carbon data, R = (1.042 ±0.020) -[1.04 ±0.91 (GeV/c 2 )~4]Q w 4 ,giving A + = <*> and A_ = 0.99 GeV/c 2 ; and A + > 1.06 and A_ >0.77 GeV/c 2 at the 95% confidence level.In conclusion, the results of this experiment are in agreement with the predictions of quantum electrodynamics as described by Bjorken, Drell, and Frautschi for the photoproduction of electron pairs.We would like to thank M. D. Rousseau for his invaluable contribution in the early stages of the experiment, and M. W. Collins and A. E. Groome for their excellent support throughout. We greatly appreciate the efforts of A. J. Egginton and the NINA machine group, and the help given by the technical services group under M. J. Moore. Finally, we acknowledge the encouragement and support of Professor A.Inelastic electron-proton scattering at high four-momentum transfer and large electron-energy loss has been used to investigate the electromagnetic structure and interactions of the proton. 1 We have measured the double differential cross section d 2 o{E,E', 6)/dQ,dE' for electrons on hydrogen in a new kinematic region made accessible by the Stanford linear accelerator. We report measurements made at 6° and 10° for several incident energies E, and for a range of scat-dan, and S. C. C. Ting, Phys. Rev. JL61, 1344(1967.

Results of electron-proton inelastic scattering at 6° and 10° are discussed, and values of the structure function W 2 are estimated. If the interaction is dominated by transverse virtual photons, vW 2 can be expressed as a function of w = 2Mv/q 2 within experimental errors for q 2 >1 (GeV/c) 2 and CJ >4, where v is the invariant energy transfer and q 2 is the invariant momentum transfer of the electron. Various theoretical models and sum rules are briefly discussed.

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Microtubules are significant therapeutic targets for the treatment of cancer, where suppression of microtubule dynamicity by drugs such as paclitaxel forms the basis of clinical efficacy. Peloruside A, a macrolide isolated from New Zealand marine sponge Mycale hentscheli, is a microtubule-stabilizing agent that synergizes with taxoid drugs through a unique site and is an attractive lead compound in the development of combination therapies. We report here unique allosteric properties of microtubule stabilization via peloruside A and present a structural model of the peloruside-binding site. Using a strategy involving comparative hydrogen-deuterium exchange mass spectrometry of different microtubule-stabilizing agents, we suggest that taxoid-site ligands epothilone A and docetaxel stabilize microtubules primarily through improved longitudinal interactions centered on the interdimer interface, with no observable contributions from lateral interactions between protofilaments. The mode by which peloruside A achieves microtubule stabilization also involves the interdimer interface, but includes contributions from the alpha/beta-tubulin intradimer interface and protofilament contacts, both in the form of destabilizations. Using data-directed molecular docking simulations, we propose that peloruside A binds within a pocket on the exterior of beta-tubulin at a previously unknown ligand site, rather than on alpha-tubulin as suggested in earlier studies.

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We have extended our earlier measurements of parity violating asymmetries in the inelastic scattering of longitudinally polarized * Work supported by the Department EY-76-C-03-0515.

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