Chlorophyll‐Amino Acid Interactions in Synthetic Models

A number of amino acids and molecules of related functionality have been covalently connected to pyrochlorophyllide a (PChla) and bacteriopyrochlorophyllide a (BPChla) to determine their impact on the properties of the chromophore. PChla and BPChla esterified with side chains containing the thio‐ether functionality (methyl‐6‐hydroxyhexylsulfide, a Met analogue) were studied in detail by NMR. Intramolecular coordination at the central metal is observed for the Zn‐PChla and Zn‐BPChla thio‐ether derivatives, but not for the Mg‐PChla derivative. The NMR data for the Zn‐PChla derivative is interpreted as providing evidence for two, distinguishable five‐coordinate complexes, with the ligand on either side of the macrocycle. In contrast, the Zn‐BPChla derivative appears to be coordinated exclusively on one side. It is proposed that stereoselective coordination may provide a simple mechanism for altering the spectroscopic properties of BChla when it binds to a protein. PChla was covalently attached to the methyl esters of L‐Trp, L‐Tyr, L‐Phe and L‐Val. Very large, similar chemical shift changes in the NMR spectra in the absence of a coordinating ligand indicate that all of these compounds form a novel dimer, in which the carbonyl group of the carbomethoxy group on the amino acid coordinates to the central metal, bridging between macrocycles. An average structure is proposed for this dimer in which the macrocycle planes are parallel, overlap in ring IV and have their x‐axes antiparallel.