Determination of the Absolute Configuration of Side Chains of Basic Amino Acid Residues Using the Water-Soluble Porphyrin-Based Exciton Chirality Method.

We demonstrated that the circular dichroism (CD) exciton chirality method, based on the supramolecular interactions of meso-tetra(4-sulfonatophenyl)porphyrin (MTPPS, M = Zn or H), was applicable for the determination of the absolute configuration between the side chains of two basic amino acid residues of stable monomeric β-hairpin peptides (tryptophan zipper: Trpzip). When MTPPS was added to an aqueous solution containing Trpzip, a bisignate CD signal was detected in the Soret band region in addition to a decrease in absorbance. These spectral changes indicated the formation of a supramolecule consisting of Trpzip and MTPPS via electrostatic interactions between the positively charged lysine residue of Trpzip and the negatively charged sulfonate group of MTPPS. On the basis of the Job plots, the supramolecular structure of Trpzip-ZnTPPS is ZnP-Tz-ZnP or ZnP-Tz-ZnP-Tz-ZnP, whereas that of Trpzip-HTPPS is -(-HP-Tz-)- (MP and Tz denote MTPPS and Trpzip, respectively). To explain the bisignate CD spectra of the supramolecules, a plausible model, that is, ZnP-Tz-ZnP, was carefully analyzed by the CD exciton chirality method: two orthogonalized electric transition dipole moments of each MTPPS and the effects of free rotation of MTPPS around the electrostatic bonding axis were considered. The exciton-coupled CD spectral pattern based on ZnTPPS reflected the absolute configuration between the side chains of two lysine residues. Our approach can be used to determine the absolute configuration of side chains of other peptides with two basic amino acid residues in the amino acid sequences.