Journal of The American Society for Mass Spectrometry

, Volume 26, Issue 9, pp 1570–1579 | Cite as

Comparison of Ultraviolet Photodissociation and Collision Induced Dissociation of Adrenocorticotropic Hormone Peptides

Research Article

Abstract

In an effort to better characterize the fragmentation pathways promoted by ultraviolet photoexcitation in comparison to collision induced dissociation (CID), six adrenocorticotropic hormone (ACTH) peptides in a range of charge states were subjected to 266 nm ultraviolet photodissociation (UVPD), 193 nm UVPD, and CID. Similar fragment ions and distributions were observed for 266 nm UVPD and 193 nm UVPD for all peptides investigated. While both UVPD and CID led to preferential cleavage of the Y–S bond for all ACTH peptides [except ACTH (1-39)], UVPD was far less dependent on charge state and location of basic sites for the production of C-terminal and N-terminal ions. For ACTH (1-16), ACTH (1-17), ACTH (1-24), and ACTH (1-39), changes in the distributions of fragment ion types (a, b, c, x, y, z, and collectively N-terminal ions versus C-terminal ions) showed only minor changes upon UVPD for all charge states. In contrast, CID displayed significant changes in the fragment ion type distributions as a function of charge state, an outcome consistent with the dependence on the number and location of mobile protons that is not prominent for UVPD. Sequence coverages obtained by UVPD showed less dependence on charge state than those determined by CID, with the latter showing a consistent decrease in coverage as charge state increased.

Graphical Abstract

Keywords

Ultraviolet photodissociation Peptide Sequence coverage Collision induced dissociation 

Introduction

Significant advances in mass spectrometry for proteomics applications have evolved from the development of increasingly powerful informatics methods as well as new ion activation techniques and more sophisticated MS/MS strategies to improve the characterization of peptides in complex mixtures. The latter include deployment of real-time decision tree methods [1, 2], use of targeted ion monitoring methods [3, 4, 5], development of hybrid ion activation methods [6, 7, 8], and strategic ion manipulation based on ion–ion reactions [9, 10] and innovative ion charging concepts [11, 12]. There has also been growing effort to understand and optimize the fragmentation pathways of peptides to improve sensitivity, to enhance recognition of peptides in database searches, and, in some cases, to exploit preferential bond cleavages to provide greater specificity [13, 14, 15]. Although collision- and electron-based methods remain the most universally popular activation methods [16, 17, 18, 19, 20, 21], a number of alternatives (surface induced dissociation [22, 23], photodissociation [24, 25], high energy cation beam activation [26], and metastable atom activation [27]) have been developed to afford higher energy deposition and expand the arsenal of ways to energize ions to create meaningful fragmentation patterns.

Photon-based activation methods (termed photodissociation) offer considerable versatility, and this has led to a number of applications for analysis of peptides and proteins. These methods include infrared multiphoton dissociation (IRMPD) [25, 28, 29, 30, 31], ultraviolet photodissociation (UVPD) [25, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59], and visible photodissociation (Vis-PD) [25, 60, 61, 62]. The most common wavelengths used for UVPD of peptides include 157, 193, 266, 351, and 355 nm, each corresponding to ones readily generated by pulsed excimer or YAG lasers. The photodissociation process requires that the peptides of interest possess a suitable chromophore, either via an intrinsic chromophore (e.g., amide bond or side-chain groups of amino acids) or ones added via derivatization of the peptides prior to analysis [63, 64, 65]. For example, peptides do not naturally absorb photons around 350 nm, but they can be tagged with appropriate chromophores to make them undergo photodissociation, ultimately producing conventional b/y-type fragment ions [43, 44, 45]. This strategy has been reported for the identification of antigen binding regions of antibodies [44], for streamlining bottom-up proteomics [45], and has also shown to be useful in de novo sequencing by simplifying spectra [43]. The amide bond absorbs 157 and 193 nm photons, thus serving as the chromophore for 157 and 193 nm UVPD of peptides and proteins. The high energy per photon (7.9 eV for 157 nm and 6.4 eV for 193 nm) can elevate ions to excited electronic states, thus accounting for the production of a wide array of a, b, c, x, y, and z ions. Both 193 and 157 nm UVPD have been reported for numerous bottom-up proteomic applications and, more recently, for top down characterization of intact proteins [36, 37]. A detailed investigation of 157 nm UVPD demonstrated that some of the fragmentation pathways were radical directed [38]. Also, a recent study has shown that high energy fragmentation methods where electrons excited to almost ionized states precede C–C backbone cleavages, which lead to formation of a and x ions [39].

