We also observed an intra-chain disulfide bond formation between two adjacent cysteines within THT peptide (Figure ). The XICs from 0 and 168 hr incubation time confirms this product was formed during the incubation as shown in Figure . Figure confirms the identity of this biotransformation product as MS level. The CID MS/MS spectrum shows high sequence coverage of this product, as shown in Figure . c and z ions were labelled in the EAD MS/MS spectrum in Figure . We noticed a couple of fragment peaks (red color b and c ions in Figure ) that have observed masses that are ~ 2 Da less than their theoretical values. These ions further confirm the formation of intra-chain disulfide bond. Interestingly, in Figure , the ion with disulfide bond fragmented overlapped with the non-fragmented precursor ion, possibly because there was no mass change for this structure upon EAD fragmentation. We hypothesize that the formation of the intra-chain disulfide bond stems from the fact that two cysteines are spatially close to each other.