The kinetics of deoxyhemoglobin S gelation have been investigated using photolytic dissociation of the carbon monoxide complex to initiate the process. Measurements over a wide range of times, 10(-3)-10(4) show that both the concentration dependence of the tenth-time (i.e., the time required to complete one-tenth the reaction) and the time dependence of the process decrease as gelation speeds up. In slowly gelling samples, where single domains of polymers are formed in the small sample volumes employed with this technique (1-2 x 10(-9) cm3), there is a marked increase in the variability of the tenth-times. These results are explained by a mechanism in which gelation is initiated by homogeneous nucleation of polymers in the bulk solution phase, followed by heterogeneous nucleation on the surface of existing polymers. At the lowest concentrations, homogeneous nucleation is so improbable that stochastic behavior is observed in the small sample volumes, and heterogeneous nucleation is the dominant pathway for polymer formation, thereby accounting for the high time dependence. At the highest concentrations homogeneous nucleation becomes much more probable, and the time dependence decreases. The decrease in concentration dependence of the tenth-time with increasing concentration results from a decrease in size of both the homogeneous and heterogeneous critical nuclei. The model rationalizes the major observations on the kinetics of gelation of deoxyhemoglobin S, and is readily testable by further experiments.

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The accessibility of the s(4)U base in native tRNA(Val) from E.coli was monitored by studying the binding of various mercurials. The relative binding order HgBr(2)[unk]HgCl(2)>>CH(3)HgOAc[unk]CH(3)HgCl[unk]PCMB parallels approximately the steric requirements of linear HgX(2) or RHgX compounds for S(N)2 displacement by sulfur, although other factors are operative. Para-chloromercuri-benzoate (PCMB) does not bind the thiolated nucleotide unless the tertiary structure of the tRNA is opened up by removal of Mg(2+) ions and heating to 40 degrees . Under these conditions, equilibrium dialysis measurements using (14)C-labeled PCMB showed one binding site (n = 0.93) with an association constant, K(1), of 9 x 10(4)M(-1).

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