Unperturbed dimensions of crosslinked histones evaluated using random‐flight statistics and rotational isomeric state theory

Unperturbed dimensions have been computed via rotational isomeric state theory for approximately 700 dimers obtained from histones 2A, 2B, and 4. Dimers result from a crosslink joining either lysyl or tyrosyl residues in different histones. Sets of statistical weights were chosen that yield either a low helicity or a helical content near 40%. These extremes correspond to helicities of histones in the commonly used acid/urea and sodium dodecyl sulfate systems, respectively. Many features of the rotational isomeric state results can be sucessfully reproduced by much simpler expressions based on random‐flight statistics. For example, the two methods are about equally effective in predicting how the radius of gyration of a given type of crosslinked dimer should vary from that of the analogous linear polypeptide chain. This result is in marked contrast to that attained using crosslinked, partially helical homopolypeptides. The unexpected success of random‐flight statistics with partially helical crosslinked histones is due to the suppression of long helical segments by helix‐breaking amino acid residues. Random‐flight statistics should be applicable to other crosslinked, partially helical proteins if their amino acid sequences contain the usual representation of helix‐breaking amino acid residues.