In contrast to 193 and 157 nm photons, 266 nm photons are not absorbed by the peptide backbone but rather by the amino acid side chains of tyrosine, tryptophan, and phenylalanine. Despite the lower energy per photon (4.6 eV), 266 nm UVPD also results in cleavages that lead to formation of a, b, c, x, y, and z ions, similar to the types of ions formed upon absorption of 157 or 193 nm photons [25]. Oh et al. reported the rich array of fragment ions upon 266 nm UVPD of protonated peptides and noted enhanced backbone cleavages adjacent to aromatic amino acids (tryptophan, phenylalanine, and tyrosine) [47]. The Kim group also showed that tryptic peptides derivatized by phenyl isothiocyanate reagents required lower laser powers for efficient photodissociation at 266 nm, thus confirming the enhanced photo-absorption cross-sections attributed to the added aromatic chromophores [49]. Kim and coworkers also analyzed phosphopeptides using 266 nm UVPD, which led to the production of highly characteristic an– 97 ions affiliated with each phosphorylated residue [57]. Park et al. used 266 nm UVPD to analyze phosphorylated peptides in an FTICR mass spectrometer, finding a consistent loss of 98 Da [50]. The Julian group has shown that absorption of 266 nm photons promotes selective homolytic cleavage of disulfide and C–S bonds, thus providing a means to map cysteine residues in peptides and proteins [51]. Ly and coworkers also utilized 266 nm photons to activate peptides or proteins containing iodinated tyrosines, leading to a process termed radical directed dissociation (RDD) which results in site-localized fragmentation and can be used as a spatially-specific probe of protein structure in the gas phase [53, 54, 56]. In another RDD strategy, Diedrich and Julian showed that phosphorylated sites in peptides could be pinpointed using a Michael addition reaction with naphthalenethiol followed by 266 nm UVPD to selectively cleave the C–S bond installed during the Michael addition reaction [56]. More recently, Tao et al. showed that D- and L-amino acids in peptides could be differentiated using an RDD process initiated by 266 nm UVPD [59].

This variety of photon-based activation methods has propelled interest in understanding the correlation between the photon wavelength and the outcomes of peptide activation in terms of dissociation efficiency, the types of fragmentation processes, and the potential for selective bond cleavages. Detailed comparisons of the fragmentation pathways and distributions of fragment ions arising from 157, 193, and 266 nm UVPD have not been extensively explored. Recently, Lai et al. compared the fragmentation caused by 193 nm UVPD and 266 nm UVPD for singly protonated peptides, including angiotensin (DRVYIHPF) analogues and bradykinin (RPPGYSPFR) [66]. They noted numerous similarities in the types of fragment ions (both the ones derived from amide and non-amide backbone cleavages and some side-chain cleavages) produced by 193 and 266 nm UVPD along with some variations in relative abundances [66]. Herein, we report the UVPD patterns of six adrenocorticotropic hormone (ACTH) peptides as a function of charge state and amino acid composition. ACTH peptides were chosen as they contain the three amino acids with aromatic side-chains (Phe, Trp, Tyr) that serve as the optimal chromophores for absorption of 266 nm photons (as well as secondary chromophores for 193 nm photons). Our goal is to extend the fundamental understanding of 266 nm UVPD in comparison with 193 nm UVPD and collisional activation.

