Analysis of spectral changes in isotopically substituted porphyrins adsorbed on melanin surfaces by solid-state carbon-13 nuclear magnetic resonance spectroscopy.
Two synthetic, water-soluble porphyrins, tetra-4N-methyl-pyridyl porphyrin (T4NMPyP) and tetra-N,N,N-trimethylanilinium porphyrin (TMAnP), and their indium(III) complexes have been isotopically substituted for evaluation by solid-state NMR of their adsorption to melanin. Methyl carbons on the pyridyl and anilinium nitrogens on the periphery of the porphyrin are one site of substitution; the four equivalent meso carbons in the porphyrin ring are the second. Chemical shifts and line widths of the carbon resonances in the bulk porphyrins change after they are adsorbed on synthetic and natural melanins. These changes may be related to the predominant binding interactions involved in the adsorption process and suggest that chemisorption, involving strong interactions comparable to chemical bonds, occurs between charged groups in both ligand and substrate. Inhomogeneous line broadening of the resonances implies that variations in binding exist, which is in keeping with the known heterogeneous nature of the melanin polymer and the different steric demands of the porphyrin ligands. These solid-phase experiments illustrate the promise of NMR for elucidating information about the binding of soluble molecules to insoluble and structurally irregular biopolymers. Furthermore, they represent the first spectroscopic probe of the binding of cationic molecules to the biopolymer melanin in the solid state. Coupled with results from molecular modelling, they indicate that cationic porphyrins bind to melanin at similar sites.
Duff
G A
GA
Department of Chemistry, Lehigh University, Bethlehem, PA 18015.
Roberts
J E
JE
Foster
N
N
D001665:Binding Sites; D002247:Carbon Isotopes; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008543:Melanins / Q000378:metabolism*; D015394:Molecular Structure; D011166:Porphyrins / Q000378:metabolism*
D002247:Carbon Isotopes; D008543:Melanins; D011166:Porphyrins; C021096:tetra(4-N-methylpyridyl)porphine; D004295:Dihydroxyphenylalanine