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Showing papers by "George M. Whitesides published in 1980"


Journal ArticleDOI
TL;DR: The rate of reduction of GSSG to glutathione (GSH) by thiolate anions follows a Br/nsted relation in pK"s of the conjugate thiols (RSH):pnu" - 0.4-0.5.
Abstract: The rate of reduction of oxidized glutathione (GSSG) to glutathione (GSH) by thiolate anions (RS-) follows a Br/nsted relation in pK"s of the conjugate thiols (RSH):pnu" - 0.5. This value is similar to that for reduction of Ellman's re- agent: 0nu" = 0.4-0.5. Analysis of a number of rate and equilibrium data, taken both from this work and from the literature. indicates that rate constants, k, for a range of thiolate-disulfide interchange reactions are correlated well by equations of the formlogk=C*0nu.p Kunu"*0"pK""*0re pKule(nuc=nucleo phile.c=central, andlg=leavinggro upsulfur):eq36-3 8 give representative values of the Br/nsted coefficients. The values of these Br/nsted coefficients are not sharply defined by the available experimental data. although eq 36-38 provide useful kinetic models for rates of thiolate-disulfide interchange reac- tions. The uncertainty in these parameters is such that their detailed mechanistic interpretation is not worthwhile. but their qualitative interpretation-that all three sulfur atoms experience a significant effective negative charge in the transition state, but that the charge is concentrated on the terminal sulfurs-is justified. Equilibrium constants for reduction oiCSSG using a,cr-dithiols have been measured. The reducing potential of the dithiol is strongly influenced by the size of the cyclic disulfide formed on its oxidation: the most strongly reducing dithiols are those which can form six-membered cyclic disulfides. Separate equilibrium constants for thiolateanion-di sulfide interchange (Ks-) and for thiol-disulfide interchange (KsH) have been esti- mated from literature data: Ks- is roughly proportionalto 2ApK. is the difference between the pKus of the two thiols involved in the interchange. The contributions of thiol pKu values to the observed equilibrium constants for reduction of GSSG with a,co-dithiols appear to be much smaller than those ascribable to the influence of structure on intramolecular ring forma- tion. These equilibrium and rate constants are helpful in choosing dithiols for use as antioxidants in solutions containing pro- teins: dithiothreitol (DTT), 1.3-dimercapto-2-propanol (DMP), and 2-mercaptoethanol have especially useful properties. 201 I

376 citations





Journal ArticleDOI
TL;DR: The rates of reduction of several disulfide derivatives of papain(SSR) by DTT were determined and data provide estimates of the Brønsted coefficient for the "central" thiol in thiol-disulfide interchange: these estimates fall in the range beta c congruent to -0.43.
Abstract: Bronsted coefficients have been determined for the rates of thiol-disulfide interchange between low molecular weight thiols and the disulfide groups of four native or modified proteins: DNase (beta nuc congruent to 0.36), lysozyme (beta nuc congruent to 0.55), adenylate kinase(SSCH3)2 (beta nuc congruent to 0.65), and papain(SSCH3) (beta nuc congruent to 0.45). These values are similar to those observed for reductions of oxidized glutathione and Ellman's reagent by a similar set of thiols (beta nuc congruent to 0.5). Glutathione is anomalously slow in reduction of certain protein disulfide groups: although this effect may in part reflect steric hinderance to attack by the relatively large glutathione molecule at disulfides shielded by protein tertiary structure, other (presently undefined) factors appear also to be important, at least in the case of DNase. The rates of reduction of several disulfide derivatives of papain(SSR) by DTT were determined. These data provide estimates of the Bronsted coefficient for the "central" thiol in thiol-disulfide interchange: these estimates fall in the range beta c congruent to -0.25 to -0.43. Rates of reduction of protein disulfide moieties were analyzed by using a Bronsted equation developed previously [Szajewski, R. P., & Whitesides, G. M. (1980) J. Am. Chem. Soc. 102, 2011] to yield pKa values for the participating thiol moieties: in particular, for papain, pKa(Cys-25) = 8.4 at pH 9 and pKa (Cys-25) = 4.1 at pH 6. The thiols of the structurally essential cysteine group of lysozyme seem to have pKa congruent to 11. The advantages and disadvantages of this method for estimating thiol pKa values are discussed.

