Ac‐Asn‐Pro‐Tyr‐NHMe is also observed, while the Tyr side‐chain rotamer distribution is similar to that of Ac‐Tyr‐NHMe. An interaction between the Asn side chain and the Pro‐Tyr‐NHMe backbone was previously shown to stabilize a β‐bend conformation at Pro‐Tyr in water. Data are also presented for Ac‐Tyr‐Pro‐Asn‐NHMe, for which local interactions do not stabilize particular backbone conformations in dimethylsulfoxide or in water. The conformations of the peptides studied here are relatively insensitive to temperatures between 27° and 62°, both in dimethylsulfoxide and in water. The sequences Asn‐Pro‐Tyr and Tyr‐Pro‐Asn occur in ribonuclease A, and these tripeptides serve as models for the interactions involved in the folding of this protein.