On electrolysis of NAD+ in aqueous solution at a potential corresponding to the initial one-electron reduction of NAD+ to a free radical, a greenish-yellow color appears which fades when electrolysis is complete. Literature ultraviolet absorption data for the resulting dimer show considerable variation. When the electrolysis is conducted in darkness, the colored product has epsilon 340 of approx. 5700 M-1 . cm-1 and epsilon 259 of approx. 31000 M-1 . cm-1. On ultraviolet and visible illumination, the color disappears, the 340-nm peak decreases and the 259-nm peak increases. On only visible illumination, the color disappears, both peaks increase, the dimer's polarographic oxidation wave decreases and the wave due to 1-substituted nicotinamide reduction increases. The data suggest that the dimer decomposes to NAD+ and 1,4-NADH.
The biologically important redox couple, &mcotinarmde adenine dmucIeot~de/l.4.&d~hydromcotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e-) transfer steps coupled with (a) dimenzation of an intcrmtiate free radical product, (b) protonation-deprotonation of an intermediate product, (c) other chermcal reactions, (d) adsorption of reactant, mtermcdlate and product specxes, and (e) medlatlon by electrode surface species. Cathodic reduction of NAD+ proceeds through two 1 e-steps well separated in potential; protonation of the free radical produced on the first step occurs prior lo the second electron-transfer; a first-order chemical reaction coupled lo the latter may involve rearrangement of an initial diiydro product to l&NADH (and some 1,6-NADH) In the apparently single stage 2 e-anodic oxidation of NADH, the initial step IS an irreversible heterogeneous electron transfer, which proceeds lo at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH+', loses a proton (first order reaction) to form 2 neutral radical, NAD-. which may participate in 2 second heterogeneous electron transfer (ECE mccharusm) or inay react with NADH+* (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD+ * Invited lecture read at the International Symposium on Bioelectrochennstry and Bioenergetics, June 28th-July 3rd 1981, Kuyat Anavim, Israel.
Revised manuscript received December ~2nd rgigThe problems involved in inferring conformation and orientation from electrochemical measurements are considered as are the implications of extrapolating the results for relatively simple nucleotides to biopolymersThe determination of conformation, e-g_. shape in solution -more particularly, when approaching the electrode -largely depends on estimation of the effective molecular cross-section as reflected in the esperimentally measured diffusion coefficient, D ; for example, formation of associated species as in base stacking is usually reflected in a variation in D and, often, in redo-u potential_ The determination of conformation at the solutionlelectrode interface is often intimately connected with the state and orientation of an adsorbed species -more particularly of its electroactive and adsorption sites -relative to the electrode surface_ Current trends in inferring such interfacial conformation for DNA and derived large nucleic acid species are summarized ; the adsorption pattern seen on oxidation of NADH at carbon electrodes is reviewed. * Invited lecture at the 5th International Symposium on Bioelectrochemistry. 3-S September rg6g. \Veimar (D-D-R_)_ 0 Permanent address : EcoIe Xormale Suptrieure de l'Ense@ement Technique. 94230 Cachan. France 00 Permanent address I J_ Heyrovsky Institute of Physical Chemistry and Ekctrochemistry. I IO oo Prague I. CzechosIovakia. o3o"--~g5S/So/or~~-r~5 Q Elsevier Sequoia 126 Bresnahan. Bfoirous. Samec and Elvinghas produced apparently conflictin, = interpretations of the causes of such behavior as related to the conformation of these entities when in solution, when approaching the aqueous solutionlmercury electrode interface, and when at the interface_ While the present authors, who have avoided involving themselves with species more complex than a dinucleotide or a relatively short oligamer, have found the results and interpretations of work on large species published by various grcups e-g_. those at Bmo. Jena and Jiilich. to be of great interest, they do not consider that, at present, they could define and appraise the contemporary situation in respect to the conformation of the nucleic acid polyeIectrolytes in solution and at the interface to the satisfaction of all of the different groups involved_ Consequently, the present paper only indicates some of the viewpoints expressed regarding large nucleic acid species, summarizes some of the principal concepts and =-.:umptions involved in obtaining information on the conformation of s___$er nucleotides in solution and at interfaces, e.g.. in regard to adsorptxon and electron transfer, and notes the resulting rmplications for more cornpIes species
Space limitations force a -rather didactic presentation ; the reader should refer to the originat references for supporting evidence and logical relationship development_No attempt has been made to assign priority for the various findings and explanations reported ; similarly, no attempt at thoroughness of coverage has been attempted-...
ABsTRAm The iIwib&m of an e!eetrode reetion consisting of two successi ve one-electron (I e) transfer ste+s, coupled with. dimcrization of the iatermcdiatc product under coaditioas of steady-state coax&e diffusion, has beea tkoretically aaalyzed for (he situation where the second electron-transfer step is aca~mpaaied or preceded by protoaation of the intermediate product. The resulting relationships have bccausedtocxamia e the ekctrochemical behavior of the biologically important NAD +/NADH redox couple. The model, wlich scezas best to fit the experimental data iavolves: (a) le reduction of NAD + to NA@ which can cfin&ze, followed at more acgative potential by concerted protoaatioa aad Ie redac~oa of Na to NADH, where the proton donor is aa HB species such as H30 + or NH: ; (b) an apparently single-step 2e oxidation of NADH to NAD +. Possible caases for the differences in electrochemical NAD+ reductioa aad NADH oxidation are considered ia tems of .the tiplicatioas of theory. INTRODU~ON There has ken increasing interest in examining the electrdchemical reduction of nicotirkide adenine dinucleotide (NAD+; DPN + ; coenzyme I), and the electm chemical oxidation of 1,4-d.ihydronicotinamide adeke dinuckotide (NADW, DPNH; coenq&eI reduced) from the analytical vievint, and & a possible basis for r&ore ticwotighly elucidating the role of the Ne+/NADe redox:muple in biolo$cal n+ox proksses (cf. ref!. cited & ref. I),-However, a&e is .still uncerta@ty cmic&&ig _th&. exact. @&re df .t@e %mSAGism invol~&, e.g. poSsible &u&S f&r the c&o&c kduction of FGD + ~kv&ing two ~&wlectrkn (ie j, s_tep$ .w& skp*atcd in pot+@:
Adam-The easier electrochemical reduction of uridme (I-/LLLtibofirranosylumcil) in dimethyl sulfoxide as compared to uracil (2, 4dihydroxypyrimidine) by ca. 0.1 V is exphcable on the basii of the electronwithdrawing effect of the ribose group. This effect and possible steric hindrance by the ribose group markedly affect the reaction sequence following the initial one-electron reduction to generate a radical anion, which abstracts a proton from the parent uridine (father-son reaction) to form the neutral uridine free radical and the uridme anion. With increasing &dine concentration, Further reduction and protonstion reactions are favored, resulting in an increase in the effective faradaic n from CLI. 0.5 to 0.8. The availability of only one proton-donating site on uridine, ie, that on N(3). allows explication of the behavior of other hydroxypyrimidiues such as uracil.
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