The photoreduction of water-soluble cationic manganese(III) meso-tetrakis(1-methylpyridium-4-yl)porphyrin (Mn(III)(TMPyP)(4+)) bound to a synthetic polynucleotide, either poly[d(A-T)2] or poly[d(G-C)2], was examined by conventional absorption and circular dichroism (CD) spectroscopy, transient absorption, and transient Raman spectroscopy. Upon binding, Mn(III)(TMPyP)(4+) produced a positive CD signal for both polynucleotides, suggesting external binding. In the poly[d(A-T)2]-Mn(III)(TMPyP)(4+) adduct case, an interaction between the bound porphyrin was suggested. The transient absorption spectral features of Mn(III)(TMPyP)(4+) in the presence of poly[d(A-T)2] and poly[d(G-C)2] were similar to those of the photoreduced products, Mn(II)(TMPyP)(4+), whereas Mn(III)(TMPyP)(4+) in the absence of polynucleotides retained its oxidation state. This indicated that both poly[d(A-T)2] and poly[d(G-C)2] act as electron donors, resulting in photo-oxidized G and A bases. The transient Raman bands (ν2 and ν4) that were assigned to porphyrin macrocycles exhibited a large downshift of ~25 cm(-1), indicating the photoreduction of Mn(III) to Mn(II) porphyrins when bound to both polynucleotides. The transient Raman bands for pyridine were enhanced significantly, suggesting that the rotation of peripheral groups for binding with polynucleotides is the major change in the geometry expected in the photoreduction process. These photoinduced changes do not appear to be affected by the binding mode of porphyrin.