| Abstract
| - The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclearpolarization) in the photoreactions of the aromatic dye 2,2‘-dipyridyl with non-native states of bovine andhuman α-lactalbumins (BLA and HLA) in aqueous solution have been studied using the time-resolved CIDNPtechnique. Non-native states have been obtained at pH 2 in the presence of 0, 8, and 10 M urea-d4 and at pH6.7 in the presence of 10 M urea-d4. The dependence of the geminate CIDNP spectra of the two proteins onthe denaturant concentration is shown to be determined by the intrinsic reactivity of the amino acid residuestoward the triplet excited dye rather than by structural changes in the proteins. Values of the proton paramagneticrelaxation times (T1) have been obtained from an analysis of the CIDNP kinetics. For tryptophan and tyrosineresidues, the T1 values change in opposite directions when the proteins are progressively denatured, reflectingthe different internal mobilities of the two types of residues. It has been found that for both BLA and HLAthe CIDNP kinetics of the non-native states formed at pH 6.7 in the presence of 10 M urea are almost identicalto those at pH 2 with no urea, suggesting that the polarizable amino acid side chains have closely similarsolvent accessibilities and motional properties in the two non-native states.
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