Stop-Flow Experiments

Two-dimensional H NMR experiments acquired in stop-flow mode represent to some extent a link between on-flow LC-NMR screening and the detailed structural elucidation of isolated compounds employing two-dimensional NMR techniques in conventional off-line probe-heads. As the LC-NMR-MS hyphenation offers the possibility of triggering the stopping of the LC pump by the MS signal, this technique is particularly well suited for the reliable detection of compounds showing only weak UV absorbances. 

MS triggering of stop-flow spectra even provides an additional feature - it is now possible to monitor the peak purity as a triggering criterion, instead of just the peak maximum in conventional UV-triggered experiments. Thus, in the case of partial co-elution, the stopping of the LC pump can be delayed until a peak elutes without major impurities. 

Such MS-triggered stop-flow experiments are preferably performed in the second LC-NMR-MS run, required for the determination of the number of exchangeable protons by D-H back-exchange. 

The complementary structural information of both NMR spectroscopy and MS detection is particularly valuable for the analysis of closely related glycosidic natural products, where compounds of the same molecular mass (isobars, see Table 5.1.1), and even with identical MS/MS fragmentation patterns, frequently occur, as this example demonstrates. 

Modern ion-trap spectrometers are capable of performing MS experiments which yield unique information on the molecular weight of individual sugar units in the oligosaccharide chain of saponins. While fragmentations up to MS5 could be performed in off-line mode, MS/MS spectra were recorded in on-flow LC-NMR-MS runs by default (auto-MS/MS mode). 

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Basically the kinetic results are consistent with the first (rapid) reaction being the addition of a hydroxide ion to the diazonium ion followed by the very fast deprotonation of the (Z)-diazohydroxide to give the (Z)-diazoate (steps 1 and 2 in Scheme 5-14). In addition, however, the stopped-flow experiments showed that the diazonium ion also reacts with the water molecule, initially forming the conjugate acid of the (Z)-diazohydroxide (ArN2OH2), which is then very rapidly deprotonated (reaction 1 in Scheme 5-14). The rate of the relatively slow (Z/E)-isomerization (reaction 5 in Scheme 5-14) can in general be measured by conventional spectrophotometry. 

Spectra which are better resolved (useful for example for the exact determination of coupling constants) can be obtained by carrying out stopped-flow experiments. Here we stop the chromatographic separation after 3 and 7.5 min, optimize the homogeneity (by shimming the magnet) and carry out the desired NMR experiments. 

Fig. 35 Proton spectra obtained from the stopped-flow experiment. Above acetal 4. Below acetal 5. In each case 16 scans, relaxation delay 1 sec...

Fig. 36 COSY spectrum of acetal 5 obtained in a stopped-flow experiment. Measurement time 11 min...

The burst phase amplitude represents the percentage change in the far-UV CD signal (216-225 nm) occurring in the dead time of the stopped-flow experiment relative to the total change on folding. 

Fig. 42. The unfolded baseline and the Cyt c burst phase. The solid curves show the equilibrium behavior of Cyt c. The equilibrium fluorescence and CD of the (unfolded) fragments (A and <>) match the unfolded holo Cyt c baseline at high GdmCl and define the continuation of the unfolded baseline to lower GdmCl concentrations. The horizontal dashed line shows the initial fluorescence and CD in the stopped-flow experiments (4.3 M GdmCl). The solid symbols indicate the fluorescence (A) and the ellipticity at 222 nm (B) reached by holo Cyt c in the burst phase on dilution into lower (or higher) GdmCl, as suggested by the arrows (starting from either pH 2 ( ) or pH 4.9 ( )). These comparisons are made on an absolute, per-molecule basis. Forster-averaged distance (Trp-59 to heme) is at the right of A. (From Sosnick et al., 1997, with permission. 1997, National Academy of Sciences, USA.)...

Try to imagine (e.g., from optically monitored stopped-flow experiments) what you can expect to get out of a rapid-freeze experiment, and preevaluate its biological relevance. 

Stopped-flow experiments of luminol chemiluminescence in the system luminol/pure DMSO/tert.butylate/oxygen 109> with independent variations of the concentrations of reactants confirmed the results obtained previously by E. H. White and coworkers 117> as to pseudo-first-order dependence of the chemiluminescence intensity upon each of the reactants. Moreover, the shapes of the decay curves obtained... 

To circumvent complex mathematical evaluations, stopped flow experiments are measured preferentially under pseudo first-order conditions. The rate of complex formation between protein A and protein or ligand B is described by... 

