Methylene blue (MB) plus light, in the presence of oxygen, mediates formation of 8-hydroxyguanine in DNA. The yield of 8-hydroxyguanine may be as much as from 2 to 4% of the guanines present. The results presented here show that treatment of supercoiled plasmid DNA with methylene blue plus light causes single-stranded nicks. However, single-stranded nicking occurs approximately 17-fold less frequently than does formation of 8-hydroxyguanine. The nicking rate is reduced in the presence of Mg ion but is not prevented by inhibitors of the iron-catalyzed Fenton reaction or by scavengers of hydroxyl free radicals. Extensive exposure of DNA to light in the presence of MB produces no detectable thiobarbital reactive material thus implicating that single strand nicking does not occur by hydroxyl free radical attack on deoxyribose. Formation of 8-hydroxyguanine is apparently not dependent upon intercalative binding of MB to DNA, since it is formed in polydeoxyguanylic acid.

The pyrolysis of trimethyl gallium has been studied in a toluene carrier flow system from 686" K to 983' K using total pressures from 6.1 mm to 31.1 mm. The progress of the reaction was followed by measuring the amount of methane, ethane, and ethylene formed. The rate constants decline rapidly if the toluene-to-alkyl ratio is decreased below 45. Part of this decrease has been shown to result from the loss of methyl radicals as ethylbenzene, propane, propylene, and xylenes. In an unseasoned vessel, the decomposition is markedly dependent on surface conditions. In the unpacked conditioned vessel, the reaction is 94% homogeneous in the first bond region and 97% homogeneous in the second bond region.The decomposition occurs in two steps:

€544. A Convenient Solid for Calibration of the Gouy MagneticSusceptibility Apparatus. By B. N. FIGGIS and R. S. NYHOLM. THE measurement of magnetic susceptibilities by the Gouy method is a relative one, the apparatus being calibrated in terms of a substance of known susceptibility, for which water, nickel chloride solution, and powdered cupric sulphate pentahydrate or ferrous ammonium sulphate hexahydrate have been used. The low suceptibility of water is often inconvenient if small tubes are being calibrated. Nickel chloride solution requires accurate analysis before use and ferrous ammonium sulphate is often of questionable purity. This substance and copper sulphate do not pack well and several different values for the susceptibilities of both solids have been reported.The required properties for a calibrant are: (1) Readily available pure;(2) an accurately known and moderate susceptibility (xg = ;(3) stability in moist air;(4) xg must vary in a known and simple way, at least at room temperature; (5) easily and reproducibly packable into the Gouy tube. The complex mercury tetrathiocyanatocobaltate HgCo(CNS), offers some advantages and its susceptibility has therefore been accurately determined. at 20' 1 being used as reference, the gram susceptibility of the complex is 16.44 (-&O.OS) x at 20".As reported elsewhere,l it obeys the Curie-Weiss law, xg cc (T + lO)-l where T is expressed

The pyrolysis of trimethylindium has bcen studied ill a toluene carrier flow sysie!n from 550' I< to '781' I< using total pressures from 6.0 to 33.5 mln. The progress of the reaction was follo\\~ed by ~lleasuri~lg thc ainount of methane, ethane, ethylene, propane, and ethylbenzene formed and in a number of cases by direct i~idium analysis. Thc cleconiposition occurs in three stcps:At all temperatures reaction [2] follows rapidly after reaction [I]. Reaction [3] occurs a t a measurable rate only a t tenlperat~~res sufficiently high that reactions [I] and [2] arc coillplctcd in a very sn~all fraction of the contact time. At lowcr temperatures the InCI-13 produccd deposits in the reaction zone as a white hIn1 of (InCH3),,.Both k1 and k3 declinc rapidly if the toluene-to-alkyl ratio is clecreascd below 150. In ail ~~nseasonecl vessel, the decomposition is mar1;edly dependent on surface conclitions. In the unpacbed conditioned vessel thc reaction is a t least 8'3% honlogeneous in the lirst bo~ltl regioil and 07% l~oinogeneous in the third bond region. Both k, and k J depcnd on the total pressure in the systein, the depencle~lce of hl being vcry slight. . ISTRODUCTIONNo previous lcinetic studies of the thermal decomposition of trimethylindiuin have been reported.A study of the reaction of triisobutylindium with 1-decene a t 150° C for 100 hours (1) yielded tridecylindiunl and 83y0 isobutylene. The tridecylindium brolte down further t o yield only 64y0 free indiuin inetal plus 1/2 CI-I2=CI-I-C81-117 and 3/2 CH,-CHs-C8I-I17. The reinailling indiuin was assumed to be in sonle polyineric form.A lcinetic study of the therinal deconlposition of trimethylgalliuu~ has been reported (2). T h e rate constants obtained were extreinely sensitive to the nature of the surface in the reaction zone. After a series of runs, the reaction zone became conditioned and reproducible results were obtained. T h e experilnental results showed D l = 59.5 lccal/mole and D 2 = 35.4 ltcal/mole. From a lcnowledge of the total bond energies, D3 = 77.5 kcal/mole was calculated.The thermal decomposition of trimethylindium is expected to be similar to that of triinethylgalliuln and the other neighboring methyl metallic alltyls that have been studied (3,4,5). The mean metal-carbon bond dissociation energy in trimethylindium has not been reported.

The thermal decomposition of diethylzinc in a toluene carrier system has been studied over the temperature range of 669 to 762 K , and percent decomposition of 3.8 to 97.2%. The decomposition occurs by a two step mechanism, the second step rapidly following the first, The overall progress of the reaction was followed by determination of the residual diethylzinc by hydrolysis followed by analysis for zinc by atomic absorption. In selected runs two additional methods were used: (i) measurement and analysis of the gas evolved during the hydrolysis and (ii) full analysis of all pyrolysis products involving the C2Hs group.In the absence of zinc oxide the decomposition is essentially homogeneous and log kl (s-1) = 14.3 + 0.6 -49 000 * 2 000 4.58Tor -205 000/19.15T if the activation energyi s expressed inj oules rather than calories. The presence of zinc oxide causes marked catalysis of the overall decomposition but does not appear to affect reaction 3 or reaction 6.[61 CzHs + C2H4 + H 9 800 + 500 log k3 (cc mol-1 s-1) = 11.12 + 0.15 -4.58T 30 300 (at 15 mm pressure) log k6 (s-I) = 11.0 -- 4.58TReaction 6 is in a strongly pressure dependent region and is based on very limited data.AHTI A. KOSKI, S. JAMES W. PRICE et BARRY C. TRUDELL. Can. J. Chem. 54, 482 (1976). On a CtudiC la dCcomposition thermique du dikthylzinc dans un systkme contenant du tolukne comme diluant ; ces Ctudes ont Ct C rCalisCes entre 669 et 762 K et amenCes des pourcentages de dCcomposition de 3.8 a 97.2%. La dCcompositio11 se produit par un mkanisme en deux Ctapes; la deuxikme Ctape suit rapidement la premiere On a suivi le progrks globale de la reaction en determinant le dikthylzinc rCsiduel par une hydrolyse suivie par une analyse du zinc par absorption atomique. Dans un certain nombre de cas spicifiques, on a utilisC deux methodes supplCmentaires: (i) la mesure et l'analyse du gas qui s'Cchappe au moment de l'hydrolyse et (ii) l'analyse compl'ete de tous les produits de pyrolyse impliquant le groupe C2H5.