We han, e shown that spedfic binding of divalent cations to a polysaechafide polyelectro]ym, leading firm cohesion between the chains, can cause characteristic effects ~n ~e c~rcutar diehroism spectrum which are understandabb in terms of modem theo~, [ l ]. For atginate, tiffs binding is a co-operative p,~o-tess that predominantly in~oNes consecuti~'e ~luro-nale residues. Some other systems ha~e now been inves,~igated in an attempt to formulate a general inSetpretafion of bio]ogics] phenomena of this type. T~e known streng~ and specificity of comp~exafion are explained m termg of an "'egg-box model" which ~s ,derN,ed from our measmemems, the known coordination geometries in mode] compounds, and the requiremen~s roar ~ooperafivity.In pectin, • .the methyl este~ of poly(ga~acturonie acid), the interactions with ca~on.~ is relatively weak because ~he chains are unch~r~ed. In contrasl to the beha~onr of alginates, in which the amplitude diminishes w~th gelatben I ] ], the broad, positive, n ~ ~* Cations are absent, however, and their cha~acte~stic in-quence on the ~ orbha~s is therefore no~ seen. After saponification, the pe!ysacchafide binds Ca 2+ wiih ge~ formation and with a large decrease ~n amplitude of the n -~ .~* band ( fig. 1) which shows a gaussian difference spectrum centered on 208 ~[rn. This is therefore [ l ] explained as a specific bind,~ng of most of the umnam residues m Ca 2+ wNch tends to reverse the sign of the n ~ ~* band. Derivatives that are or,.ly partly saponified, as in "%w me~hoxy pectins" used industr~ally and p~esent in MomeNtal systems, show similar specUoseopic changes during gela~on wi~.~ Ca 2÷, except that an additional peak remains with ~he position (210 nm) and ampfitude expected fo,~ methyl galactaronate Nsidues. Being uncha~ged ~Lhe~,c groups wosld noi pa{~¢ipaie directly in ion bin~!k~ng, and the ~esiduai peak therefore confirms our v~ew that the extreme spectra," changes arise horn ~pecific pert~rbatio~ of bound ~e~idues by proximity of the ionic cha~ge: *~buIed between a nnmbez Of rotations] states about .... Po]ymarin~ionate as well as po]yg~]mmnate an~ ,~(5)--C(5) which, due to ~hc d~rent ofienta~ion~ of polyga~acturona~e sequences can be perturbed spotthe ~emboxy] ehromo~no~e Lq the ~yn~met~Je erl.~i~on-m:enI of ~he ~aga~, ring, ha~e ~ ->~* &rods. of ~arying .
