OPTICALLY ACTIVE BIARYLS 1455 m.p. 139-141.5', infrared spectrum identical with that of startingmaterial, [ a ] * 6~ 0.0' ( G 1.1,i 2 , benzene). B.-Thermostated solutions ( 10jo) of (-)-I in redistilled o-xylene were examined polarimetrically (2-dm. tube) over a period of a t least one half-life. Readings were taken a t 435 m r and the results were plotted as log at ws. time. Excellent straight-line relationships resulted from the minimum of fifteen readings which were obtained for each run. The values of k obtained from the slopes of the curves are listed inTableI. Anal. Calcd. for ClsHls02: C, 81.17; H, 6.81. Found: C, 81.44; H, 6.94. The (+)-isomer ([a]*% -k 25' (C 0.99 chloroform)) Was obtained by decomposition of the salt ([aIp8D -68' (C 1.0, chf.)) which precipitated from (*)-I and W n i n e in acetone.The product was not further investigated.A.-A 1% solution of ( -1-

Using both circular dichroism (CD) and differential scanning calorimetry (DSC), several laboratories find that the thermal unfolding transitions of alpha alpha and beta beta homodimeric coiled coils of rabbit tropomyosin are multistate and display an overall unfolding enthalpy of near 300 kcal (mol dimer)(-1). In contrast, an extant CD study of beta beta and gamma gamma species of chicken gizzard tropomyosin concludes that their unfolding transitions are simple two-state transitions, with much smaller overall enthalpies (98 kcal mol(-1) for beta beta and 162 kcal mol(-1) for gamma gamma). However, these smaller enthalpies have been questioned, because they imply a concentration dependence of the melting temperatures that is far larger than observed by CD. We report here DSC studies of the unfolding of both beta beta and gamma gamma chicken gizzard homodimers. The results show that these transitions are very similar to those in rabbit tropomyosins in that 1) the overall unfolding enthalpy is near 300 kcal mol(-1); 2) the overall delta C(rho) values are significantly positive; 3) the various transitions are multistate, requiring at least two and as many as four domains to fit the DSC data. DSC studies are also reported on these homodimeric species of chicken gizzard tropomyosin with a single interchain disulfide cross-link. These results are also generally similar to those for the correspondingly cross-linked rabbit tropomyosins.

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