A slow rate exchange process allows the detection of both carbonyl (C=O) and hydroxyl (-OH) O-17 nuclear magnetic resonances of the carboxylic group

In this study we report, for the first time, on the detection of both the c arbonyl (-C=O) and the hydroxyl (-OH) O-17 resonances of the carboxylic gro up by O-17 NMR spectroscopy. Two well separated peaks, at 340.3 and 175 ppm , were detected for the -C=O and -OH groups, respectively, of Boc[O-17]Tyr( 2,6-diClBzl)-OH in DMSO-d(6). This finding is attributed to the participati on of the carboxylic group in a rather strong hydrogen-bonded interaction. These resonances disappeared in the presence of a small quantity of TFA (tr ifluoroacetic acid), which, however, did not cleave the Boc-group and a bro ad resonance was detected for both oxygens. In fact, this result indicates that the proposed hydrogen-bonded interaction is not stabilized in the pres ence of TFA, which is a very strong hydrogen bond breaker. Only one broad s ignal was observed for both carboxylic oxygens at 251.8 ppm for the HCl[O-1 7]Tyr(2,6-diClBzl)-OH in DMSO-d(6) solution, suggesting that the carbonyl o xygen of the Boc-group is probably the proton acceptor group of the carboxy lic hydrogen, stabilizing a gamma -turn like structure. A single relatively sharp resonance appearing in chloroform, (247.5 ppm) upheld shifted by sim ilar to 14.5 ppm, compared to the resonance found in DMSO-d(6) for Boc-[17( O)]Tyr(2,6-ClBzl)-OH in the presence of TFA, is attributed to a gamma -turn fraction being in a fast exchange with the open form. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.