Spectroscopic and crystallographic data are presented for a series of tetrapeptides and analogous depsipeptides that can form a minimal β-hairpin (two intramolecular hydrogen bonds). These model compounds have been used to test the hypothesis that "mirror image" β-turns promote β-hairpin formation. This hypothesis was inspired by a statistical survey of β-hairpins in globular proteins (Sibanda, B. L.; Thornton, J. M. Nature 1985, 316, 170), which showed that mirror image β-turns (type I′ and type II′), although rare in general, are very commonly associated with β-hairpins containing a two-residue loop between the strand segments. Each of our four-residue molecules contains proline at the second position, to promote a central β-turn. The β-turn is induced to be either "common" or "mirror-image", relative to the outer residues, by choice of residue configuration (L vs D). In methylene chloride, end-capped tetrapeptide Ac-L-Val-D-Pro-D-Ala-L-Leu-NMe 2 folds largely into the β-hairpin conformation, while the diastereomer Ac-L-Val-L-Pro-L-Ala-L-Leu-NMe 2 displays little or no β-hairpin folding. For each diastereomer, the hydrogen-bonded driving force for β-hairpin folding is identical, and the dramatic difference in folding behavior therefore reflects a variation in the intrinsic conformational properties of the diastereomeric backbones. Similar behavior is seen for the diastereomeric peptide pair Ac-L-Val-D-Pro-Gly-L-Leu-NMe 2 vs Ac-L-Val-L-Pro-Gly-L-Leu-NMe 2 , and for the analogous depsipeptides with a lactic acid or glycolic acid residue at the third position. Thus, our results show not only that mirror-image Pro-X turns strongly promote β-hairpin folding, but also that common β-turns strongly discourage formation of a β-hairpin with a two-residue loop.

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This study compared three different ways autobiographical memories are elicited involuntarily: (1) cued by an active goal common to memory and retrieval contexts in combination with sensory information associated with this goal-directed activity; (2) cued by sensory information that does not relate to goal-directed activity common to both memory and retrieval contexts; and (3) activated when no identifiable cue present in retrieval context. Two hundred and twenty eight participants recorded details of a single autobiographical memory that resulted naturally from a spontaneous, nondeliberate retrieval. Nearly all recorded memories described specific events (83%) with very few memories less than 7 days old (8%) and many memories more than 5 years old (44%). No significant differences resulted between the three retrieval types for the age, specificity or prior rehearsal of memories reported. However, the level of attention at retrieval was significantly more diffuse for retrievals without a clearly identifiable cue. The authors discuss the implications of these findings for current models of involuntary autobiographical memory retrieval.

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