The thermal stability and reduction kinetics of graphene oxide were studied by measuring the electrical resistivity of single-layer graphene films at various stages of reduction in high vacuum and by performing temperature programmed desorption (TPD) measurements of multilayer films in ultrahigh vacuum. The graphene oxide was exfoliated from the graphite oxide source material by slow-stirring in aqueous solution, which produces single-layer platelets with an average lateral size of ∼10 µm. From the TPD measurements, it was determined that the primary desorption products of the graphene oxide films for temperatures up to 300 °C are H 2 O, CO 2 , and CO, with only trace amounts of O 2 being detected. Resistivity measurements on individual single-layer graphene oxide platelets resulted in an activation energy of 37 ( 1 kcal/mol. The TPD measurements of multilayer films of graphene oxide platelets give an activation energy of 32 ( 4 kcal/mol.

Poly [ADP-ribose] polymerase 1 (PARP-1) is a highly abundant chromatin-associated enzyme. It catalyzes the NAD + -dependent polymerization of long chains of poly-ADP ribose (PAR) onto itself in response to DNA damage and other cues. More recently, the enzymatic activity of PARP-1 has also been implicated in the regulation of gene expression. The molecular basis for the functional switch from chromatin architectural protein to transcription factor and DNA damage responder, triggered by PARP-1 automodification, is unknown. Here, we show that unmodified PARP-1 engages in at least two highaffinity binding modes with chromatin, one of which does not involve free DNA ends, consistent with its role as a chromatin architectural protein. Automodification reduces PARP-1 affinity for intact chromatin but not for nucleosomes with exposed DNA ends. Automodified (AM) PARP-1 has the ability to sequester histones (both in vitro and in cells) and to assemble nucleosomes efficiently in vitro. This unanticipated nucleosome assembly activity of AM-PARP-1, coupled with the fast turnover of the modification, suggests a model in which DNA damage or transcription events trigger transient histone chaperone activity.

Background: Poly(ADP-ribose) polymerase 1 (PARP-1) modulates chromatin structure and is activated upon DNA damage.Results: PARP-1 differentiates between nucleosomes and DNA in its binding affinity and is activated to different degrees.Conclusion: PARP-1 engages different DNA-binding modules with nucleosomes and DNA.Significance: The role of PARP-1 as a chromatin architectural protein and responder in DNA repair is reflected in different binding modes.

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