of force-extension curves of dextran. However, CMA — oppositely to dextran, which has no intrinsic torsion — adopts preferentially a pseudo-helical conformation, and this has already been pointed out in Ref. 71 to be a possible source of error when using freely jointed chain models to interpret data. More recently, Kuttel and Naidoo 75 suggested that in amylose stretching the elongation might be due to a complex rotation of glycosidic linkages, and that chair to boat transitions could play little role in the process. Finally we would like to mention that $^{4}$C$_{1}$ to $^{1}$C$_{4}$ transitions could be in principle investigated by means of AFM spectroscopy of, e.g. , pectin $^{71}$. Their more complex two-step transition prevented so far an estimate of the puckering free energy, but it still represents — to the best of our knowledge — the only approach which could provide direct estimate of inverted chair free energies of the pyranoid form of aldohexoses other than idose and altrose. In the field of computer simulations, ab-initio predictions of the puckering free energy of pyranoses are even more scarce: we are only aware of a Car–Parrinello metadynamics of glucose in vacuum 76 (where, unfortunately, use of non-optimal collective variables has been made of $^{77}$), a Hartree-Fock/coupled clusters calculation (MP2/cc-pVTZ) with inclusion of vibrational, rotational and solvation free energies contributions $^{78}$, and a calculation at the B3LYP/6 − 311 + +G ∗∗ level $^{79}$. The calculations of Appell and coworkers79 predicted a free energy difference for the $^{1}$C$_{4}$ conformer of α -d- Glc and β -d- Glc of 20.40 and 29.18 kJ/mol, respectively. It has to be noticed, that calculations performed by including solvation effects, although only at the MP2/cc-pVTZ level 78 led to a free energy of β -d- Glc $^{1}$C$_{4}$ conformer of about 57 kJ/mol. This very estimate appears to be substantially higher than all other ones reported so far. In absence of similar ab-initio calculations for idose, that could be employed as benchmark system, it is in our opinion difficult to assess the confidence level of this estimate. However, the possibility that the values reported in Ref. 62 and Ref. 63 could underestimate the free energy of inverted chairs for stereoisomers other than idose can not be completely ruled out. Given the results briefly reviewed so far, it is obvious that clear-cut experimental reference values for sugar ring puckering free energies are not yet available. The theoretical estimates presented in Ref. 62 and in Ref. 63 are mutually consistent, in agreement with recent ab-initio calculations $^{79}$, and were able to predict with reasonable accuracy the population of chairs in α , β -d- Ido, and possibly altrose $^{70}$. The results of AFM pulling on dextran are also compatible with these estimates, in the sense that the free energy of boat conformers is expected to be 7