We studied the parameters of binding of 5,10,15,20-tetra-(N-methyl-3-pyridyl)porphyrin (TMPyP3) to the anti-parallel human telomeric G-quadruplex d(TTAGGG)4, the oligonucleotide dTTAGGGTTAGAG(TTAGGG)2 that does not form a quadruplex structure, as well as to the double stranded d(AC)8 x d(GT) and single stranded d(AC)8 and d(GT)8 DNAs. The analysis of absorption revealed that the binding constants and the number of DNA binding sites for TMPyP3 were d(AC)8 < d(GT)8 < d(AC)8 x d(GT)8 = d(TTAGGG)4 < dTTAGGGTTAGAG(TTAGGG)2. We demonstrated for the first time that the binding constant of TMPyP3 with the non-quadruplex chain dTTAGGGTTAGAG(TTAGGG)2 (1.3 x 10(7) M(-1)) is approximately 3 times bigger than the binding constant with the quadruplex d(TTAGGG)4 (4.6 x 10(6) M(-1)). Binding of two TMPyP3 molecules to d(TTAGGG)4 led to a decrease of thermostability of the G-quadruplex (deltaT(m) = -8 degrees C). Circular dichroism spectra of TMPyP3:d(TTAGGG)4 complexes revealed a shift of DNA structure from the G-quadruplex to an irregular chain. We hypothesize that partial destabilization of the telomeric G-quadruplex by TMPyP3 might be a reason for relatively low potency of this ligand as a telomerase inhibitor, as well as its marginal cytotoxicity for cultured tumor cells.