Determinatio nof -Helix and -Sheet Stability in the Soli dState :A Solid-State NM RInvestigatio nof Poly(L-Alanine)

Th erelativ estabilit yof -helix and -sheet secondary structure in the solid state was investigate dusin gpoly(L-alanine )(PLA )a sa mode lsystem .Protei nfoldin gan dstabilit yha sbee nwell studie di nsolution ,bu tlittl ei sknow nabou tsolid-stat eenvironments ,suc ha sth ecor eo fa folde dprotein, wher epeptid epackin ginteraction sar eth edominan tfacto ri ndeterminin gstructura lstability. 13 C cross-polarizatio nwit hmagi cangl espinnin g(CPMAS )NM Rspectroscop ywa suse dt odetermin ethe backbon econformatio no fsoli dpowde rsample so f15-kD aan d21.4-kD aPL Abefor ean dafte rvarious sampl etreatments .Reprecipitatio nfro mhelix-inducin gsolvent strap sthe -helical conformation of PLA ,althoug hth emetho do freprecipitatio nals oaffect sth econformationa ldistribution .Grinding convert sth esecondar ystructur eo fPL At o afina lsteady-stat emixtur eo f55% -sheet and 45% -helix a troo mtemperatur eregardles so fth einitia lsecondar ystructure .Grindin gPL Aa tliqui dnitrogen temperature slead st oa simila rsteady-stat emixtur ewit h60% -sheet and 40% -helix, indicating that mechanica lshea rforc ei ssufficien tt oinduc esecondar ystructur einterconversion .Coolin gth esample i nliqui dnitroge no rsubjectin gi tt ohig hpressur eha sn oeffec to nsecondar ystructure .Heatin gthe sampl ewithou tgrindin gresult si nequilibratio no fsecondar ystructur et o50% -helix/50% -sheet at 100° Cwhe nstartin gfro ma mostly -helical state. No change was observed upon heating a -sheet sample ,perhap sdu et okineti ceffect san dth edifferen theatin grat euse di nth eexperiments .Thes eresults ar econsisten twith -sheet approximately 260 J/mol more stable than -helix in solid-state PLA. ©200 2Wile yPeriodicals ,Inc .Biopolymer s64 :24 6-254 ,2002