Deuterium exchange rate

The hydrogen-deuterium exchange rates for 1,2-dimethylpyrazolium cation (protons 3 and 5 exchange faster than proton 4 Section 4.04.2.1.7(iii)) have been examined theoretically within the framework of the CNDO/2 approximation (73T3469). The final conclusion is that the relative reactivities of isomeric positions in the pyrazolium series are determined essentially by inductive and hybridization effects. 

Mandelate racemase, another pertinent example, catalyzes the kinetically and thermodynamically unfavorable a-carbon proton abstraction. Bearne and Wolfenden measured deuterium incorporation rates into the a-posi-tion of mandelate and the rate of (i )-mandelate racemi-zation upon incubation at elevated temperatures. From an Arrhenius plot, they obtained a for racemization and deuterium exchange rate was estimated to be around 35 kcal/mol at 25°C under neutral conditions. The magnitude of the latter indicated mandelate racemase achieves the remarkable rate enhancement of 1.7 X 10, and a level of transition state affinity (K x = 2 X 10 M). These investigators also estimated the effective concentrations of the catalytic side chains in the native protein for Lys-166, the effective concentration was 622 M for His-297, they obtained a value 3 X 10 M and for Glu-317, the value was 3 X 10 M. The authors state that their observations are consistent with the idea that general acid-general base catalysis is efficient mode of catalysis when enzyme s structure is optimally complementary with their substrates in the transition-state. See Reference Reaction Catalytic Enhancement... 

N- Methylbenzodihydroazocines (83), (84), and (85) have been studied from the standpoint of possible stabilization of the IOtt- electron anions available by deprotonation (71JA4016). The kinetic acidities, determined by deuterium exchange rates, indicate that (83) and (84) are converted to the anions about 80 and 30 times faster, respectively, than the corresponding 1,2- and 1,4-dihydroquinoline models the enhancement in (85) was slight. By this criterion the benzazocinyl anion (86) was judged to have some degree of aromatic character. 

The kinetic acidity of phosphonium salts was first studied by NMR measurements of the hydrogen-deuterium exchange rate for a series of phosphonium salts565 ... 

Little work has been reported. From studies of proton/deuterium exchange rates, 2-methylpyrrolo[2,l-6]thiazole (478) was estimated to have a pKa of 6.4,394 a value comparable with that of 2-methylindolizine (pKa = 5.9).394 In the same way, the basicity of 3,4-dimethylimidazo-[l,5-a]benzimidazole (468b) (pA = 6.01) resembles that of imidazo-[l,2-a]pyridines (pA = 5.05-5.96).398 It would seem, therefore, that the basicity of azapentalenes parallels that of related indolizine derivatives [Eq. (40)]. 

The hydrogen-deuterium exchange rates of indolizine and some methyl derivatives have been measured at 50 °C in D20/dioxane. It has been shown that the exchange rates reflect the regioselectivity of protonation. Kinetic data have been discussed using HMO and PPP calculations (71T4171). Some pKa values of indolizines and azaindolizines are given in Table 9. 

The reactivity of these five-membered systems with nucleophiles has received note. Zatsepina and Tupitsin and their co-workers recorded the use of deuterium exchange rates in methyl groups of five-membered heterocycles in ethanolic KOEt as a source of o values, with a p value of 4.7.271 Spinelli and his co-workers revealed that even for tri- and tetrasubstituted five-membered heterocycles the Hammett equation employing er values where appropriate gives good correlations. The systems studied are 32-37, in all cases the slow step... 

In solution chemistry too attempts have been made to protonate xenon to XeH+ in the superacid media.150 1038 Evidence for the protonation comes from suppression of proton-deuterium exchange rates of deuterium gas in the presence of xenon in strong acid medium.342... 

Hydrogen-deuterium exchange rates of H2 and in some 3-substituted pyrazine 1 -oxides in Na0D-D20 have been correlated with a constants, and the log of the H2 exchange rates have been shown to be rectilinearly related to the pK values of these compounds (745). A considerable exchange-rate enhancement was caused by replacement of the =C4-H function in a pyridine A-oxide by a =N4 with formation of a pyrazine A-oxide (745). 

Hydrogen-deuterium exchange rates ofH2 in 3-cyanopyrazine 1-oxide (pA 1.12 ... 

Let kunfold he the rate constant for unfolding, kfdd the rate constant for folding, and be the hydrogen/deuterium exchange rate for an amide... 

GpG, d-GpG and d-pGpG Platinum Complexes. Only one complex is obtained for each dinucleotide. The comparative NMR data are collected in Table I. The anti - anti configuration has been previously assigned to these three complexes (23) on the basis of the following results - a - The comparison of the 3 - and 5 -H8 deuterium exchange rates in D2O (pD 10) with those of the... 

A combination of different homo- and heteronuclear 2D NMR techniques at 500 MHz have enabled complete assignment of the H and signals of the two conformers of RA-VII in CDCI3 (major conformer conformer A minor one conformer B) [94]. The structures of the two conformers (A and B) in CDCI3 were elucidated based on temperature effects on NH protons, deuterium exchange rate, vicinal coupling constants and NOE experiments (Fig. 27). These conformational analyses showed that the structure of these conformers is a result of geometrical isomerization and that the predominant conformer A exhibits a typical type II P-turn between Ala and Tyr, similar to the crystal structure as analyzed by X-ray diffraction. The minor conformer B exhibits a type VI... 

Dynamics of Protein Structure from Proton or Deuterium Exchange Rates... 

Freitas, M.A., Hendrickson, C.L., Emmett, M.R., Marshall, A.G. (1999) Gas-phase Bovine Ubiquitin Cation Conformations Resolved by Gas-phase Hydrogen/ Deuterium Exchange Rate and Extent. Int. J. Mass Spectrom. 185/186/187 565-575. 

The kinetics of the protophilic dedeuteriation of thieno[2,3-6]-[2- H]- and -[3- H]-thiophen, thieno[3,2-6]-[2- H]- and -[3- H]-thiophen, and benzo[fc]-[2- H]- and -[3- H]-thiophen have been measured. The activation parameters for the deuterium-exchange reactions of the isomeric thienothiophens with t-butyl alcohol catalysed by potassium t-butoxide were determined. The relative deuterium-exchange rate constants for the 2-position in thiophen, the isomeric thienothiophens, and benzo[/>]thiophen were found to be 1 10 9 4, whereas the corresponding relative values for the 3-positions are 1 94 10 65. These partial rate factors were compared with those obtained upon acidic exchange and interpretation was attempted. ... 

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