The possible aggregation of the opioid pentapeptide leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu) has been studied by steric exclusion chromatography and electrospray mass spectrometry. The results obtained with both techniques indicate the presence of multiple aggregates, whose apparent size and relative amount was environment and time-dependent. However, the mismatches in the relative ratio between aggregated and monomeric peptide, in the dependency of aggregation from environment conditions, as well as in the apparent size distribution of the aggregates indicated by the two techniques do not allow interpreting the data obtained as different descriptions of the same phenomena. On the contrary, these data appear to be consistent with the hypothesis that the chromatography-detected aggregates formed in the liquid phase are mostly destroyed during injection in the mass spectrometer, whereas those detected by this latter technique are - at least principally - formed upon interactions only established in the gas phase. Nevertheless, the same data are also unconsistent with a complete destruction of the liquid phase-formed aggregates during the injection procedure, since a fraction of these aggregates appears to survive injection, and be detected by mass spectrometry.
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