Nucleation, Growth, and Form in Crystals of Peptide Helices

A model for the nucleation of crystallization in peptide helices is presented. The crystal structures of four polymorphic forms of a hydrophobic helical decapeptide Boc-Leu-Aib-Phe-Phe-Leu-Aib-Ala-Ala-Leu-Aib-OMe (I) exemplify alternative packing modes in cylindrical molecules. Three crystal forms of peptide I are monoclinic $P2_1$, while one is orthorhombic $P22_12_1$. The five different helical molecules characterized have very similar backbone conformations over much of the peptide length. A survey of 117 helical peptide structures with a length \geq 8 residues reveals a preponderance of the triclinic (P1), monoclinic $(P2_1)$, and orthorhombic $(P2_12_12_1)$ crystal forms. Models for the formation of critical nuclei are based on helix association driven by solvophobic forces, resulting in the formation of raftlike structures. Raft association can be further driven by the imperative of minimizing solvent accessible surface area with the formation of blocks, which can be subsequently fitted in Lego set fashion by multiple hydrogen bond interactions in the head-to-tail region. This model provides a rationalization for observed crystal formation based on a postulated structure for an embryonic nucleus, which is determined by aggregation patterns and unconstrained by the dictates of symmetry