Abstract: We studied an effect of the binding of tetracationic meso-tetrakis(4-Nmethylpyridyl)  porphyrin (TMPyP) in chromatin as post-translational histone modification in dark-bound TMPyP and upon visible light irradiation. TMPyP was applied into HeLa normal and hyperacetylated mononucleosomes at various base pair/porphyrin ratios (r).

Hyperacetylated mononucleosome was characterized in agarose and Triton-Acetic acid-Urea gel electrophoresis, which was shown by subspecies bands in all histones. The process of nucleosome denaturaion was observed by measuring the curve of thermal stability of the phase transition temperature with molar basepair ratio at r=50, r=25, and r=10. Within normal and hyperacetylated mononucleosomes, there are three phase transition temperatures, namely Tm1 and Tm2 related to the histone – DNA interaction, and Tm3 for dsDNA disruption. And each phase transition was analyzed into three following parameters : the midpoint temperature of denaturation (Tm), the absorbance amplitude, and a half-bandwitdh. The results reveal that hyperacetylated nucleosome has Tm1, which is higher shifted and Tm2’s amplitude which is reduced 50% compared to normal nucleosome. Dark-bound TMPyP destabilizes the conformation of DNA-histone interaction, which is shown by a lower thermal denaturation of both normal nucleosome (?Tm1=50C; ?Tm2=6.70C) and hyperacetylated nucleosome (?Tm1=5.70C; ?Tm2=00C). While dsDNA phase transition (Tm3) is quite stabil at around 80- 840C. Further, changes in amplitude of all Tm1, Tm2 and Tm3 could be observed in a manner consistent with the increasing molar basepair ratios as following results : 25%, 40%, 100% for normal and 4.8%, 37.5%, 100% for hyperacetylated nucleosomes. Last effect, which we obtained, is enlarger and broader the half-bandwidth of each phase transition temperatures.  Thus, Upon visible light irradiation all changes relating to DNA/histone, histone octamer and dsDNA disruptions are more pronounced.