Experimental

Materials

The six ACTH (human) peptides, ACTH (1-10, sequence SYSMEHFRWG, Mr 1298.4), ACTH (1-14, sequence SYSMEHFRWGKPVG, Mr 1679.9), ACTH (1-16, sequence SYSMEHFRWGKPVGKK, Mr 1936.3), ACTH (1-17, sequence SYSMEHFRWGKPVGKKR, Mr 2092.4), ACTH (1-24, sequence SYSMEHFRWGKPVGKKRRPVKVYP, Mr 2932.5), ACTH (1-39, sequence SYSMEHFRWGKPVGKKRRPVKVYPNGAEDESAEAFPLEF, Mr 4540.1), as well as alpha-bag cell peptide (1-9 sequence APRLRFYSL, Mr 1122.3), [Tyr5] Bradykinin (sequence RPPGYSPFR Mr 1076.2), [Phe2, Nle4] ACTH (1-24 sequence SFS(Nle)EHFRWGKPVGKKRRPVKVYP, Mr 2899.5), and adamtsostatin-16 (sequence SPWSQCATSCGGGVQTR with a disulfide between the two cysteines Mr 1722.9) were purchased from American Peptide Company (Sunnyvale, CA, USA). LC-MS grade acetonitrile and water were obtained from EMD Millipore (Darmstadt, Germany). LC-MS grade formic acid was purchased from Fisher Scientific (Fair Lawn, NJ,USA).

Mass Spectrometry

All experiments were carried out using a Thermo Velos Pro dual linear ion trap mass spectrometer (Thermo Scientific, San Jose, CA, USA) or a Thermo Orbitrap Elite mass spectrometer (Thermo Scientific, Bremen, Germany). The dual linear ion trap was used for the MS/MS comparisons based on 266 nm UVPD, 193 nm UVPD, or CID, and the Orbitrap instrument was used primarily to confirm specific fragment ion assignments based on high resolution, high accuracy measurements. The dual linear ion trap was modified to allow UVPD with a Continuum Minilite Nd:YAG laser (Santa Clara, CA, USA) set to 266 nm or a Coherent Excistar excimer laser (Santa Clara, CA, USA) set to 193 nm. The set-up and implementation of UVPD was similar to that described previously [31]. Peptides were diluted in a 50/50 acetonitrile/water solution with 0.1% formic acid to a final concentration of 1 μM and were infused at a flow rate of 3 μL/min. CID was performed on every observed charged state of each of the six ACTH peptides using a normalized collision energy of 35%. 266 nm UVPD was performed on each charge state observed for the six ACTH peptides using 1, 2, 5, or 10 pulses (nominally 6 mJ per pulse) at a 10 Hz laser pulse rate. 193 nm UVPD was also performed for all observed charge states of the six ACTH peptides using 2 pulses and 2 mJ per pulse at 500 Hz laser pulse rate. Only a fraction of the light enters the ion trap because neither focusing nor collimating optics is used, and the laser beam is divergent as it emerges from the laser. Due to the relatively low photon flux through the ion trap, the extent of secondary (or consecutive) fragment ion dissociation is minimal.

Analysis of Spectra

All CID and UVPD spectra were analyzed manually. Lists of fragments were obtained using both Protein Prospector v. 5.12.2 (UCSF) and Proteomics Tool kit (Institute for Systems Biology). Data for the fragment ion distributions was processed using Microsoft Excel. Abundance information was extracted from the raw data and used to calculate the percentage distributions for each type (b and y for CID and a, b, c, x, y, and z for UVPD).

