COLD-DENATURED ENSEMBLE OF APOMYOGLOBIN - IMPLICATIONS FOR THE EARLY STEPS OF FOLDING

The dynamics of protein-refolding experiments initiated by a temperatu re jump depend critically on the nature of the initial cold-denatured ensemble. The cold-denatured state of equine apomyoglobin has been inv estigated in aqueous buffers by near-and far-UV circular dichroism, fl uorescence, infrared, and NMR spectroscopies at temperatures ranging f rom -20 to 98 degrees C. Cold denaturation of apomyoglobin is well des cribed by a cooperative transition below 3 degrees C and differs in ma ny aspects from acid-induced unfolding. As a reference system, the N-t erminal A-peptide fragment of equine apomyoglobin has also been studie d in aqueous-and trifluoroethanol solutions. The A-peptide has a low h elix-forming propensity in the absence of any stabilizing tertiary int eractions. The results show that cold denaturation breaks the AGH-hydr ophobic interface of equine;ne apomyoglobin. Furthermore, at least som e GH-helical structure appears to be preserved at the expense of the l ess stable A-helix.