Showing papers in "Journal of the American Chemical Society in 1972"

Protection of hydroxyl groups as tert-butyldimethylsilyl derivatives

Abstract: There is a need for development of chemical agents to protect hydroxyl groups. The agents developed must combine stability under varying circumstances with susceptibility to easy removal by a specific agent. Laboratory derivation of such a substance--dimethyl-tert-butylsilyl--is described and the structure of the chemical is diagrammed. The ethers of this substance are stable in water or alcohol bases under normal conditions and are also stable to hydrogenolysis and mild chemical reduction. This group of ethers should prove useful in a wide variety of hyroxyl-protecting applications. The group is becoming more and more useful in the synthesis of PGs (prostaglandins).

Calculation of ground and excited state potential surfaces of conjugated molecules. I. Formulation and parametrization

Abstract: A formulation is developed for the consistent calculation of ground and excited state potential surfaces of conjugated molecules. The method is based on the formal separation of u and 7r electrons, the former being represented by an empirical potential function and the latter by a semiempirical model of the Pariser-Parr-Pople type corrected for nearest-neighbor orbital overlap. A single parameter set is used to represent all of the molecular properties considered; these include atomization energies, electronic excitation energies, ionization potentials, and the equilibrium geometries and vibrational frequencies of the ground and excited electronic states, and take account of all bond length and bond angle variations. To permit rapid determination of the potential surfaces, the u potential function and SCF-MO-CI energy of the r electrons are expressed as analytic functions of the molecular coordinates from which the first and second derivatives can be obtained. Illustrative applications to 1,3butadiene, 1,3,5-hexatriene, a,w-diphenyloctatetraene, and 1,3-cyclohexadiene are given. detailed interpretation of electronic transitions and A concomitant photochemical processes in conjugated molecules requires a knowledge of the ground and excited state potential surfaces. The determination of such surfaces has long been a goal of theoretical chemistry. Difficulties in a reliable a priori approach to the problem for a system as simple as ethylene2 are such that calculations for more complicated molecules are prohibitive at present. Consequently, a variety of methods that utilize experimental data have been introduced. Completely empirical treatments, in which the energy surface is expressed as a function of potential parameters fitted to the available information (1) Supported in part by Grant EY00062 from the National Institute of Health. (2) U. Kaldor and I. Shavitt, J . Chem. Phys., 48, 191 (1968); R. J. Buenker, S. D. Peyerimhoff, and W. E. Kammer, ibid., 55, 814 (1971). (equilibrium geometry, vibrational frequencies, etc.), have had considerable success in applications to molecules for which a localized electron description is app l i~ab le .~ The great advantage of this type of approach, which leaves open questions of reliability when extended from one class of molecules to another, is the ease and speed of the calculations; this had made possible applications to systems as large as certain nucleic acids and globular proteins. For conjugated molecules, however, the importance of delocalization introduces difficulties into such an empirical treatmenL5 (3) (a) See, for example, J. E. Williams, P. J . Stand, and P. v. R. Schleyer, Annu. Reu. Phys. Chem., 19, 531 (1969); (b) S. Lifson and A. Warshel, J . Chem. Phys., 49, 5116 (1968); A. Warshel and S . Lifson, ibid., 53, 8582 (1970). (4) M. Levitt and S. Lifson, J. Mol. B i d , 46, 269 (1969); M. Levitt, Nature (London), 224, 759 (1969). ( 5 ) C. Tric, J . Chem. Phys., 5 1 , 4778 (1969). Journal of the American Chemical Society 1 94:16 1 August 9, 1972

Electrogenerated chemiluminescence. IX. Electrochemistry and emission from systems containing tris(2,2'-bipyridine)ruthenium(II) dichloride

Abstract: Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Electrogenerated chemiluminescence. IX. Electrochemistry and emission from systems containing tris(2,2'-bipyridine)ruthenium(II) dichloride Nurhan E. Tokel, and Allen J. Bard J. Am. Chem. Soc., 1972, 94 (8), 2862-2863• DOI: 10.1021/ja00763a056 • Publication Date (Web): 01 May 2002 Downloaded from http://pubs.acs.org on February 19, 2009

Lateral diffusion in spin-labeled phosphatidylcholine multilayers.

