Protein stability in aqueous solutions is important in numerous fields, particularly biotechnology and food-science. To shed new light on the protective effect of carbohydrates on proteins, we studied saccharide-structure effects in aqueous solutions on the coil-to-globule transition occurring at the lower critical solution temperature (LCST) of poly-N-isopropylacrylamide (PNIPA), an isomer of polyleucine, as a simple model representing certain key behaviors of proteins (e.g., denaturation/renaturation). We systematically selected sugars and polyols to relate structural and physical characteristics of these carbohydrates to their effect on PNIPA solutions. Using isothermal titration-microcalorimetry, we showed that no significant binding of saccharides to the polymer occurs. Using micro-DSC, we studied the decreasing polymer LCST temperature with rising carbohydrate concentration. Beyond the expected observation that steric exclusion is important, we observed previously-unreported significant differences among the effects of isomeric aldohexoses and also among the effects of isomeric diglucoses on PNIPA LCST. We found good correlation between the sugar hydration number and its effect on LCST. We conclude that the larger and denser the hydrated cluster a carbohydrate forms, the worse a cosolvent is for the polymer, and the stronger it's lowering effect of the coil-to-globule transition. Such favoring of the compact globule state provides a protective effect against denaturation of globular proteins. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2307-2318, 2008