166 citations


Journal ArticleDOI
TL;DR: The rate of reaction of organic halides with magnesium is proportional to organic halide concentration and magne- sium surface area as mentioned in this paper, and the two most plausible limiting rate-determining steps are halogen atom abstraction and electron transfer.
Abstract: The rate of reaction of organic halides with magnesium is proportional to organic halide concentration and magne- sium surface area. All organic iodides examined and many secondary alkyl bromides appear to react with magnesium at mass- transport or diffusion-controlled rates in diethyl ether. The rates of less reactive organic bromides (neopentyl, phenyl, cyclo- propyl) contain contributions both from mass transport and from chemical reaction. Most organic chlorides react at rates which are not mass transport limited. Analysis of rate-structure profiles indicates that free carbanions and carbonium ions are not intermediates, and that the transition-state structure does not resemble an Sy2 reaction. Insertion of magnesium into the carbon-halogen bond also does not seem to occur, Soluble Mg(1) can be excluded as the only reactive species responsible for disappearance of alkyl halide. The two most plausible limiting rate-determining steps are halogen atom abstraction from the organic halide by the metal and electron transfer to the organic halide from the metal.

77 citations




Journal ArticleDOI
TL;DR: In this paper, the authors present constants for the reduction of the carbon bronrinc bond into a tri-duty-butybutyveto (AlBN) with nragncsium.
Abstract: llarlntctt constants for rcuction of'substiturcd arllbronridcs with nragncsium in scvcrillstllvcnt s lirlltlrl:dicthy lcthcr (p:l.l):di-n-but ylcthcr(r,,= 1.9):-l:lr:r'di- rr-butllethcr crclohcranc(p:1. 0):tctrahvdrol'u ran(p-0):J:lv:vt ctrahl'- drolunrn hcxanrcrirvlphos phonrnride (/r r 0):dicthrlcthcr contuining 0.lJ N{ clclopcntl'lnrrt gnesiunl bronlidc (ir - 0) In thc pollr sglvcnts. rcactlgn uppcars ttt bc nrass trrrnsport lirnitcd. The rrrtc ol'rcrtction ol'lrrll iodidcs with ntagncsiunl in dicthrl cthcr is probably also mass tnrnsport lintitcd (p : 0). Thc ratc of'rcaction ol'chlorobcnzcnc with rnagncsium (pr - 1.0) is ca. l0a slowlcr than that ol'bronrobenzenc in dicthll cthcr. and is not nrass transport lrmitcd. The rcactittn ol'rtrvl bronlidcs with tri-rr-buty'ltin hldride (AlBN. hr,) has p = l.-1. Analysis olthcsc valucs of p narrows thc choicc f or the ratc-dctermining stcp in the rcaction ol'bromobcnzenc with magncsiunr in dicthy'l cthcr to onc of threc limiting possibilitics: clcctron Lransfer from thc nrctal tp thc a11,l brontidc (3). ury'l radical tirrnration b),bronrinc abstraction from thc aryl brornidc by thc mctal (6). or. lcss probabll. inscrtion olu nugncsiunr atonr into thc carbon bronrinc bond (7).

43 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that cyclopentyl bromide is de- pendent on the first power of the organic halide concentration, the magnesium surface area, and the rate at which the solution is stirred, and is inversely dependent on the solution viscosity.
Abstract: Three observations indicate that the rate of reaction of cyclopentyl bromide wjith metallic magnesium in several ethe- real solvents is limited by the rate of its encounter with the metal. First, the rate of disappearance of cyclopentyl bromide is de- pendent on the first power of the organic halide concentration, the magnesium surface area, and rate at which the solution is stirred, and is inversely dependent on the solution viscosity (-d(RBr)/d?a (RBr)', A', @I, 7-l). Second, the activation energy for the reaction in diethyl ether (E, N 2.3 kcal/mol) is consistent with mixing and diffusion as rate limiting. Third, a number of representative alkyl halides react with magnesium in diethyl ether at very similar rates; no organic halides examined react faster than this rate. The dependence of the rate of reaction of neopentyl bromide on solution dielectric constant illustrates transition from transport-limited to non-transport-limited rates, and suggests appreciable charge separation in the chemical step which breaks the carbon-bromine bond.

