Time-of-flight secondary ion mass spectrometry of Fmoc-amino acids linked to solid supports through ionic interactions

Abstract

The TOF-SIMS (time-of-flight-secondary ion mass spectrometry) spectra of Fmoc-amino acids linked to solid supports through ionic interactions have been studied to examine if easy desorption of preformed ions from the surface occurred. Preliminary results indicated that the structure of the polymer affected the nature of the recovered ions. However, in the case of widely used polystyrene based supports, characteristic ions of the amino acids were clearly observed with noticeable abundances. Moreover, an analytical criterion to differentiate Fmoc-amino acids linked to various solid supports through either covalent or ionic bonds was established. # 1998 John Wiley & Sons, Ltd. Received 9 September 1998; Accepted 9 September 1998 Very few spectroscopic methods are available to analyze samples in the solid state whereas such techniques present numerous applications in the fast-growing field of solidphase chemistry. Indeed, rapid and efficient product characterization should be obtained with the organic moieties still anchored to the support, thus avoiding product release in solution by any time-consuming and eventually deleterious chemical treatments.Cross polarization-magic angle spinning (CP-MAS) NMR 1 and IR 2 have been used to follow polymer supported syntheses but the main drawback of such techniques consists of the overlapping of the polymer signals with the signals of the expected compound.We have developed an alternative analytical method based on time-of-flight-secondary ion mass spectrometry (TOF-SIMS) to monitor step by step solid-phase peptide syntheses. 3,4 We report the results obtained when this technique was applied to samples where the amino acids were not covalently linked as previously described but simply anchored onto various supports through ionic interactions. EXPERIMENTAL ChemicalsProtected amino acids and the MBHA-resin were purchased from Senn Chemicals AG (Germany). The pin support consists of a derivatized crown, upon which synthesis is performed, and an inert polypropylene stem. The aminomethylated polystyrene-grafted (PS-pin) and methacrylic acid/dimethylacrylamide-grafted (MD-pin) crowns were supplied by Chiron Technologies Pty. Ltd. (Australia). The crowns and the resin which bear free amino groups were immersed overnight in a 0.4 M dimethylformamide (DMF) solution of Fmoc-protected amino acid (either Fmoc-Phe-OH or Fmoc-Leu-OH) and then washed once with DMF for 1 minute. Mass spectrometryCrown samples analyzed by TOF-SIMS were cut out directly from the the pins with a razor blade to obtain thin slices, which were then immediately placed on a stainless grid. The resin beads were analyzed directly, as such. TOF-SIMS measurements were performed on a TRIFT I spectrometer from the PHI-Evans Company (Eden-Prairie, MN, USA). Spectra were recorded using a pulse (1 ns, 12 kHz) liquid metal source ( 69 Ga, 15 keV) operating in the bunched mode of operation in order to provide good mass resolution (m/Dm = 2000, measured at m/z 43). Due to large charging effects on the insulating materials pres...

Controlling cell morphology on amino acid-modified cellulose

“…Using TOF-SIMS the presence of the Fmoc group on the surface of the samples was monitored by detection of secondary ions at m/z (mass/charge ratio) of 179 D. 14 Individual amino acids, and their associated surface coverage, were monitored by specific secondary ion peaks (associated with C 9 H 8 N + and C 14 H 11 + ) as reported in Fig. 2a for tryptophan (Trp).…”

Characterization of Nα‐Fmoc‐protected dipeptide isomers by electrospray ionization tandem mass spectrometry (ESI‐MSn): effect of protecting group on fragmentation of dipeptides

“…Both the isomers show (Table and also Supporting Information) an ion at m/z 157 ([M + H–Fmoc + H–CH 3 OH] + ). Besides, the former shows product ions at m/z 171 ([M + H–Fmoc + H–H 2 O] + ), 132 (y 1 + ) and 129 ([M + H–Fmoc + H–CH 3 OH–CO] + ) and the latter shows a low abundance ion at m/z 161 ([M + H‐Fmoc + H‐CO] + ) and an abundant ion at m/z 72 corresponding to the immonium ion (a 1 + ), a characteristic ion for the presence of valine at the N‐terminus. The formation of the y 1 + ion for 5 and the immonium ion (a 1 + ) for 7 is in agreement with higher proton affinity of valine compared to that of glycine .…”

“…Goel and Kenny also reported the mechanism for the formation of the b 1 ‐1 and corresponding a 1 ‐1 ions in the product ion mass spectra of N‐para ‐ferrocenylbenzoyl dipeptide esters. A literature survey revealed that there are very few reports on mass spectral studies of Fmoc protected peptides/amino acids . To examine the effect of the Fmoc‐ group on the fragmentation of the dipeptides, we have studied a series of Fmoc‐protected dipeptide positional isomers using both positive and negative ion ESI‐MS/MS.…”

Characterization of Nα‐Fmoc‐protected ureidopeptides by electrospray ionization tandem mass spectrometry (ESI‐MS/MS): differentiation of positional isomers

“…These ions are absent for 3 . Instead, the latter shows an abundant ion at m / z 120 corresponding to the immonium ion of phenylalanine21 (Scheme ), a characteristic ion for the ureidopeptides derived from phenylalanine at the N‐terminus. A low abundance ion is observed at m / z 104 corresponding to the loss of combination of PhCH 2 CN and CO.…”

“…Furthermore, the MS 3 CID of m / z 201 of 5 and 7 gives (Supporting Information) low abundance ion at m / z 158 (loss of HNCO). Besides, the former shows an abundant ion at m / z 172 by the loss of CH 2 = NH (Scheme ) and the latter gives abundant ions at m / z 90 (loss of (CH 3 ) 2 CHCH 2 CN and CO) and m / z 86 corresponding to the immonium ion of leucine21 (Scheme ). Thus, the MS 3 spectra of m / z 201 ion of 5 and 7 are also useful to distinguish between the ureidopeptides derived from glycine and leucine at the N‐terminus.…”