Results

UVPD using 266 nm photons has been less frequently employed for analysis of peptides than 193 nm UVPD, and it seemed particularly important to evaluate the fragmentation induced by 266 nm photon absorption as well as the similarities and differences to that caused by 193 nm photon absorption. In order to investigate the variations in peptide fragmentation obtained by 266 nm UVPD, each charge state of the six ACTH peptides was activated using one to ten laser pulses. MS/MS spectra were also acquired by conventional CID and by 193 nm UVPD (two pulses) to compare the types and distribution of fragment ions obtained by each of the three activation methods. Examples of the MS/MS spectra produced by 266 nm UVPD (five laser pulses), 193 nm UVPD, and CID are shown in Figure 1 for ACTH (1-17) (3+ charge state). Both the 266 and 193 nm UVPD spectra show a rich array of fragment ions, ranging from the conventional a/b/y ions observed upon CID to c/z ions typically observed upon electron-activated (radical type) dissociation and x ions that are only commonly seen upon UVPD. The fragment ions in the visually congested UVPD spectra can be confidently assigned based on high accuracy measurements. Supplemental Figure 1 shows an expansion of the region from m/z 760 to 890 for peptide ACTH (1-24) (4+). UVPD (both 266 and 193 nm) and CID mass spectra for the other ACTH peptides are shown in Supplemental Figures 2–6. Histograms summarizing the distribution of fragment ions obtained by 266 nm UVPD, 193 nm UVPD, and CID are shown in Figure 2 for ACTH (1-17) for each of its five charge states (2+, 3+, 4+, 5+, 6+), and additional histograms for all of the MS/MS data for each activation method are shown in Figures 3 (266 nm UVPD), 4 (193 nm UVPD), and 5 (CID). The most prominent fragment ions produced by each activation method for each charge state of each peptide are summarized in Supplemental Table 1.
Figure 1

Examples of 266 nm UVPD, 193 nm UVPD, and CID mass spectra of ACTH (1-17, sequence SYSMEHFRWGKPVGKKR, Mr 2092.4), 3+ precursor ion. (a) 266 nm UVPD (five pulses; six mJ per pulse), (b) 193 nm UVPD (two pulses; 2 mJ per pulse), and (c) CID (NCE 35). The precursor ion is labeled with an asterisk. There is little surviving precursor in the CID spectrum

Figure 2

Distribution of 266 nm UVPD, 193 nm UVPD, and CID fragment ions for all observed charge states of ACTH (1-17, sequence SYSMEHFRWGKPVGKKR, Mr 2092.4). Five pulses at 6 mJ per pulses was used for 266 nm UVPD, two pulses at 2 mJ per pulse was used for 193 nm UVPD, and 35 NCE was used for CID

Figure 3

Distribution of 266 nm UVPD fragment ions for all observed charge states of (a) ACTH (1-10), (b) ACTH (1-14), (c) ACTH (1-16), (d) ACTH (1-17), (e) ACTH (1-24), and (f) ACTH (1-39). UVPD was carried out using five pulses at 6 mJ per pulse

Based on the graphical distributions shown in Supplemental Figures 7 and 9 and the representative series of spectra in Supplemental Figure 8, it is clear that the number of laser pulses has a relatively modest impact on the overall distribution of fragment ions for our UVPD setup. Visual comparison of the spectra in Supplemental Figure 8 confirms that no new abundant ions are produced with an increasing number of laser pulses, suggesting that the majority of fragment ions are produced directly from the precursor ion, not via secondary dissociation of primary fragment ions. (This outcome depends on the laser power and the overlap of the laser beam with the ion cloud, so it would be expected to be instrument-dependent). For all spectral comparisons discussed in the next section, five laser pulses were used for the 266 nm UVPD spectra and two laser pulses were used for the 193 nm UVPD spectra.

