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3.4. Identification of high molecular weight proteins By “on polymer”digestion
To verify tentative protein identifications on PE-UHMW material, adsorbed proteins have to been zymatically digested
and the generated peptides have to be sequenced by mass
spectrometry. To perform enzymatic treatment comparable
to “on-tissue” digestion described in literature, the possibility of analyte delocalization after trypsin application was
investigated. Using the piezo printer a20 pM albumin solution was applied at a resolution of 100 m covering 25 mm2
homogeneously on an ITO surface and dried at room temperature. Trypsin was applied in a small rectangular shape
(2×5mm,100 m resolution) in the central area of the albumin square with the piezo printer before the sample was
incubated overnight at37 ◦C in an atmosphere saturated
with ethanol/water(1:1,v/v). Solvents were removed on the
next day by vacuum drying and MALDI matrix was applied
as described again with the piezo printer. In MSI analysis
the localization of albumin related peptides was investigated
(Fig.5). Albumin wasidentified by peptide mass fingerprinting
and selected peptides further identified by PSD fragmentation. It could be demonstrated that albumin fragments were
primary located in the area of previous trypsin application at
a lateral resolution of 80 m. The intensity distribution of the
non-normalized data set shown in Fig.5 reveals several signals outside the defined area, which can be related to artifacts
or slight peptide diffusion. Data normalization based on the
TIC levels those signals out and reveals them as noise.
After proofing the peptide localization and protein identification process of the MSI approach with albumin, proteins
adsorbed to PE-UHMW were investigated.For enzymatic
treatment of PE-UHMW after synovia incubation,trypsin
was applied on the total polymer surface. For this, buffer
adjustment turned out to be very critical as PE-UHMW is
sensitive to basic pH and thus polymer hydrolysis can occur
already at pH8.5 (a pH usually adjusted forefficient tryptic digestion).The addition of Rapigest solution to enhance
proteolytic degradation by denaturing proteins allowed pH
adjustment to7.5.At this pH value sufficient enzymatic cleavage of protein was observed while polymer hydrolysis was
prevented.
During the MALDI process PE-UHMW acts as an insulating
material. This effect usually results in peak broadening due
to increased energy transfer for ion desorption/ion ization and
therefore increased energy distributions as well asdecreased
ion extraction efficiencies.While this disadvantageous behavior is not critical for intact protein analysis as pointed out
before,it clearly limits the possibilities of unambiguous
protein identification because of low absolute signal intensities and partialloss of mass resolution for the generated
protonated peptides.Onaverage120m/z signals were obtained
within each mass spectrum generated for each position
of the MSI experiment. Mass spectra were compared to
m/z lists derived from in silico digests of already identified
proteins. It was possible to correlate m/z values with peptides belonging to proteins presentin synovia. Fig.6 shows
an exemplary profile spectrum of an“ on-polymer” digest
and the annotation of tryptic peptides for each identified
protein.
For unambiguous identification peptides were fragmented
By PSD experiments, however, signal intensities were only
sufficient to generate neutral losses from the precursor ion
and fragments related to the first few amino acids from the
termini, which was not sufficient for automated data base
identification but confirmed protein identity when PSD spectra were interpreted manually.
Detected peptide signals were again analyzed for their
distribution on the polymer samples. Similar localization preferences were found as for the respective proteins. Peptide
distribution was particularly correlated with folded PE-UHMW
regions, rough areas and sharp edges. Fig.7 shows the distribution of selected peptide signals associated with albumin
(m/z 1024, [500–508]k. CCTESLVNR.r) and apolipoprotein E(m/z
1104, [97–106]k.RLAVYQAGAR.e).The list of associated peptide fragments(Fig.1) includes all high abundance proteins
and the majority of low abundance proteins were also found
in SDSPAGE analysis.
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