The forced degradation of RP carried out as per ICH guidelines results in the formation of six degradation products which have been characterized using LC/MS/MS in combination with accurate mass measurements.

An isocratic liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) method has been developed for the separation and identification of stress degradation products (DPs) of carvedilol (CAR), a non-selective b-blocker with vasodilatory properties. CAR was subjected to hydrolytic (acidic, alkaline and neutral), oxidative, photolytic and thermal stress conditions as per ICH guidelines Q1A (R2). The drug degraded under acidic, basic and photolytic stress conditions while it was stable under neutral, thermal and oxidative stress conditions. A total of five DPs were formed which were all well resolved from the parent drug as well as from each other on an Agilent XDB C-18 column (4.6 Â 150 mm, 5 mm) using a mobile phase comprising of 20 mM ammonium acetate (pH adjusted to 6) (solvent A) and acetonitrile (solvent B) in an isocratic elution method. The structures of the degradation products were elucidated by using LC-ESI-MS/MS combined with accurate mass measurements.

[11] DFT calculations were performed using Gaussian 09 for Windows (Gaussian Inc.).[12] The structures of the [M+H] + ions, intermediates, ion-neutral complexes and acyl cation adducts were optimized using B3LYP/6-31G(d), and in some cases B3LYP/6-31+G(d,p) basis sets. Vibrational corrections were calculated and relative enthalpies of the ions were used for comparison. The complete data with optimized structures are given in the Supporting Information. The ESI-ion-trap mass spectrum (+ ion mode) of compound 1 showed the protonated molecules [M+H] + at m/z 390, and the fragment ions [M+H-162] + at m/z 228 (base peak) and

The present study demonstrates that ESI tandem mass spectrometry is very useful for differentiating positional isomers of hybrid peptides containing D-Ala and APyC amino acids. While the protonated peptides give rise to characteristic sequencing ions, the cationized peptides produce additional rearrangement ions ([b(n-1) + OCH(3) + Na](+) and [b(n-1) + OH + Na](+)) which helps distinguish between the presence of D-Ala and APyC amino acids at the C-terminus.

A new class of Boc-N-protected hybrid peptides derived from L- Ala and ε-Caa (L-Ala = L-Alanine, Caa = C-linked carboamino acid derived from D-xylose) have been studied by positive ion electrospray ionization (ESI) ion-trap tandem mass spectrometry (MS/MS). MS spectra of protonated and alkali-cationized hybrid peptides produce characteristic fragmentation involving the peptide backbone, the tert-butyloxycarbonyl (Boc) group, and the side chain. The dipeptide positional isomers are differentiated by the collision-induced dissociation (CID) of the protonated and alkali-cationized peptides. The CID of [M + H] ion of Boc-NH-L-Ala-ε-Caa- OCH (1) shows a prominent [M + H - CH] ion, which is totally absent for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH (6), which instead shows significant loss of t-butanol. The formation of the [M + Cat - CH] ion is totally absent and [M + Cat - Boc + H] is prominent in the CID of the [M + Cat] ion of Boc-NH-L-Ala-ε-Caa- OCH (1), whereas the former is highly abundant and the latter is of low abundance for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH (6). It is observed that 'b' ions are abundant when oxazolone structures are formed through a five-membered cyclic transition state in tetra-, penta-, and hexapeptides and the cyclization process for larger 'b' ions led to an insignificant abundance. However, the significant 'b' ion is formed in ε,α-dipeptide, which may have a seven-membered substituted 2-oxoazepanium ion structure. The MS spectra of [M + Cat - Boc + H] ions of these peptides are found to be significantly different to those of [M + H - Boc + H] ions. The CID spectra of [M + Cat - Boc + H] ions of peptide acids containing L-Ala at the C-terminus show an abundant N-terminal rearrangement ion, [b + 17 + Cat], which is absent for the peptide acids containing ε-Caa at the C-terminus. Thus, the results of these hybrid peptides provide sequencing information, the structure of the cyclic intermediate involved in the formation of the rearrangement ion, and distinguish a pair of dipeptide positional isomers.