40 citations



Journal ArticleDOI
TL;DR: Ratc-structurc prof ilcs arc comparcd for four RCactions: reductittn of alkyl chlorides and brornides with tri-nbutyltin hydridc (Et:O.l'icicnt r ) 0.ttO) as mentioned in this paper.
Abstract: Ratc-structurc prof ilcs arc comparcd for four rcactions: reductittn of alkyl chlorides and brornidcs with tri-n-butyltin hydridc (Et:O. -15 oC and 0 oC-. rcspcciivcll). rcducrion oi alky'l brornides with lithrum'1,4'-din rcthylbenzopheno nc tiHf.,24.a), and rcacrisn ol'alkllchloridc iwith mctallic nragncsiunr (t:t:O,0'C').All iour ratc structure profiles are lin- carly'currclatcd (corrclation cocl.l'icicnt r ) 0.ttO). Thc sinrilarity of thcse profilcs suggcsts that the carbon'halogcn bond is signil'ic1ntl1 brokcn in the tnrnsition statc lor each typc of'rcaction. (:lcctrochcrnica l rcduction potcntials of a linlitcd nunlbcr ol:alkyl hllidcs alsocorrclate lincarll'with thcsc ratcs. The sinrilarit;'olth c ratc structurc prof ile l'or reaction of alky'l halidcs at a ntagncsiunr surfacc to thosc of hornogeneous rcactions supports thc inl'crcncc that rcaction at thc nragnesium surface is not complicatcd b1'adsorption or othcr hctcrogeneous phenonlena.

Journal ArticleDOI
TL;DR: The acceptance of oxoreductases as catalysts in organic synthesis has been slowed by the expense of the nicotinamide cofactors required by many of these enzymes, so it is necessary to develop methods for producing them and for stabi l izing them during use.
Abstract: The acceptance of oxoreductases as catalysts in organic synthesis2 has been slowed by the expense of the nicotinamide cofactors required by many of these enzymes. Effective procedures for nicotinamide cofactor recycling have decreased the effective cost of these substances by al lowing them to be regenerated in situ.3 The nicotinamide cofactors are, however, intr insical ly unstab le in so lu t ion,4 and the economic advantage to be ga ined by any recycl ing scheme is l imited. I t is thus also necessary to develop methods for producing them lcss expcnsivelr s and for stabi l izing them during use. Herc we reporr a combined cel lfrec enzymat ic and chemica l sy 'n thcs is o i \ , \D s tar t ing l ronr readi l r ava i lab le AMP (Scheme I ) . Th is sy ,n thes is i a srep roward rh t : deve loprnent o f a pract ica l nonfermentat ion route to NAD and NADP.6 It also i l lustrates the ut i l i t l , of enzymaric methods for the synthesis of useful quanti t ies of complex substances and provides a f lexible route to derivatives ol ' NAD. The key in termediate in th is synthes is , n icot inamide mononuc leot ide (NMN), was prepared f rom AMP in three s teps. t Ribose -5-phosphate (r--5-P) was obtained b1 acid-catalvzed hvdrolysis of AMP.8 Treatment of r-5-P *i th anhydrous ammoniae in dry ethylene glycol provided a solut ion of r ibos. 'lamine 5phosphate (rA-5-P). This substance was nor isolated. bur u,as condensed with Nt-(2,4-dinitrophenyl )3-carbamoylpvridin ru m chloride (NDC)'0 to afford NMN in 25oto yield based on r-,5-P, The NMN (a lso not iso la ted) was coupled wi th ATPIr in a s tep catalyzed by NAD pyrophosphorylase12 (EC 2.1.7.1) immobil ized in PAN gel.r3 This enzymatic coupling is an equilibrium reacrion and was driven to completion by hydrolyzing the pyrophosphate formed by us ing pyrophosphatase (EC 3.6.1 .1) in PAN. t r The yield of NAD was 90-97olo based on NMN.