Discussion

The MS/MS spectra shown for ACTH (1-17) in Figure 1 and the stacked histograms in Figure 2 highlight some of the characteristic features observed for each activation method. The distributions of N-terminal (a, b, c) versus C-terminal ions (x, y, z) ions produced by 266 nm UVPD versus 193 nm UVPD are remarkably similar for each ACTH peptide in every charge state. For example, for the longer peptides [e.g., ACTH (1-24) and ACTH (1-39)], the portion of C-terminal ions formed upon 266 nm UVPD or 193 nm UVPD remains around 50%–55%, regardless of charge state, and the histogram graphs in Figure 3 (for 266 nm UVPD) and Figure 4 (for 193 nm UVPD) mirror each other for each peptide. The distribution of C-terminal versus N-terminal ions changes more dramatically for the shorter ACTH peptides [such as ACTH (1-10) and ACTH (1-14)], for which the portion of C-terminal product ions consistently increases from approximately 30% for the singly charged peptides to upwards of 75% for the 3+ charge state. An increase in the portion of C-terminal ions with increasing charge state is similarly observed for ACTH (1-10), ACTH (1-14), ACTH (1-16), and ACTH (1-17) for both UVPD methods. This increase in the portion of C-terminal (x, y, z) ions with increasing charge state is likely attributed to the predominance of basic sites on the second half of the peptide sequence, thus increasing the probability of charge retention for the C-terminal ions, but this effect does not necessarily reflect charge-modulated fragmentation directed by mobile protons as anticipated for CID. For the larger ACTH peptides, fluctuations in the distribution of N-termini versus C-termini ions upon UVPD are less notable, with variations of less than 10% for most of the ion types (a, b, c, x, y, z).
Figure 4

Distribution of 193 nm UVPD fragment ions for all observed charge states of (a) ACTH (1-10), (b) ACTH (1-14), (c) ACTH (1-16), (d) ACTH (1-17), (e) ACTH (1-24), and (f) ACTH (1-39). UVPD was carried out using two pulses at 2 mJ per pulse

The distributions of fragment ions produced by CID are shown in the stacked histograms of Figure 5. The CID results are much more dependent on the initial charge state of the precursor peptide, with swings as much as 50% for C-terminal versus N-terminal ion types [e.g., 32% C-terminal ions for doubly-protonated ACTH (1-14) versus 82% C-terminal ions for triply protonated ACTH (1-14)] and distributions that seesaw dramatically depending on the precursor charge state [e.g., 65% C-terminal ions for singly protonated ACTH (1-14), 32% C-terminal ions for doubly-protonated ACTH (1-14), and 82% C-terminal ions for triply-protonated ACTH (1-14)]. The shifts and seesaws in the C-/N-terminal fragment ion distributions for the CID data as a function of precursor charge state are not surprising. CID pathways are known to be highly charge-mediated with the number of mobile protons (i.e., ones not sequestered at highly basic sites) dictating the preferred fragmentation processes [67]. The lack of significant variation in the C-terminal/N-terminal fragment ion distributions for either 266 nm UVPD or 193 nm UVPD reflects the reduced impact of proton-mediated pathways and the greater contribution from dissociation processes that may occur directly from excited states and/or radical-modulated ones. In addition, the rich diversity of UVPD fragment ions, as evidenced by the multi-segmented bars in Figures 2, 3 and 4, underscores the wide variety of competing (and possibly consecutive) pathways that minimize substantial variations as a function of charge state.
Figure 5

Distribution of CID fragment ions for all observed charge states of (a) ACTH (1-10), (b) ACTH (1-14), (c) ACTH (1-16), (d) ACTH (1-17), (e) ACTH (1-24), and (f) ACTH (1-39). CID was carried out using NCE of 35

