Ac‐L‐Ala‐Aib‐L‐Ala‐OMe: X‐Ray Analysis of a Distorted β‐Bend and Magnetic Nonequivalence of Aib‐Methyl Groups

Ac-L-Ala-L-Ala-OMe (1) crystallizes in the space group P21 with a = 10.796(2), b = 10.788(3), c = 14.625(6) A, β = 92.74(3)°, and Z = 4 (R value for 1590 independent reflexions 0.05). In contrast to comparable Aib oligopeptides the two independent tripeptide molecules 1 in the asymmetric unit form very distorted β-turns. The 41 intramolecular hydrogen bridges are extremely weak (3.34 and 3.67 A, respectively) in the solid state. This widening of the β-turn seems to result from strong intermolecular hydrogen bonds (2.94 and 2.85 A) linking the two independent molecules 1 to form dimeric units. These units are linked with further hydrogen bonds (3.01 and 2.83 A) to a two-dimensional network parallel to the bc-plane. In solution the tripeptide 1 exhibits a particularly large magnetic nonequivalence of the two Aib-Cβ atoms (Δδ = 2.6 ppm). This indicates a highly populated conformation with a preferred orientation of the pro-S-Aib-Cβ relative to the Aib peptide carbonyl group. Ac-l-Ala-L-Ala-Ome: Rontgen-Strukturanalyse eines verbogenen β-Turns und Magnetische Nichtaquivalenz der Aib-Methylgruppen Ac-L-Ala-Aib-L-Ala-OMe (1) kristallisiert in der Raumgruppe P21 mit a = 10.796(2), b = 10.788(3), c = 14.625(6) A, β = 92.74(3)° und Z = 4 (R-Faktor fur 1590 unabhangige Reflexe 0.05). Im Gegensatz zu vergleichbaren Aib-Oligopeptiden bilden die beiden unabhangigen Molekule 1 in der Elementarzelle stark verbogene β-Turns. Die intramolekularen H-Brucken (41) sind extrem schwach (3.34 bzw. 3.67 A). Diese Aufweitung des β-Turns scheint aufgrund starker intermolekularer H-Brucken (2.94 und 2.85 A) innerhalb dimerer Einheiten zweier unabhangiger Molekule 1 zu erfolgen. Die Dimeren sind durch weitere starke H-Brucken (3.01 und 2.83 A) zu einem zweidimensionalen Netzwerk parallel der bc-Ebene verbunden. In Losung zeigt das Tripeptid 1 eine besonders ausgepragte magnetische Nichtaquivalenz der beiden Aib-Cβ-Atome (Δδ = 2.6 ppm). Diese deutet auf eine Konformation mit stark bevorzugter Orientierung der pro-S-Aib-Methylgruppe relativ zu der Aib-Peptidcarbonylgruppe hin.

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