7-N-(mercaptoalkyl)mitomycins: implications of cyclization for drug function

The Kyowa Hakko Kogyo and Bristol-Myers Squibb companies reported that select mitomycin C(7) aminoethylene disulfides displayed improved pharmacological profiles compared with mitomycin C (1). Mechanisms have been advanced for these mitomycins that differ from 1. Central to many of these hypotheses is the intermediate generation of 7-N-(2-mercaptoethyl)mitomycin C (5). Thiol 5 has been neither isolated nor characterized. Two efficient methods were developed for mitomycin (porfiromycin) C(7)-substituted thiols. In the first method, the thiol was produced by a thiol-mediated disulfide exchange process using an activated mixed mitomycin disulfide. In the second route, the thiol was generated by base-mediated cleavage of a porfiromycin C(7)-substituted thiol ester. We selected four thiols, 7-N-(2-mercaptoethyl)mitomycin C (5), 7-N-(2-mercaptoethyl)porfiromycin (12), 7-N-(2-mercapto-2-methylpropyl)mitomycin C (13), and 7-N-(3-mercaptopropyl)porfiromycin (14), for study. Thiols 5 and 12-14 differed in the composition of the alkyl linker that bridged the thiol with the mitomycin (porfiromycin) C(7) amino substituent. Thiol generation was documented by HPLC and spectroscopic studies and by thiol-trapping experiments. The linker affected the structure of the thiol species and the stability of the thiol. We observed that thiols 5 and 12 existed largely as their cyclic isomers. Evidence is presented that cyclization predominantly occurred at the mitomycin C(7) position. Correspondingly, alkyl linker substitution (13) or extension of the linker to three carbons (14) led to enhanced thiol stability and the predominant formation of the free thiol species. The dominant reaction of thiols 5 and 12-14 or their isomers was dimerization, and we found no evidence that thiol formation led to mitosene production and aziridine ring-opening. These findings indicated that thiol generation was not sufficient for mitomycin ring activation. The potential pharmacological advantages of mitomycin C(7) aminoethylene disulfides compared with 1 is discussed in light of the observed thiol cyclization pathway.     

Simultaneous characterization of the reductive unfolding pathways of RNase B isoforms by top-down mass spectrometry

A novel method for characterization of the simultaneous reductive unfolding pathways of five isoforms of bovine pancreatic ribonuclease B (RNase B) is demonstrated. The results indicate that each isoform unfolds reductively through two three-disulfide-containing structured intermediates before proceeding to the fully reduced form, as in the reductive unfolding pathways of the A variant lacking the carbohydrate chain. The rates of reduction of bovine pancreatic ribonuclease A (RNase A) and RNase B and the formation and consumption of their reductive intermediates are identical, indicating that the unfolding events necessary to expose disulfide bonds for reduction are not affected by the oligosaccharide. The method utilizes top-down mass spectrometry and a naturally occurring tag on the protein, viz. the carbohydrate moiety, to obtain unfolding information of an ensemble of protein isoforms and is a generally applicable methodological advance for conducting folding studies on mixtures of different proteins.     

Formation of new acid sites in dilute oxide solid solutions: A predictive model

A predictive model for the formation of new acid sites in a dilute solid solution of two component oxides is being developed by examining the electrostatic potential at the substituting cation site and the changes in the matrix necessary to balance the stoichiometry. When the prediction was used to compare with experimental results, the satisfactory comparison suggests that the electrostatic potential and the balance of stoichiometry are the two dominant factors in new acid site formation. The implications of this are discussed.     

The ground-state infrared spectra of aluminium monodeuteride: the mass-independent molecular parameters of group IIIa hydrides

The infrared spectra of the monodeuteride of aluminium has been measured in the gas phase in its ground electronic state (¹Σ) using a diode laser spectrometer. These experimental results were combined with those available for AlH from infrared Fourier transform spectra of the ν=2→0 sequence bands in order to calculate a set of mass-independent parameters and mass scaling coefficients. A set of Dunham parameters was also determined from the AlD data. The values of the mass scaling coefficients of the hydrides of four group IIIa metals are discussed.     

Fredholm determinants and the mKdV/Sinh-Gordon hierarchies

For a particular class of integral operatorsK we show that the quantity

General convergence results for stochastic approximations via weak convergence theory

The paper treats general convergence conditions for a class of algorithms for finding the minima of a function f(x) when f(x) is of unknown (or partly unknown) form, and when only noise corrupted observations can be taken. Such problems occur frequently in adaptive processes, and in many applications to statistics and estimation. The algorithms are of the stochastic approximation type. Several forms are dealt with—for estimation in either discrete or continuous time, with and without side constraints, and with or without periodic search renewal. The algorithms can be considered as sequential Monte Carlo methods for systems optimization. The innovations partly concern the method of proof. However, an interesting “constrained” and “renewed” algorithm is also considered. By using ideas from the theory of weak convergence of probability measures, we can get relatively short proofs, under much weaker conditions than heretofore required. For example, the noise can be correlated, and there are fewer restrictions on the step size. Furthermore, the nature of the method permits generalizations to more abstract cases (which occur for example, if we are optimizing a distributed parameter system). The results can be extended in many directions and variations of the technique can be used to get bounds on rates of convergence. Special forms of the method can be applied to many well-known “adaptive” procedures.     

Frequency shifts due to the interference of resolved peaks in magnitude-mode Fourier-transform ion cyclotron resonance mass spectra

We have obtained relationships for frequency shifts resulting from the interference of spectral components for the magnitude mode Fourier transform. The approximation of a weak perturbation of well resolved peaks has been used. Both the low- and high-pressure limits for Fourier-transform ion cyclotron resonance (FTICR) operation have been considered. We have found that the shifts can be either negative or positive, depending on the initial phase and/or the choice of the time-domain interval. The magnitude of shifts generally does not exceed the peak width. In the approximation of small perturbations the shifts produced by multiple peaks are additive. We have compared theoretical results with experimental shifts for isotopic clusters of multiply charged insulin. Up to 1 ppm frequency variations were experimentally observed for the insulin 5+ charge state, consistent with theoretical estimates. The peak interference is of particular significance in the case of bio-molecular mass spectra having a large number of peaks and covering a considerable dynamic range (i.e., relative abundance). We conclude that the common mass measurement procedure based on the location of the magnitude mode maxima of well resolved peaks can result in systematic mass measurement errors. The relationships obtained provide corrections for the frequency shifts and thus improve the mass measurement accuracy.