POLYPEPTIDE STRUCTURE AND DYNAMICS CONTROLLED BY PHOT0-ISOMERIZATION

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In order to clarify the mechanism of the helix-sense inversion of poly(β-n-propyl L-aspartate)(PnPLA), the conformational transition of PnPLA and its derivative with azobenzene group were investigated by means of NMR.1) The right-handed α-helix (αィイD2RィエD2)of PnPLA can change thermoreversibly to the left-handed (αィイD2LィエD2) with increasing temperature from 30 to 70℃ in CDClィイD23ィエD2. The midpoint is 50℃, the transition free energy is 110 kJ/mol, and the exchange rate between both conformations is 5 /s at about 50℃. All the ィイD113ィエD1C signals of PnPLA can separate completely between the right- and left-helices, although only the NH and βィイD2sィエD2 proton resonances show the conformational dependence. These results indicate that the side-chain conformation of αィイD2RィエD2-helical PnPLA is different from that of αィイD2LィエD2-helix, as well as the main chain conformation. The αィイD2RィエD2-helix conformation of PnPLA changes to the left-handed, and subsequently to the random-coil with increasing the trifluoroacetic acid concentration. The coil conformation of PnPLA can change to the αィイD2LィエD2-helix with increasing the temperature, and the αィイD2RィエD2-helix is not detected in the high temperatures.2) PnPLA containing 30% p-phenylazobenzene group in the side chain do not show the helix-sense inversion with the photo-isomerization of azobenzene group. The PnPLA containing more than 50% p-phenylazobenzene group is preparing for further experiments.3) The internal rotational angle from the spin-spin coupling constant ィイD13ィエD1Jαβ is necessary to determine the side chain conformation. The monomer of L-asp-βィイD2RィエD2-d was successfully synthesized from the fumaric acid in DィイD22ィエD2O/NDィイD24ィエD2OD with the enzyme of L-aspartase to remove the contribution of the residual geminal ィイD12ィエD1Jββ in the signal of βィイD2sィエD2 proton. PnPLA-βィイD2RィエD2-d is preparing for further experiments.