TY - JOUR
T1 - Studies on phosphatidylserine by tandem quadrupole and multiple stage quadrupole ion-trap mass spectrometry with electrospray ionization
T2 - Structural characterization and the fragmentation processes
AU - Hsu, Fong Fu
AU - Turk, John
N1 - Funding Information:
This research was supported by U.S. Public Health Service grants P41-RR-00,954, R37-DK-34,388, P60-DK-20,579, P01-HL-57,278, P30-DK-56,341, and a grant (996003) from the Juvenile Diabetes Foundation. The authors thank the reviewers for their critical comments on this manuscript.
PY - 2005/9
Y1 - 2005/9
N2 - Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M - H]- and [M - 2H + Alk]- in the negative-ion mode, as well as in the forms of [M + H]+, [M + Alk]+, [M - H + 2Alk]+, and [M - 2H + 3Alk]+ (where Alk = Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination. Following CAD, the [M - H]- ion of PS undergoes dissociation to eliminate the serine moiety (loss of C3H5NO2) to give a [M - H - 87]- ion, which equals to the [M - H]- ion of a phoshatidic acid (PA) and give rise to a MS3-spectrum that is identical to the MS2-spectrum of PA. The major fragmentation process for the [M - 2H + Alk]- ion of PS arises from primary loss of 87 to give rise to a [M - 2H + Alk - 87]- ion, followed by loss of fatty acid substituents as acids (RxCO2H, x = 1,2) or as alkali salts (e.g., RxCO2Li, x = 1,2). These fragmentations result in a greater abundance of [M - 2H + Alk - 87 - R2CO 2H]- than [M - 2H + Alk - 87 - R1CO 2H]- and a greater abundance of [M - 2H + Alk - 87 - R2CO2Li]- than [M - 2H + Alk - 87 - R 1CO2Li]-; while further dissociation of the [M - 2H + Alk - 87 - R2(or 1)CO2Li]- ions gives a preferential formation of the carboxylate anion at sn-1 (R1CO 2-) over that at sn-2 (R2CO2 -). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of Rx′CH=CO, x = 1,2). This results in a more prominent [M - 2H + Alk - R2′CH= CO]- ion than [M - 2H + Alk - R1′CH=CO]- ion. Ions informative for structural characterization of PS are of low abundance in the MS2-spectra of both the [M + H]+ and the [M + Alk]+ ions, but are abundant in the MS3-spectra. The MS2-spectrum of the [M + Alk]+ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving rearrangement steps. The [M - H + 2Alk] + ion of PS yields a major [M - H + 2Alk - 87]+ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M - H + 2Alk - 87 - R1CO 2H]+ than [M - H + 2Alk - 87 - R2CO 2H]+. Similarly, the [M - 2H + 3Alk]+ ion of PS also yields a prominent [M - 2H + 3Alk - 87]+ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair.
AB - Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M - H]- and [M - 2H + Alk]- in the negative-ion mode, as well as in the forms of [M + H]+, [M + Alk]+, [M - H + 2Alk]+, and [M - 2H + 3Alk]+ (where Alk = Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination. Following CAD, the [M - H]- ion of PS undergoes dissociation to eliminate the serine moiety (loss of C3H5NO2) to give a [M - H - 87]- ion, which equals to the [M - H]- ion of a phoshatidic acid (PA) and give rise to a MS3-spectrum that is identical to the MS2-spectrum of PA. The major fragmentation process for the [M - 2H + Alk]- ion of PS arises from primary loss of 87 to give rise to a [M - 2H + Alk - 87]- ion, followed by loss of fatty acid substituents as acids (RxCO2H, x = 1,2) or as alkali salts (e.g., RxCO2Li, x = 1,2). These fragmentations result in a greater abundance of [M - 2H + Alk - 87 - R2CO 2H]- than [M - 2H + Alk - 87 - R1CO 2H]- and a greater abundance of [M - 2H + Alk - 87 - R2CO2Li]- than [M - 2H + Alk - 87 - R 1CO2Li]-; while further dissociation of the [M - 2H + Alk - 87 - R2(or 1)CO2Li]- ions gives a preferential formation of the carboxylate anion at sn-1 (R1CO 2-) over that at sn-2 (R2CO2 -). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of Rx′CH=CO, x = 1,2). This results in a more prominent [M - 2H + Alk - R2′CH= CO]- ion than [M - 2H + Alk - R1′CH=CO]- ion. Ions informative for structural characterization of PS are of low abundance in the MS2-spectra of both the [M + H]+ and the [M + Alk]+ ions, but are abundant in the MS3-spectra. The MS2-spectrum of the [M + Alk]+ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving rearrangement steps. The [M - H + 2Alk] + ion of PS yields a major [M - H + 2Alk - 87]+ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M - H + 2Alk - 87 - R1CO 2H]+ than [M - H + 2Alk - 87 - R2CO 2H]+. Similarly, the [M - 2H + 3Alk]+ ion of PS also yields a prominent [M - 2H + 3Alk - 87]+ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair.
UR - http://www.scopus.com/inward/record.url?scp=23844509905&partnerID=8YFLogxK
U2 - 10.1016/j.jasms.2005.04.018
DO - 10.1016/j.jasms.2005.04.018
M3 - Article
C2 - 16023863
AN - SCOPUS:23844509905
SN - 1044-0305
VL - 16
SP - 1510
EP - 1522
JO - Journal of the American Society for Mass Spectrometry
JF - Journal of the American Society for Mass Spectrometry
IS - 9
ER -