The MS/MS spectra for the ACTH peptides are frequently dominated by several highly abundant fragment ions, which are indicative of preferential cleavages. The most predominant fragment ions produced by UVPD and CID are compiled in Supplemental Table 1 for each charge state of each ACTH peptide. For ACTH (1-10), several of the most dominant fragment ions, namely a2, y4, y5, and y8, are observed for all three activation methods, with cleavage of the backbone at the Y–S site leading to y8 and a2 ions being particularly enhanced. The UVPD spectra exhibit special enhancement of a ions that are not favored in the CID spectra, and these may arise from secondary dissociation of b ions formed with excess internal energy or ones produced directly from the precursor peptide, potentially via dissociation of ions in excited electronic states. The similarities between the 266 nm UVPD and 193 nm UVPD spectra (Figures 1, 3, and 4), both in terms of the identities and distributions of fragment ions, is remarkable. The level of similarities even extends to the formation of a few low abundance z-type fragment ions, which are absent from the corresponding CID mass spectra. Apparently absorption of 266 nm (4.7 eV) or 193 nm (6.4 eV) photons allows access to excited states (whether the same ones or different ones for each of these wavelengths), and the activation process leads to similar fragmentation processes. Absorption of 193 nm photons may occur at the amide backbone as well as the aromatic side chains of W, Y, and F (the sites where 266 nm photons are absorbed), with W having a greater photoabsorption cross-section than Y or F [48]. From previous studies of UVPD, it is believed that dissociation may occur directly from excited electronic states as well as after internal conversion and intramolecular vibrational redistribution [39, 68]. Based on the present results that show a mixture of CID-like fragment ions (b/y) and radical-directed fragment ions (a/x, c/z), it appears that UVPD involves more than one type of process.

The UVPD and CID spectra collected for ACTH (1-14) showed an even more dramatic preferential cleavage of the Y–S bond, leading to the dominant y122+ ion for the 3+ and 4+ charge states (and complementary a2 and b2 ions). The lower charge states (1+, 2+) instead exhibited enhanced b11 and b13 ions upon 266 nm UVPD and CID, with the former representing a proline-type cleavage that also accounts for the prominent complementary y3 ion. Interestingly, despite the overlap in the sequences of ACTH (1-10) and ACTH (1-14), the resulting MS/MS spectra had few similarities, an outcome that is largely attributed to the C-terminal ion dominance in the spectra that masks the more subtle variations in the N-terminal ion abundances.

For ACTH (1-16), the Y–S cleavage (formation of y14) remained the most consistently prominent process upon UVPD and CID for the higher charge states (3+, 4+, and 5+) (see Supplemental Figure 4). Cleavage of the backbone between K–K (resulting in b15) was also a significant pathway, which is a hallmark charge-modulated process common for both UVPD and CID. Selected a, c, and z ions were observed in the UVPD (266 and 193 nm) spectra, which again reflected the greater diversity of pathways upon UV photoactivation.

ACTH (1-17) is an interesting example, having just a single additional arginine residue at the C-terminus compared with ACTH (1-16). The resulting UVPD mass spectra (Figure 1) exhibited many of the same preferential cleavage sites as observed for ACTH (1-16), such as the enhanced Y–S cleavage and cleavage adjacent to a basic residue. Similar to ACTH (1-16), the ACTH (1-17) ions in lower charge states (2+ and 3+) underwent a dominant cleavage of the last amino acid in the sequence (K–R bond) resulting in the formation the b16 ion. UVPD of ACTH (1-17) also show prominent formation of the y15 ions, which corresponded to the Y–S cleavage for 3+, 4+, 5+, and 6+ charge states, as well as a modest increase in C-terminal ions as the charge state increased.

Four charge states (3+, 4+, 5+, and 6+) were observed for ACTH (1-24), and representative fragmentation patterns generated by UVPD and CID are shown in Supplemental Figure 5. The relative portion of N-terminal ions compared with C-terminal ions produced by UVPD was greater for all charge states compared with the distributions observed for the shorter ACTH peptides. Again, the Y–S bond cleavage was particularly favored, yielding the y22 (and a2) ions noted previously. The a22 ion, arising from a V–Y backbone cleavage, was a significant product ion upon UVPD, and it has been observed previously that cleavages C-terminal to valine and N-terminal to tyrosine may be enhanced [66].

