A st at ist ical t ho-m odjw a m ic t heor.)> is developed t o in vat igat e t h 6) c ffivts Q f soliit c> e.xclz4ded voliime on the stability cf globiilur proteins. Proteins are moddcd as two states in chemical eqiiilibrizim: the denatirred state is modeled as ajle.xible chain of tungent hard spheres (pearlneckluce chain) iihilc thc native state is modded us a single hurd spherc. Study qfmodel protrim bovine puticwutic tr)p.sin inhibitor and lvsozvme in a n/fcA4illan-Mayer tnodel solution of hard spheres indicatcs tliut the e.xcliided volume of soliites has three distinct types (f &cts on protein stubilit)?: ( I ) small-size solutes strong1.v denature proteins, ( 2 ) medium-size soltiles stabilize proteins at low solute concentrations and destabilize them at high concentrations. and
(3) large-sizL> .so/iite.s stuhiliie native-stute proreins across thr whole liquid region. The study aI.so.find~s that inrwasing the chain length ofhard-chain polymer .soliite.s has an effect on protein .stabilitj* that is similur to increasing the diameter of spherical solutes. This work qua1itativel.v c.xp1ain.s why stubilixxs tend to hc' large size moleciiles such as .sugars, pol.vmer.s, polynols, nonionic, and anionic swfactants while denaturants tend to be small .size molecules such us alcohols, glycols. amides, ,fi)rmamides, ureus. and gtranidiium salts. Qziantitative comparison hrtwc.cn theoretical predictions and experimental resiilts,for,folding,free encygv changes shows that the c.xcliidi~d-,~oliimc c:ffect is at least as important as the binding and/or electrostatic effects on .solicte-as.si.stcd protein-denaturation processes. Our theory may ul.~o be able t o explain the i.fl;..r q/'~.xcliid~d vohrme on the condensation qf DNA. 0 I996 John W i l q & Sons, Jnc.