As described for stopped flow experiments above, all commercially available SPR systems work under (pseudo) first-order conditions as well. This is realized either by a large excess of free ligand (in the large volume of the cuvette) compared with a nanoliter volume of the sensor layer [156] or by continuous replacement of free ligand in a flow injection system (e.g.,BIAcore [157]). 

The time resolution of stopped flow experiments is typically 1-2 ms,21 and is determined by the time required to mix the solutions, flow the mixed solution to the detection chamber, and stop the flow. Smaller detection cells can be used to decrease the time resolution at the expense of the signal-to-noise ratio of the detected signals. Various kinetic traces have to be averaged to achieve good kinetic profiles and sample volumes of milliliters with concentrations of micromolar to millimolar are required. 

The non-linear dependence of the relaxation process on the DNA concentration was also observed in stopped-flow experiments and the same mechanism, i.e. fast pre-equilibrium followed by a slow intercalation step, was proposed." This latter study did not report values for the individual rate constants. The mechanism proposed in Scheme 4 was employed in subsequent studies despite the criticism on the accuracy for the data related to the fast kinetic component (see below). The original temperature jump study also showed that the relaxation kinetics depend on the structure of the DNA.117 The slower intercalation rate for 5 with T2 Bacteriophage DNA when compared to ct-DNA was ascribed to the glucosylation of the former DNA (Table 3). 

The dissociation of 7 (Scheme 5) from poly[(G-C)] showed three relaxation times and the amplitude corresponded to the total signal, while the dissociation of 7 from poly[(A-T)] was faster and only two relaxation times, corresponding to 70% of the total signal were observed in the stopped-flow experiment. The biological activity of this class of molecules was correlated to the presence of four relaxation times when the dissociation process is measured with DNA, in particular the presence of the long-lived component of hundreds of milliseconds.86,104 105,132 143 The difference in the dissociation kinetics observed with the two polydeoxynucleotides indicates that intercalation into G-C sites is responsible for the biological activity. The dissociation of 7 from ct-DNA led to four relaxation times, a result that is in line with the relaxation times observed with poly[(G-C)] and poly[(A-T)]. 

CDs can form complexes with stoichiometries different from 1 1. Stopped-flow experiments were employed to study the binding dynamics of a 2 2 complex between pyrene and y-CD.196 Both, 1 1 and 2 2 complexes are formed and the 2 2 complex exhibits excimer-like emission. The association rate constant for the 2 2 complex was found to be 6 x 107M-1 s-1, while the dissociation rate constant was 73 s-1. These values correspond to a decrease of up to 5 orders of magnitude when compared to the dynamics for the 1 1 complex. 

Stopped-flow experiments, 171-172 cyclodextrins (CD), binding dynamics of guests binding to, 205, 207, 208 DNA, binding dynamics of guests binding to, 187, 189-191, 194, 195-196... 

Addition of NaN02 (50 pM) had no effect on the reaction profile with NO present, and no reaction was observed (on the time scale of the stopped-flow experiment) when NO was absent. However, at higher concentrations, anions, including the conjugate bases of various buffers (B ), slowed down the reaction. This was attributed to the competition between water and the anions for the labile 5th coordination site of Cu(dmp)2(H20)2+. 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

A further consequence of the complexing of Pn+ by the monomer is that there may be a difference between the ionic population in stopped-flow experiments (and others) in which m is very small and in normal kinetic experiments with m in the range 0.1-2 mold"1. Therefore the p+ obtained from the two types of experiment need not necessarily be the same see for example Leon et al. (1980). 

Various groups of workers have attempted to determine the k, by the use of common-ion salts so as to repress the dissociation of the ion-pairs at the growing end. In this context the two types of initiator pose different, but related, problems. If the initiator is a protonic acid AH, as in most of the stopped-flow experiments, the greatest part of the AH does not participate in the initiation (see Section 4.3.2). In these systems, therefore, conjugate ions A2H can be formed from the added salt Bs+A" and therefore the effective [A ] < [Bs+A ]0. In the usual plots [Equation (23) below] of kp (defined in Section 2.5.3) versus [Bs+A ]01/2 this uncertainty will not affect the intercept k, at [Bs+A ]0 1/2 = 0, but it will make the slope, from which the kp can be found, smaller, so that the resulting kp will be an underestimate. 

The common finding, discussed in the previous section, that when cationic polymerisations are initiated with protonic acids, HA, as in the stopped-flow experiments, the [Pn ] [HA]0 can be explained along the same lines, as will be shown in Section 4.3.2. We have here a useful discriminatory test, because in the pseudocationic polymerisations initiated by HA (with or without a modifier, such as an organic sulphide), the concentration of growing ester is equal to [HA]0 (Plesch, 1992). 

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