Staggered molecular packing in crystals of a collagen-like peptide with a single charged pair

The crystal structure of the triple-helical peptide, (Pro-Hyp-Gly)(4)-Glu-L ys-Gly-(Pro-Hyp-Gly)(5) has been determined to 1.75 Angstrom resolution. Th is peptide was designed to examine the effect of a pair of adjacent, opposi tely charged residues on collagen triple-helical conformation and intermole cular interactions. The molecular conformation (a 7(5) triple helix) and hy drogen bonding schemes are similar to those previously reported for collage n triple helices and provides a second instance of water mediated N-H ... O =C interchain hydrogen bonds for the amide group of the residue following G ly. Although stereochemically capable of forming intramolecular or intermol ecular ion pairs, the lysine and glutamic acid side-chains instead display direct interactions with carbonyl groups and hydroxyproline hydroxyl groups or interactions mediated by water molecules. Solution studies on the EKG p eptide indicate stabilization at neutral pH values, where both Glu and Lys are ionized, but suggest that this occurs because of the effects of ionizat ion on the individual residues, rather than ion pair formation. The EKG str ucture suggests a molecular mechanism for such stabilization through indire ct hydrogen bonding. The molecular packing in the crystal includes an axial stagger between molecules, reminiscent of that observed in D-periodic coll agen fibrils. The presence of a Glu-Lys-Gly triplet in the middle of the se quence appears to mediate this staggered molecular packing through its indi rect water-mediated interactions with backbone C=O groups and side-chains. (C) 2000 Academic Press.