Circular Dichroism Studies of Isoleucine
Oligopeptides in Solution*
3 IUILIIAJ7 GOODA\MS, 1:RED SAIDER, Polytechnic Institute o j Btdilgn, B ~. o ~/ i l y n , N . Y., id CIAUDIO TOSIOLO, Institute oJ' Otyanic Clremistiy, [ -) 1 iuersity of I'atlua, I'atlua, Italy Synopsis A c.i)tifoi.iiiatiotial :tii:ilysis WBS caarried out 011 it series of x,-isoleuciiie oligoiners 1i:ivitig tlie geiieritl formilla 130C-(Ileu)n-OAle (n = 2-8). These ohgopeptides were examined in tiifliioloethaiic)l, tiifliioroethatiol-sc.id and mixed organic-water media. The results iiidicale that these oligomers form @-corifoimatioiis begiritiitig at the hep1,amer. The slability of the @-ccttiformatic)tis wits foiiiid to be greater than those formed by oligopeptides derived from r,-alatiitie.
In the preceding paper lve discussed tlie synthesis, polarimetry arid ultraviolet spect.roscopy (UV) of a series of L-isoleucine oligomers. Our preliminary results indicated that in trifluoroethariol secondary structures first appear a t tlitl Iieptiimer. Our UV iiivestigatioii suggests tli:it tlirse secondary structures may be p-coiiforni:itioris. 111 order to confirm our preliminary conformatioiial an:ilysis, \ve undertook :in examination of the isoleucine ohgopeptides using circular dicliroism (CD). In tliis paper \i-e present the findings of CD studies in trifluoroethanol (TkX), Iiexafluoroacetoiir: s wquiliydrate (HP'A), sulfuric :wid, m d several mixed solvent systems.
Apparatus and Measurements
Circular Dichiwism Studies
Circular dichroism studies were carried out using a Cary ti0 spectropohrimeter modified nitli a Model ti001 circular dichroism attaclimerit. The experimental solutions were prepared by \vrigliing tlie desired sample iiit,o ii volumetric flask and adding the solvent. Conceiitr;ition studies were carried o u t by st:wtiiig with :L stock solution of tlir: oligomw aiid diluting to thc clrsiiwl coiiwtitrat ion. The striclics pt~ifortii(d in t , I i c v:irious cosolvciit syst ( m s \ v ( w ' coiiduct(d by first tlissolviiig t I I V oligornr~ i t i rl'l~i~;. rl'li(b cosolvc.iil (siill'uric: acid, \v:tt cr, lic~s:~Aut~ro:ict.totit~) \v:is 1 1w1i :dtlctl : i l i t l t lit. i x w i l tiiig soltit ion \v:is st irred lor 20-30 iiiiiis. t o (ailsure equilibriuni. ' l ' h spcc:tr;~ \verc obt:iiiicd using 0.1 inin, 0.5 nim :md 1 cni pith lengtli wlls. D r y prtb-sIrll(~1 r1re. * This represeiits p:iper SSSV iii ~i i r series OII [~[)iif(Ji,iii:ili[)tl:il Aspec*ts of I'olypeptidc 1 7 19 0 1!)71 by Jo11ii Wiley & S < ) i i h , IIIV.