In addition to hydrolysing RNA, bovine pancreatic ribonuclease splits esters of pyrimidine nucleoside 3'-phosphates, including dinucleotides. For a series of 3':5'-linked dinucleotides of general structure CpN, where N is a 5' linked nucleoside, kcat for the release of N varies enormously with the precise structure of N. Structural studies have been interpreted to indicate that the group N interacts with a subsite, B2, on the enzyme that comprises Gln69, Asn71 and Glu111. We report studies by site-directed mutagenesis that indicate that Gln69 is not involved in productive interactions with any of the dinucleotide substrates and that Asn71 is an important component of subsite B2 for all dinucleotide substrates tested. Glu111 appears to be functionally involved in catalysis for dinucleotide substrates solely when N is guanosine.

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systems (4-9, respectively) and the syn-1-bridge bromo peroxides in the [2.2.1], [3.2.1], and [4.2.1] systems (10)(11)(12). The lifetimes of the radical cations decreased in the order 7+ (no decomposition seen by CV) > 8+ > 4+ » 5+ > 6+, 9+ (no reduction wave seen by CV). Accurate E0' values could be measured only for 7 (2.27 V), 8 (2.35 V), and 4 (2.40 V) because of large AEp values at fast scan rates or low temperatures. ESR spectra with resolved proton hyperfine splittings are reported for 4+, 7+, and 8+.We recently showed that bicyclic tertiary dialkyl peroxides 1 and 2 give long-lived enough radical cations at room temperature to allow observation of reduction waves in their cyclic voltammograms, so that their thermodynamically significant E°' values could be measured with reasonable accuracy.1 2345Di-fert-butyl peroxide (3) cation radical, in contrast, decomposed so rapidly at room temperature that only an irreversible oxidation wave was observed. Although 3+ was longer lived at -78 °C, and both oxidation and reduction waves were observed, the difference in oxidation and reduction peak potentials, , was so large at low temperature that E0' could not be very accurately determined.In this work we report the cyclic voltammetric behavior of nine secondary bicyclic peroxides 4-12, compare their solution oxidation potentials with gas-phase ionization potential data from the literature,2-5 and give ESR data for three secondary peroxide radical cations. Results and DiscussionElectrochemistry. Peroxides oxidize above +2 V vs. SCE, and we observe large background currents in this range in methylene chloride. Following Parker,6 we em-(1)

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Bromierung des aus Cyclopentadien (I) erhältlichen Hydroperoxids (IIa) [daneben (IIb)] gibt das Dibromid (III), das mit AgzO bzw. Ag‐trifluoracetat (Ag‐Tfa) die Produkte (IV) (Hauptprodukt) bzw. (Va) + (Vb) + (VI) (Isomerengei misch) liefert.

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Two complementary syntheses provide an entry to isomeric bicyclic peroxides having saturated medium‐sized rings. The new approach is demonstrated for (1) and (2).