Abstract: The paramagnetic resonance spectrum of a highly concentrated region of spin-labeled phosphatidylcholine included in oriented bilayers of phosphatidylcholine (PC) changes dramatically in time. This time dependence of the spectra is due to the lateral diffusion of the oriented labeled molecules spreading in the planes of the corresponding monolayers. The resonance spectra can be analyzed in terms of a time-dependent superposition of spectra corresponding to the different concentrations of spin label. It is possible to estimate the diffusion constant: D ‘v 1.8 f 0.6 x cm2/sec at room temperature (25”) . If lateral diffusion is assumed to be due to pairwise exchange of neighboring molecules, then this diffusion coefficient corresponds to an exchange frequency which is of the order of lo7 sec-l. This rate is high enough to suggest that the lateral translation of molecules bound to membranes may sometimes have biological significance. he membrane of a biological cell is involved in many T essential processes that require molecular motion. For example, the active transport of ions and molecules through membranes doubtless requires the molecular motion of membrane components. There is much current interest in the question of the rate of lateral difSusion of molecules in membranes. This question arises in connection with membrane biosynthesis and, as discussed later, the possible lateral diffusion of “messenger” molecules in membranes. Since virtually all biological membranes contain lipids and many biological membranes are thought to cmtain lipid bilayers, we have carried out a study of the rate of lateral diffusion of a spin-labeled lipid in a phospholipid bilayer system. It is very difficult to make an a priori estimate of the rate of lateral diffusion of lipids in phospholipid bilayers. Much evidence indicates that in, for example, egg lecithin bilayers the hydrocarbon chains are in a highly “fluid” state. That is, chain isomerizations take place rapidly (2 107 sec-’) and with relatively high probability. However, this high degree of molecular motion is most pronounced toward the terminal methyl groups of the fatty acid chains, 3 , 4 whereas near the polar head groups the hydrocarbon chains appear to be relatively more rigid and tightly packed. The rate of inside-outside transitions of spin-labeled phospholipids through bilayers is remarkably of the order of -(24 hr)-l. Thus, the bilayer appears to have the combined properties of a fluid and a rigid structure. This makes it difficult to give any plausible estimate of how rapid lateral diffusion might be. The first attempt to measure the rate of lateral diffusion in phos(1) (a) Supported by the National Science Foundation under Grant No. GP 26456. (b) NATO Postdoctoral Fellow, on leave from Laboratoire de Physique des Solides, Faculti des Science de Paris. This work has benefited from facilities made available by the Advanced Research Projects Agency through the Center for Materials Research at Stanford University. (2) For a discussion of this subject and references to earlier work, see W. L. Hubbell and H. M. McConnell, J . Amer. Chem. Soc., 93,314 (1971). (3) B. G. McFarland and H. M. McConnell, Proc. Nut. Acud. Sci. U. S., 68, 1274 (1971). (4) P. Jost, L. J. Libertini, V. G. Hebert, and 0. H. Griffith, J. Mol. B i d , 59, 77 (1971). See also J. Seelig, J . Amer. Chem. Soc., 92, 3881 (1970). This increase in molecular motion toward the terminal methyl groups applies also to intact biological membranes. See W. L. Hubbell and H. M. McConnell, Proc. Nut. Acud. Sci. U . S., 64,20 (1969). (5) R. D. Kornberg and H. M. McConnell, Biochemistry, 10, 1111 (1971). pholipid bilayers was made by Kornberg and McConne1L6 These investigators studied the N-methyl proton line broadening in phosphatidylcholine vesicles brought about by low concentrations of the spin label PC I.