For the first time, the molecular weight distribution and extent of side reactions have been quantified in polyurethane prepolymers. A combination of NMR, size exclusion chromatography, and mass spectrometry has been used to measure the molecular weight distribution. The values obtained are consistent with Flory's step polymerization theory. The type and amount of side reactions as a function of reaction temperature have also been investigated. At low temperature of reaction, the amount of branches arising from allophanate linkages is negligible. With only a relatively modest change in reaction temperature, this type of side reaction increases dramatically, so that at 145 °C as much as 10% of the nitrogens participate in allophanate linkages. The effect of this reaction on the molecular weight distribution was also measured. Upon formation of these side products, the molecular weight distribution broadens and the average increases. In this prepolymer system, the reactivity is independent of molecular weight.

UV resonance Raman spectroscopy was used to detect and estimate the frequency of the unfavored imino tautomer of the transition mutagen 5-hydroxy-2-deoxycytidine (HO 5 dCyt) in its anionic form. In DNA, this 2-deoxycytidine analog arises from the oxidation of 2-deoxycytidine and induces C 3 T transitions with 10 2 greater frequency than such spontaneous transitions. An imino tautomer marker carbonyl band (Ϸ1650 cm ؊1 ) is enhanced at Ϸ65°C against an otherwise stable spectrum of bands associated with the favored amino tautomer. This band is similarly present in the UV resonance Raman spectra of the imino cytidine analogs N 3 -methylcytidine at high pH and N 4 -methoxy-2-deoxycytidine at pH 7 and displays features attributable to the imino form of C residues and their derivatives. The fact that the imino tautomer of HO 5 dCyt occurs at a frequency consistent with its high mutagenic enhancement lends strong support to the hypothesis that unfavored base tautomers play important roles in the mispair intermediates of replication leading to substitution mutations.The tautomerism of nucleic acid bases is of great interest because of the putative role of their unfavored tautomers in substitution mutagenesis (1, 2). Thus, whereas the dominant amino and keto forms of the base residues enable correct gene replication by complementary base pairing between A and T and between G and C, the minor tautomeric imino and enol species allow for mispairing of A with C, G with T, and also are involved in mispairing of A with A, G with G, A with G, and G with A, resulting in transition and transversion mutations, respectively.Experimental and theoretical investigations involving pK measurement of various tautomer analogs (3), relaxation methods (4), and quantum mechanical calculations (5) have afforded estimates of the frequency of these rare tautomers. However, because of the very low level of minor tautomeric species in solution (Ϸ10 Ϫ4 -10 Ϫ5 ; refs. 6-8), it has not been possible to observe them directly by spectroscopy. Here, we report evidence obtained by UV resonance Raman spectroscopy (UVRR) for the minor tautomer of a highly mutagenic analog of the 2Ј-deoxycytidine (dCyt) residue, 5-hydroxy-2Ј-deoxycytidine (HO 5 dCyt).Earlier, it was found that C 3 T mutations resulting from transition metal-mediated damage to DNA by reactive oxygen species most frequently occur opposite C residues (9). The major mutagenic modification of these C residues is HO 5 dCyt, an oxidation product. HO 5 dCTP is incorporated efficiently into DNA by a DNA polymerase in vitro (10). On transfection of this DNA into Escherichia coli, it causes C 3 T transitions at a frequency of Ϸ2.5%, very much higher than that observed for any previously identified base analog-induced DNA lesion. A possible mechanism for these transitions involves a much greater frequency of the imino tautomer of HO 5 dCyt, which would increase the tendency to pair with A (2).In fact, our results show that the frequency of the minor tautomer of the anionic form of HO 5 dCyt, wh...

Infrared spectroscopy has been used to study the evolution of polyurethane foam structure, providing measures of relative reaction kinetics, hard segment growth, the onset of phase separation, the formation of order, and the development of final morphology. Changes in free, monodentate, and bidentate hydrogen-bonded urea groups dominate the organization of the entire ensemble. Hard segments formed by reaction of 2,6-toluene diisocyanate (2,6-TDI) and by a mixture of 80% 2,4-TDI and 20% 2,6-TDI displayed very different local segmental alignment, a factor crucial in the development of morphology. Phase separation occurred faster, with fewer and shorter hard segments, in the systems with well-ordered straight chains. The formation and time evolution of monodentate ureas suggest that phase development may be incomplete, or trapped, in systems with ill-defined urea structures. A low degree of spatial order exists in the systems containing these structures.

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