6105kcal/mol at 300 K). This change is small compared with the calculated solvation enthalpy (-23 kcal/mol) at infinite dilution.1°Comparison of the internal entropy differences of the conformers in solution shows that the values are of the same magnitude as those found in vacuum (see Table I). The solvated polyproline conformation appears to have somewhat less freedom than the a-helical conformer, which is very similar in entropy to the C7eq conformer.This study of a peptide model system has demonstrated that molecular dynamics simulations can be used to estimate the configuration entropy differences in conformational transitions and in solvation processes. Such internal entropy changes have to be included in any calculations of the free energy differences between Acknowledgment. We thank B. Brooks, M. Pettitt, W. Reiher, P. Rossky, and S. Swaminathan for helpful discussions. Part of the calculations reported here were performed at the Rowland Institute and Los Alamos National Laboratory. Registry No. CH,CONHCHCH,CONHCH,, 227 15-68-0. ~~ ~~~ (12) Mezei, M.; Mehrotra, P. K.; Beveridge, D. L., private communication. (13) Pettitt, M., Karplus, M., unpublished results. Dinuclear elimination of groups from two different metal atoms involving the formation of metal-metal bonds or bridged metal ( I ) For several examples, see: Collman, J. P.; Hegedus, L. S. "Principles (13) Puddephatt, R. J.; Azam, K. A,; Hill, R. H.; Brown, M. P.; Nelson, C. D.; Moulding, R. P.; Seddon, K. R.; Grossel, M. C. J. Am. Chem. SOC. 1983, 105, 5642. J = 11 Hz, 1 H). The coupling between the hydride and the methyl protons (14) ' H N M R (CD2C12, -25 "C) 5 -0.49 (t, J = 4 Hz, 3 H), -10.92 (t, could not be resolved. (15) Tatsumi, K.; Hoffmann, R.; Yamamoto, A,; Stille, J. K. Bull. Chem. SOC. Jpn. 1981, 54, 1857.( 1 6) For a related Pt complex containing a pdppm and a q'-dppm ligand, see: Braunstein, P.; de Meric de Bellefon, C.; Lanfranchi, M.; Tiripicchio, A.