North Curolitia 22riif 4 Synopsis
We have perfornied I)otetitiotiiet,ric tit.rat.ions of poly-I,-lysitie. From 1,liese d a h we have calcrilat.ed t,he free eiiergy and enthalpy changes for t.he folding of the raiidoni coil to the a-helix in 10% ethanol ( -120 and -120 cal/niole) and from the random coil t,o t,he 8-stmcture in water (-140 and 870 cal/mole) and in 10% et,hanol (-180 and 980 cal/mole). Comparison of these values with each other and with values for the coil ---f ahelix transition in water (-78 and -880 cal/mole) led to the following conclusions. The stabilization by ethanol of the a-helix with respect to the coil is that predicted from the known free energy of transfer of the peptide group from water to 10% ethanol. Similar d a b t,o explain the enthalpy difference are not available. The thermodynamic friitctions for the transition from a-helix to &structure, obtained by subtracting those for t.he coil 4 a-helix and coil + p-struct,ure transitions, are explained from a consideration of the st,rnctural differences : non bonded int,eractions of the polypeptide backbone are less favorable in t,he p-st,ructure than in the a-helix, causing an increase in the energy, while hydrophobic contact.s between side chains raise the entropy of t,he 6-structure as compared with t,he a-helix, so that t,he free energy difference between the two structures is small, but enthalpy and entropy differences are large. The observation of only small differences in t~he free energy and enthalpy changes for the transition from coil to p-st.rricture ripon going from water t,o 10% ethanol is expected by considering both the free eiiergy of t,rtttisfer of {,he peptide group (as for the a-helix) atid the free energy and eiithalpy of traiisfer of t.he apolar part of the side chaiii involved in hydrophobic bond forniat.ioii. * To whom requests for reprints should be addressed. 2133 0 1'371 by John Wiley & Sons, Iric.. * The occurrence of phase separation in polymer solutions does not indicate the existence of any crystallized specie^.^ However, in the present case, the participation of the fl-stnicture warrants a description in terms of crystallization.