As the largest peptide in the series, ACTH (1-39) generated five charge states (3+, 4+, 5+, 6+ and 7+) upon ESI and some rather unique results for both 266 nm UVPD and CID (Supplemental Figure 6). This peptide sequence is unusual in that there are no basic sites found within the final 18 residues (C-terminus). For 266 nm UVPD the cleavage of the S–Y bond was not observed for any charge state; in fact there was no consistent preferential cleavage for any of the charge states upon UVPD. CID resulted in the dominant formation of a b35 ion, which arises from cleavage of the F–P bond, a process consistent with the well-known rule of peptide cleavage N-terminal to proline. N-terminal b ions were highly favored upon CID of all charge states, presumably because the most basic charge sites, which were clustered closer to the C-terminus for the shorter peptides, are actually situated in the mid-region of ACTH (1-39). This means that cleavages at backbone sites in the mid-section of the peptide more likely result in fragment ions with protons localized in the first 21 residues, thus enhancing N-terminal ions (such as the dominant b29, b33, b35, b38 ions, among others, noted in Supplemental Table 1). Interestingly, the distribution of N-terminal versus C-terminal ions for ACTH (1-39) is almost evenly split (50%/50%) upon 266 nm UVPD and 193 nm UVPD. In contrast, N-terminal fragments make up 70%–75% of the fragment ions upon CID. The C-terminal ions that are observed are long ones that contain basic residues from the N-terminal half of the peptide. The notable differences in C-terminal versus N-terminal fragment ion distribution further supports the lower degree of charge-site dependence on the fragmentation pathways observed upon UVPD.

In an effort to elucidate the reason for the apparent preferential cleavage of the Y–S bond observed upon activation of the ACTH peptides, three additional peptides were subjected to CID, 193 nm UVPD, and 266 nm UVPD. The dominant Y–S cleavage could be due to its position in the peptide (the second and third residues), a special lability of the Y–S bond, or a chromophore effect (although it is noted that this bond cleavage is also observed upon CID, not just UVPD). Bradykinin [Tyr5] and alpha-bag cell peptide were chosen because they both have a Y–S bond in the middle of the peptide or near the C-terminus rather than near the N-terminus. The cleavage of the Y–S bond was fairly prominent for both peptides and was only surpassed by proline cleavages (Supplemental Figures 10 and 11), giving evidence that the preferential Y–S cleavage observed for the ACTH peptide is not solely due to location of the Y–S residues near the N-terminus. Another peptide selected was ACTH (1-24) [Phe 2 Nle 4] for which the Tyr in ACTH at position 2 is replaced by Phe. The fragments observed from both UVPD and CID for this peptide are nearly identical to those seen for the original version of the ACTH (1-24) peptide, with the y22 and a2 ions produced in great abundance (Supplemental Figure 12). The fact that cleavage between the F–S residues parallels the preferential cleavage observed for the Y–S residues suggests that the preferential cleavage may arise from an aromatic residue/Ser motif. The third peptide analyzed was adamtsostatin-16, which contained a W and S residues at the third and fourth positions, respectively. The resulting UVPD and CID mass spectra are shown in Supplemental Figure 13. In this case, cleavage of the W–S bond is not overly favored. In total, it appears that a new preferential cleavage associated with X–S in which X is Y or F occurs upon UVPD or CID, a feature that might be prevalent in larger scale bottom-up proteomic studies.

Among the series of ACTH peptides, the UVPD spectra of the smaller peptides were more “CID-like” in terms of the portion of a, b, and y ions, particularly for ACTH (1-10) (3+) and ACTH (1-14) (4+). For the longer ACTH peptides and/or ones in lower charge states, the UVPD spectra exhibited the greater diversity of fragment ions that is the established hallmark of UV photoactivation. In terms of sequence coverages (i.e., expressed as a percentage based on the number of backbone cleavages relative to the total number of backbone positions), UVPD and CID typically gave similar coverages for the lower charge states of each peptide (see values in Supplemental Figures 1418).

