Synthesis of redox derivatives of lysine and their use in solid-phase synthesis of a light-harvesting peptide

Article abstract of DOI:10.1016/0040-4020(94)01018-U

Redox-active amino acids were synthesized for incorporation into peptide assemblies to study photoinitiated electron or energy transfer. 4'-Methyl-2,2'-bipyridine-4-carboxylic acid was obtained in 72% yield by consecutive SeO₂ and Ag₂O oxidation without isolation of intermediates. The side chain ε-amino group of Boc-L-lysine methyl ester or γ-carboxyl group of Boc-L-glutamic acid α-methyl ester was coupled to a redox moiety (transition-metal chromophore, electron donor, electron acceptor, metal ligand, or triplet-energy transmitter) using 4-(dimethylamino)pyridine, (1-benzotriazoleoxy)tris(dimethylamino)phosphonium hexafluorophosphate, N-methylmorpholine, and 1-hydroxybenzotriazole. Use of one equivalent of 4-(dimethylamino)pyridine provided the amide coupling product in 80-97% isolated yield. Selective hydrolysis of the methyl esters with lithium hydroxide provided the redox Boc-amino acids in 70-98% yield. These redox modules are suitable for solid-phase assembly of light-harvesting peptides, as illustrated by the synthesis of the partially α-helical 11-residue redox triad that contains a phenothiazine electron donor, a ruthenium(II)tris(bipyridine) chromophore, and an anthraquinone electron acceptor. Upon laser excitation at 420 nm, the peptide triad underwent photoinduced electron transfer to create a charge-separated state with a lifetime of 53 ns and decayed with a first-order rate constant of 1.9 x 10⁸ s^-1.