The photosystem I reaction center core containing P700 and iron‐sulfur center FX has been isolated from a Synechococcus photosystem I particle with 6.8 M urea at pH 10.0 followed by sucrose density ultracentrifugation. The reaction center core has retained > 90% of FX and 100% of P700 (determined by optical spectroscopy) but is totally devoid of iron‐sulfur centers FA and FB (determined by optical and ESR spectroscopy). SDS‐PAGE indicates the retention of the 57 kDa reaction center polypeptide(s) but the total absence of the 16.4 and 8.1 kDa polypeptides. The loss of FA and FB is further reflected in the decline of acid‐labile sulfide from 11.8 ± 0.4 S2− /P700 in the control particle to 4.6 ± 0.3 S2− /P700 in the reaction center core. This preparation represents the first isolation of an intact reaction center core incorporating the components P700 and FX but totally lacking FA and FB.

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The rates of reaction of the iodides (Me,Si),C(SiMe,C,H,Y) (SiMe,l), 1 (Y = p-OMe, p -M e , H, p-CI, or m-CF,), with (CF,),CHOH and CF,CH,OH (both containing CDCI,), MeOH (containing CCI,), and H, O (mixed with dioxane and CCI,) have been measured. In the reaction with the two fluorinated alcohols the spread of rates is very large, 1, Y = p -O M e being 1.9 x 1 O5 and 6.5 x 1 O4 times, respectively as reactive as 1, Y = rn-CF,, showing that the aryl group provides anchimeric assistance to the leaving of the iodide ion in the rate-determining step, thought t o be formation of the 1,3-aryl-bridged cation II. Unexpectedly, since this process is analogous to an electrophilic aromatic substitution, the substituent effects show excellent correlations w i t h a-constants. In the methanolysis and hydrolysis of 1 such an SN1 process appears to operate alongside an S, 2 process involving direct nucleophilic displacement of iodide b y the solvent, the S, 2 process being facilitated b y electron-withdrawal and the SN1 by electron-release b y Y; estimated values of the rate constants for the SN1 processes show reasonable correlation with o+-constants. Rates of the SN2 reactions of 1 with KSCN in MeCN have also been measured, and the substituent effects, involving an increase in rate b y a factor of only 2.3 o n going from I , Y = p -O M e t o 1, Y = m-CF,, are found t o be consistent with normal through-bond transmission of electronic effects.It was shown previously that in reactions of the iodides (Me,Si),C(SiR,I) with electrophiles, such as Ag' salts, iodine monochloride, or trifluoroacetic acid, rearranged products of the type (Me,Si),C(SiR,Me)(SiMe,X) can be formed, either exclusively or along with unrearranged products (Me,Si),-CSiR,X, apparently via a cation of the type I, Z = Me.' Analogous 1,3-migrations of a vinyl group were observed in the reaction of the iodide (Me,Si),C(SiMe,CH=CH,)(SiEt,I) with Ag' salts, and the much greater ease of reaction of the iodide (Me3Si),C(SiMe,CH=CH,)(SiMe,I) than of (Me,Si),CSiMe,I towards CF,CH,OH or CF,CO,H was attributed to nucleophilic assistance by the vinyl group to the ratedetermining departure of the iodide ion to form the cation I, R = Me,Z = CH=CH,.,The corresponding migration of a phenyl group was subsequently observed in the reaction of (Me,Si),C(SiMe,Ph)-(SiEt,I) with Ag' salts, and attributed to formation of the cation 11, R = Et, Y = H., That observation did not, however, show

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