Proton Nuclear Magnetic Resonance Spectrometric and Spectrophotometric Studies on Hydrophobic and Electrostatic Interactions of Cationic Water- Soluble Porphyrin with Nucleotides

Abstract

The interactions of cationic porphyrin, tetrakis(4-N-methylpyridyl)porphine (H₂TMPyP), with 5′-adenosine triphosphate (ATP), 5′-adenosine diphosphate (ADP), 5′-adenosine monophosphate (AMP), adenosine (A), adenine (AN) and metal complexes of ATP were evaluated by UV-VIS and ¹H-NMR spectroscopy at 25°C and an ionic strength of 0.1mol dm^-3 (NaNO₃). The formation constants (log K/mol^-1 dm³), defined as L+H₂TMPyP_??_L(H₂TMPyP), were found to be 2.92±0.06, 2.92±0.16, 2.59±0.07, 2.20±0.07, 2.10±0.09, 2.83±0.03, 2.93±0.04 and 2.80±0.07 for L=ATP^4-, ADP^3-, AMP^2-, A, AN, Mg(ATP)^-2, Ca(ATP)^2- and Ni(ATP)^2-, respectively. The binding of magnesium(II), calcium(II) or nickel(II) to ATP does not affect the formation of the molecular complex of H₂TMPyP with ATP. The addition of ethanol or the protonation of an adenine base, however, destabilizes the molecular complex. The predominant factor (75%) for stabilization of the molecular complex is a hydrophobic interaction between the porphyrin plane and the adenine base; the remaining factor (25%) is an electrostatic interaction between the negatively charged phosphates of nucleotides and the positively charged N-methylpyridyl groups of H₂TMPyP. A hydrophobic interaction was also confirmed by 2-D ¹H-NMR experiments, in which large upfield shifts were observed for the chemical shifts of H-2 and H-8 protons of adenine base, and H-1′ of ribose.