Experimental investigations on the backbone folding of proline-containing model tripeptides |
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Authors: | Guy Boussard Michel Marraud Andr Aubry |
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Institution: | Guy Boussard,Michel Marraud,André Aubry |
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Abstract: | Some proline-containing tripeptides with the general formulas R0CO-L -Pro-X-NHR3 (X = Gly,Sar,L -Ala,D -Ala) and R0CO-X-L -Pro-NHR3 (X = Gly,L -Ala,D -Ala) have been investigated in solution by ir and 1H-nmr spectroscopies. Their favored conformational states depend mainly on both the primary structure and the chiral sequence of the molecules. In inert solvents the βII-folding mode is the most favored conformation for the L -Pro-D -Ala and L -Pro-Gly tripeptides, while the βII′-turn is largely preferred by D -Ala-L -Pro derivatives. Under the same conditions only about one-third of the whole conformers of L -Pro-L -Ala molecules adopts the βI-folding mode. Semiopened C7C5 and C5C7 conformations are appreciably populated in the L -Pro-L -Ala sequence, on the one hand, and in the Gly-L -Pro and L -Ala-L -Pro derivatives, on the other hand. In L -Pro-Sar and X-L -Pro models, the cis–trans isomerism around the middle tertiary amide function is observed. Thus cis L -Pro-Sar and L -Ala-L -Pro conformers are folded by an intramolecular i + 3 → i hydrogen bond, whereas cis D -Ala-L -Pro and Gly-L -Pro molecules accommodate an open conformation. In dimethylsulfoxide the βII- and βII′-folding modes are not essentially destabilized, as contrasted with the βI conformation, which is less populated. In water solution all the above-mentioned conformations, with the possible exception of the βII′-folding mode for D -Ala-L -Pro molecules, seem to vanish. Solute conformations are also compared with the crystal structures of four proline-containing tripeptides. |
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