engleski

Kinetics of H/D Exchange by Mass Spectrometry. I. Analysis of a Single Exchange Profile

An exchange profile is a sequence of fragment fractions with 0, 1, ..., n deuterons introduced by hydrogen/deuterium exchange into a given molecular fragment. Derivation of the site exchange probabilities p1, p2, ..., pn, responsible for the experimentally observed exchange profile, is analyzed. A relation is established between p1, p2, ..., pn and the zeros of the polynomial whose coefficients are equal to the fractions in the exchange profile. When the zeros are complex, the probabilities are usually determined by the least squares fitting or by the maximum likelihood estimation. It is proved that in such cases the calculated probabilities can not be all distinct. Analytical expressions for the gradient and the Hessian of a sum of squared deviations between theoretical and experimental values are given. A case with only two exchanging sites (n=2) is examined in detail. Numerical simulations were performed to estimate the incidence of complex polynomial zeros for n=3-10 with varied number of detected fragment ions N=1000-1000000.

H/D exchange ; mass spectrometry ; exchange profile

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