Debate synthesis

Area Chair Brief for Editor-in-Chief

Manuscript overview

RIPUP is a methods paper combining Arg-C Ultra and r-Chymotrypsin digestion with TMT labeling for histone PTM analysis, benchmarked against Trypsin+propionylation. Headline claims: (1) faster (~3h) workflow with comparable/superior PTM coverage; (2) TMT's tertiary amine "rescues" detection of acidic acylations (succinylation, glutarylation) via charge compensation; (3) proof-of-concept application to rat hippocampal tissue (231 PTM sites). All five specialist reports converged at score 4 (ethics at 5, no compliance issues raised or debated).

Issue 1: Collision-energy confound in the charge-compensation mechanism (central to debate)

Evidence cited: Methods state fixed 30% NCE for non-TMT samples vs. stepped 30/40/50% NCE for TMT samples. The succinylation/glutarylation "dark epigenome" claim (58/31 sites, Abstract) rests on this TMT-vs-propionylation comparison.

Convergence note: This point was raised independently by data_analysis, reporting_reproducibility, and scientific_validity reviewers using different textual entry points (Methods parameters, mechanism narrative, missing unlabeled-vs-labeled comparison respectively). The skeptic argued this is genuine corroboration rather than model-multiplicity noise, since each reviewer located different supporting evidence for the same structural problem. This is a defensible reading, though the AC notes all three reviewers likely share the same underlying model and the convergence should be weighted as strong but not fully independent confirmation.

Advocate's case: Stepped NCE for TMT is standard practice (cited prior work, ref 23), not an ad hoc oversight; the empirical outcome (differential site counts) stands regardless of which mechanism is correct; the manuscript already frames the tertiary-amine explanation as inference, not fact.

Skeptic's case: The confound is stated in the authors' own Methods, not inferred by reviewers; it directly undermines the ability to attribute the effect to charge compensation rather than more thorough fragmentation; zero orthogonal or synthetic-peptide validation is offered anywhere to break the tie.

Concessions: Advocate conceded the confound is real and unresolved for isolating mechanism. Skeptic conceded the digestion-efficiency advantage of Arg-C Ultra (Figure 2C) is measured independently of this issue, and that the authors are candid about other limitations.

Status: Unresolved, load-bearing but not fatal to the paper as a whole. The confound is real and acknowledged by both sides; it specifically undermines the mechanistic explanation for the abstract's flagship discovery, not the descriptive outcome (TMT-labeled samples detect more succinylation/glutarylation sites). Resolution requires either matched-energy control experiments or explicit softening of causal language in the Abstract/Discussion. The skeptic noted a buildable-but-unperformed check (comparing unlabeled vs. propionylated samples, both at fixed NCE, from Figure 5A/5B data already in the manuscript) that could partially isolate energy from labeling — this remains an open, low-cost recommendation for revision.

Issue 2: Internal inconsistency between Abstract and Results on Arg-C Ultra+TMT vs. Trypsin+Prop

Evidence cited: Abstract states Arg-C Ultra+TMT "exceeds Trypsin-based approaches" in total PTM detection; Results state TMT-labeled samples achieved "comparable PTM numbers to conventional 'Trypsin + Prop' methods (~120 PTMs)" (flagged independently by scientific_validity reviewer, raised by skeptic in debate).

Advocate's case: Reads as imprecise Abstract wording summarizing a multi-condition comparison, not a fabricated number since the Results text is clear a few paragraphs later; resolvable by editorial revision.

Skeptic's case: This is a plain textual contradiction in the paper's central quantitative claim, not an interpretive dispute.

Concession: Advocate conceded this is a real inconsistency worth fixing.

Status: Resolved as fixable, not fatal. Both sides agree this is a wording/consistency problem requiring the authors to reconcile Abstract and Results language, not a deeper evidentiary failure.

Issue 3: Arg-C Ultra vs. Trypsin comparison confounded by labeling chemistry (raised in reports, not engaged in debate)

The contribution_context reviewer noted that Arg-C Ultra is only ever compared to Trypsin when paired with different labeling strategies (TMT vs. propionylation), and that the one same-label comparison available (Arg-C Ultra+Prop: 254 peptides vs. Trypsin+Prop: 179) shows only a modest advantage — undermining the strength of the protease-superiority claim independent of the labeling confound. This did not come up in the debate and stands unaddressed.

Issue 4: Missed-cleavage redistribution confounding the NAM quantitative experiment (raised in reports, not engaged in debate)

Both data_analysis and reporting_reproducibility reviewers raised that NAM treatment induces dose-dependent missed-cleavage redistribution, meaning fold-changes in the quantitative peptidoform analysis (Figure 7) may reflect cleavage-efficiency shifts rather than true PTM occupancy changes. Reviewers proposed a concrete check (restrict to zero-missed-cleavage peptidoforms, or stratify significant peptidoforms by modified/unmodified status). This is a substantive, independently-raised concern across two reports that never entered the debate and should not be read as resolved.

Issue 5: Lack of orthogonal/synthetic validation for succinylation and glutarylation sites (raised in reports, touched on but not resolved in debate)

Multiple reviewers (contribution_context, data_analysis, reporting_reproducibility, scientific_validity) independently flagged that the 58 succinylation and 31 glutarylation sites — the paper's headline novel finding — rest entirely on computational assignment with no synthetic peptide or orthogonal-method validation. The debate touched this only in the context of the NCE confound; the standalone absence of validation was not separately debated but is repeatedly noted across reports and compounds the NCE issue rather than being resolved by it.

Concerns from reports not engaged in debate at all

• Formylation presented as a possible finding but not experimentally distinguished from known sample-prep artifact (contribution_context, reporting_reproducibility).
• Rat hippocampal proof-of-concept (231 sites) lacks per-animal reproducibility breakdown and any comparison to prior literature or orthogonal method (multiple reviewers).
• Propionylation efficiency benchmark may be unfairly low due to buffer choice (ammonium bicarbonate vs. optimized TEAB-based protocols) (multiple reviewers).
• Cost analysis excludes instrument/researcher time and r-Chymotrypsin's uncertain "Early Access" pricing (contribution_context, scientific_validity).
• Dual-protease "complementary coverage" claim not quantified (unique vs. shared PTM sites per protease) (contribution_context, scientific_validity).