Crystal structure and a twisted β‐sheet conformation of the tripeptide L‐leucyl‐L‐leucyl‐L‐leucine monohydrate trimethanol solvate: Conformation analysis of tripeptides

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Abstract
In order to test the helical preference of short oligo-L-leucines, we crystallized the tripeptide L-leucyl-L.-leucyl-L-leucine (LLL) and carried out x-ray diffraction studies of it (L-leucyl-L-leucyl-Lleucine)2. 3CH3OH. H2O, (C39H84N6O12). Crystallized in the monoclinic system, space group P21, cell parameters: a = 12.031(2), b = 15.578(3), c = 14.087(2) Å, α = 90°, β = 97.29(1)°, γ = 90°, V = 2618.6 Å3. MW = 829.1, Dc = 1.051 gcm−3. R index of 0.057 for 4213 reflections (λcukα = 1.5418 Å) > 2σ. LLL takes tip the β-sheet rather than a helical conformation in the crystalline stale. The three methanol molecules and the water molecule that constitute the solvent of crystallization form a network of hydrogen bonds to the LLL molecules and to one another. It is rather remarkable that though A and L have stronger helical preferences than G, neither AAA nor LLL form the crystalline helix but GAL does, indicating that the helical preferences depend on the sequence context. The residue L2 in molecule A and the residues L1 and L3 of molecule B do not show the preferred conformation for forming helices. Further, very remarkably. LLL exhibits a unique super secondary feature of the protein folding topology, namely the twisted β-sheet. Where as most short peptides show only the classical p-sheet conformation. Thai even the tripeptide LLL is able to exhibit the twisted β-sheet conformation, and with the correct left-handed twist this suggests that even very short peptide segments possess the ability to assume several of the characteristic topological features exhibited by proteins. An extensive review of tripeptide conformations has been carried out and some results of this study have been included here. © 1995 John Wiley & Sons, Inc.

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