However, for higher charge states, UVPD generally yielded higher coverages compared with CID, especially for 193 nm UVPD. For example, the 2+ charge state of ACTH (1-16) yielded a sequence coverage of 87% (13 backbone cleavage sites out of 15 sites) for both CID and 266 nm UVPD. However, for the 5+ charge state the sequence coverage decreased to only 33% for CID and 47% for 266 nm UVPD. The sequence coverage obtained upon 193 nm UVPD showed less variation, ranging from 67% to 100% across the four charge states. For ACTH (1-17), Figure 6 shows a sequence coverage map for all five charges states for 193 nm, 266 nm, and CID fragmentation. A maximum of 75% coverage was obtained by CID and 266 nm UVPD for the lowest 2+ charge state, whereas the coverage decreased to 38% for CID and 56% for 266 nm UVPD of the highest 6+ charge state. The sequence coverage produced by 193 nm UVPD was again less charge-state dependent, ranging from 75% to 88% for ACTH (1-17). As anticipated, the level of coverage varied more dramatically for CID as a function of charge state compared with UVPD. For instance, the sequence coverage ranged from 70% (3+) to 26% (6+) for CID of the ACTH (1-24) peptide, whereas the range varied from 65% (3+) to 52% (6+) for 266 nm UVPD and from 74% (3+) to 61% (6+) for 193 nm UVPD of the same peptide. The variation in sequence coverage was most notable for the longer ACTH peptides, those that contained basic lysine or arginine residues at the C-terminus or in the middle of the sequences. The change in sequence coverage obtained by CID for the longer peptides as a function of charge state was not surprising given the fact that CID is more highly modulated by proton mobility than UVPD.
Figure 6

Sequence coverage maps for observed ions in all observed charge states of ACTH (1-17). Any backbone cleavage site producing N-terminal a, b, c ions is represented by ┐. Any backbone cleavage producing C-terminal x, y, z ions is represented by └. Cleavages that result in both N-terminal a, b, c ions and C-terminal x, y, z ions are designated by Open image in new window

Conclusions

Both 266 nm UVPD and 193 nm UVPD generated similar fragment ion distributions for each ACTH peptide, spanning a variety of a, b, c and x, y, z ions. Comparison of the MS/MS results for ACTH (1-10), ACTH (1-14), ACTH (1-16), ACTH (1-17), and ACTH (1-24) revealed that UVPD and CID consistently showed preferential cleavage of the Y–S bond for nearly every charge state. For all of the peptides, the production of C-terminal versus N-terminal ions and overall sequence coverage was far more dependent on the charge state and location of basic sites for CID than for UVPD, an outcome that reflects the reduced prominence of charge-mediated pathways for UVPD. In general, UVPD of the longer peptides [ACTH (1-16), ACTH (1-17), ACTH (1-24), and ACTH (1-39)] showed relatively little change in the distributions of fragment ion types as a function of the charge state of the peptide; there were much greater changes observed for CID. UVPD demonstrated a modest degree of preferential cleavage adjacent to amino acids containing aromatic side chains, suggestive of a chromophore effect, as well as enhanced cleavages adjacent to proline akin to the well-known proline effect documented for CID. This systematic comparative study has demonstrated many similarities in the types and distributions of fragment ions produced by 266 nm UVPD and 193 nm UVPD, with greater overall sequence coverage afforded by 193 nm UVPD. A comparison of the results for 193 nm and 266 nm UVPD shows that both provide better sequence coverage compared with CID for higher charge states of larger peptides; 193 nm UVPD provides better coverage on average than 266 nm UVPD, and the sequence coverages obtained by UVPD did not exhibit the significant dependence on charge state that was observed upon CID.

Notes

Acknowledgments

The authors acknowledge funding from the NSF (CHE-1402753) and the Welch Foundation (F-1155).

Supplementary material

13361_2015_1186_MOESM1_ESM.pdf (3 mb)
ESM 1(PDF 3103 kb)

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© American Society for Mass Spectrometry 2015

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of TexasAustinUSA

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