Biological interactions between polysaccharides and divalent cations: The egg‐box model

Abstract

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,~otess that predominantly in~oNes consecuti~'e ~luronale 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 require-men~s roar ~ooperafivity.In pectin, • .themethyl 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 ~ ~* gsnd 5_~ • ~he.cireular dickro~sm spectrum o'f rthe so] in-