Journal ofThermal Analysis, Vol. 41 (1994)1515-1518 COMPARISON OF THE RESULTS OF TH'FRMAL DENATURATION OF 13-LACrOGLOBULIN OBTAINED BY DSC AND UV-SPECTROSCOPY N. Poklar, G. Vesnaver and S. Lapanje Department of Chemistry, University of Ljubljana, A kerceva 5, P.O. Box 537, 61001 Ljubljana, Slovenia Abstract The thermal denaturation of [3-1aetoglobulin in the presence of urea and alkylurea solutions were measured. In the presence of a high concentration of urea this protein shows not only heat but also cold denaturation. For studying the effect of temperature two methods were used, diffe- rential scanning calorimetry (DSC) and UV-speetroseopy. DSC provides direct model-inde- pendent determination of the transition enthalpy in comparison with UV-speetroscopy, which gives only apparent or van't Hoff enthalpy of transition. The UV-melting curves were analyzed on the basis of a two-state approximation. The apparent standard enthalpies of thermal denatur- ation, A/-~z,., were compared with calorimetric ones. Keywords: 13-1actoglobulin, cold denaturation, DSC, thermal stability, urea, UV-spectroscopy Introduction The subject of the study is the thermal denaturation of 13-1actoglobulin in the presence of urea and alkylurea solutions. The part of these results were pub- lished in a previous paper [1]. It is known that 13-1actoglobulin in the presence of high concentration of urea and guanidine hydrochloride (GuHCI) shows not only heat but also cold denaturation [2 - 4]. GuHCI and urea are known to play the following roles: They lower the stability curve so that the low-temperature instability is brought into the range of experimental accessibility, they accelerate the unfolding reaction and they lower the freezing temperature of water so that the subzero temperature range may be explored [5]. Experimental For studying the effect of temperature two methods, differential scanning calorimetry (DSC) and UV-spectroscopy, were used. Equilibrium thermal un- folding of 13-1actoglobulin was monitored calorimetrically with Bio-DSC micro- John Wiley & Sons, Limited, Chichester Akad~miai Kiad6, Budapest