Tyrosyl-serine

The structure of the tripeptide L-lysyl-L-tyrosyl-L-serine acetate was determined by X-ray diffraction. The crystals are triclinic space group P1, with two peptide molecules in the unit cell. The peptides are in zwitterionic form with positive charges both in the amino terminal and epsilon-amino groups of lysine. A negative charge is found in one of the carboxylic groups, whereas the other one is protonated. Both peptides show very similar backbone torsional angles, in the beta pleated sheet region, but different tyrosine and serine side-chain conformations. The two lysine side chains have a similar conformation g + tg + t, which had not been previously found. In the unit cell we also find one water molecule, one isopropanol molecule and four acetic acid molecules, three of them likely to be present as acetate anions. These molecules form layers which separate the beta-pleated sheets. The whole structure looks like an ordered solution of peptides in the beta-sheet conformation. An extensive network of hydrogen bonds stabilizes the crystal structure.