Calculation of the characteristic ratio of randomly coiled poly(L-proline)

Allowing for rotation about the Calpha-C' bond (i.e., variation of psi) and for some degree of freedom about the peptide bond (i.e., small variations of omega), the characteristic ratios, (R2)o/nl2, of the form I (cis) and form II (trans) poly(L-proline) chain have been calculated by a Monte Carlo method in which the conformational energies were used as weighting factors. The Monte Carlo method enabled short-range interactions (beyond those involved in a single residue) to be taken into account. The effect of the presence of a small amount of one form (say cis in a trans-rich chain) on (R2)o/nl2 was also investigated. The results for the trans-rich form are in good agreement with values observed experimentally in solvents in which the poly(L-proline) chain is predominantly in form II; the presence of a small amount of cis residues reduces the characteristic ratio of the trans-rich form of poly(L-proline) significantly.