SYNTHESIS AND COMFORMATIONAL ANALYSIS OF PEPTIDES ANALOGUES OF THE CENTRAL SEQUENCE OF LULIBERIN B-TURN INVESTIGATION BY CHARGE TRANSFER COMPLEXES
Abstract
IN ORDER TO VISUALIZE SPECIFIC SIDE CHAIN-SIDE CHAIN INTERACTIONS IN PEPTIDES VIA CHARGE TRANSFER COMPLEXES BETWEEN TRP AND NVA(NIC+) WITH THE AIM OF FORMULATING A RATHER SIMPLE WAY TO EVALUATE THE PROBABILITY OF A B-TURN CONFORMATION THREE MODEL TETRAPEPTIDES WERE PREPARED BY THE SOLID PHASE TECHNIQUE: AC-TRP- TYR-X-NVA(NIC+)-NH2.CL- WHERE X STANDS FOR D-ALA, GLY, OR L-ALA, AND REFLECTS A GRADUAL DECREASE OF FOLDING. THESE MOLECULES WERE ANALYSED BY UV-VIS AND 1H-NMR SPECTROSCOPIES. ALL THE THREE ANALOGUES SHOWED A CHARACTERISTIC CHARGE-TRANSFER BAND AS A SHOULDER IN THE LONG WAVE LENGTH REGION OF THE UV-VIS SPECTRA. THE DIFFERENCES IN THE INTENSITY AT 380 NM REVEAL AN INCREASED SIDE CHAIN- SIDE CHAININTERACTION IN THE ORDER OF L-ALA<GLY<D- ALA FOR X. ON THE OTHER HAND, ON THE BASE OF THE NMR DATA IT CAN BE CONCLUDED THAT AN OPEN REVERSE B-TURN OCCURS, OFTHE TYPE II WHEN X=D-ALA AND OF THE TYPE I WHEN X=L-ALA. THE TYPE OF THE TURN IS FURTHER SUPPORTED BY SELECTIVE NOE MEASUREMENTS. IN ...
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