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Guanidine hydrochloride (Gdn-HCl) is the most commonly used denaturant for proteins and its effects on myoglobin unfolding have been widely studied (Pace et al. 1979). In particular, it has been assumed for a long time that increasing Gdn-HCl concentration destabilises the protein structure up to a complete co-operative unfolding at critical concentration C, (Pace, 1986). Recently Hagihara and co-workers (Hagihara et al. 1993) have found that Gdn-HCl, at low concentrations, refolds acid-unfolded apomyoglobin and cytrochrome c at pH=2, stabilising the molten globule state. These opposite effects of Gdn-HCl on protein stability have been discussed recently (Myers et al. 1995) and have been related to possible changes in protein accessible surface area. Furthermore a particular behaviour of the 0.5 MGdn-HCI concentration has been pointed out by optical spectroscopy measurements (Natali et al. 1998) In this work we have explored the conformational landscape of MbCO in native and partly denatured states induced by different concentration of Gdn-HCI. In particular following characteristic absorption peaks related to the conformational parameter Fe-CO bonding angle, we have investigated the protein dynamics using XANES technique t