distillation №8
Selank — C-terminal amidation of Pro-7 (free -COOH → -CONH2)
3D structure
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AI analysis
tldr
Fold #8 distills C-terminal amidation of Selank (TKPRPGP → TKPRPGP-NH2), a modification hypothesized to block carboxypeptidase-mediated degradation and extend plasma and CNS half-life. The structural prediction returned a high-confidence backbone (pLDDT 0.90), confirming that amidation does not disrupt the N-terminal tuftsin pharmacophore or the Pro-rich geometry of the C-terminal glyprolin motif. No receptor complex was modeled, so binding impact remains unquantified. The signal is promising but gated by fundamental pharmacokinetic unknowns that only wet-lab work can resolve.
detailed analysis
Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) is a synthetic heptapeptide derived from the endogenous immunomodulatory tetrapeptide tuftsin, extended with a C-terminal Pro-Gly-Pro glyprolin motif that was itself designed to improve metabolic stability over the parent compound. Its pharmacological profile spans anxiolysis, nootropic enhancement, BDNF regulation, GABAergic modulation, and immunomodulation — all effects attributable, at least in part, to the N-terminal Thr-Lys-Pro-Arg pharmacophore engaging a putative tuftsin receptor (candidate: Neuropilin-1, though molecular identity remains contested). The C-terminal extension is understood to confer structural resilience, yet carboxypeptidase-mediated trimming of terminal proline residues remains a plausible and largely uncharacterized degradation route in plasma and CNS.
The modification tested in this distillation — C-terminal amidation of Pro-7 (free -COOH → -CONH2) — addresses this vulnerability by neutralizing the terminal carboxylate, eliminating it as a carboxypeptidase substrate. This is one of the most well-precedented pharmaceutical modifications in peptide chemistry, appearing in endogenous and therapeutic peptides including oxytocin, LHRH analogues, and numerous neuropeptides. The modification also removes a negative charge at physiological pH, which is expected to modestly increase lipophilicity and passive membrane permeability — potentially relevant for a peptide administered intranasally and required to traverse the blood-brain barrier.
This fold is a natural extension of the lab's prior work. Fold #1 explored N-terminal acetylation of Semax — another Pro-rich cognitive peptide — achieving a refined verdict (pLDDT 0.80) and establishing the principle that capping the N-terminus preserves backbone geometry while potentially blocking aminopeptidase access. Fold #8 mirrors that logic at the opposite terminus for Selank, making the two folds orthogonally complementary: together they bracket the N→C exopeptidase vulnerability space for this class of proline-rich cognitive peptides. The structural prediction here returned an even higher confidence score (pLDDT 0.90), consistent with the expectation that a terminal amide substitution on a short, conformationally constrained Pro-rich heptapeptide will not perturb the backbone.
The predicted structure shows the canonical extended/polyproline-II-like geometry expected from a Pro-rich heptapeptide. The N-terminal Thr-Lys-Pro-Arg pharmacophore is preserved in conformation, and the C-terminal Pro-Gly-Pro segment retains its turn-like character. The pTM of 0.168 reflects the absence of a receptor chain — this is a monomer-only prediction and carries no docking or interface information. No Chai-1 agreement or Boltz-2 affinity data were generated, meaning we cannot make any quantitative claim about how amidation affects receptor binding affinity. This is the primary limitation of this fold.
Heuristic sequence-based profiling returns a stability score of 0.696 (moderate), aggregation propensity of 0.0 (favorable), a BBB penetration score of 0.102 (low but not negligible for a 7-mer administered intranasally), and a half-life estimate of 15–45 minutes (short). These are algorithmic estimates derived from sequence composition, not experimental measurements, and should be interpreted with proportional skepticism. The half-life estimate for the amidated form was not compared computationally against native Selank in this run — the delta remains theoretical.
The literature assessment identifies a critical evidentiary gap that limits interpretation of this fold: no published study has directly measured Selank's plasma or CNS half-life, nor characterized whether carboxypeptidase-mediated C-terminal cleavage is actually the rate-limiting degradation step. If endopeptidase activity elsewhere in the sequence dominates degradation kinetics, C-terminal amidation will fail to meaningfully extend half-life regardless of how well the structural prediction looks. Additionally, the Pro-Gly-Pro glyprolin motif may have independent functional contributions beyond metabolic stabilization — if it engages proline-recognizing binding domains on target proteins, amidation of the terminal Pro could alter those interactions unpredictably.
The verdict is PROMISING rather than REFINED because the structural signal is strong but the biological interpretation is gated by gaps that in silico tools cannot bridge in this run. The modification is chemically rational, structurally sound by prediction, and supported by broad pharmaceutical precedent. But without a receptor-complex prediction, a direct pharmacokinetic comparator, or any published SAR data for Selank C-terminal analogues, the functional hypothesis remains plausible but unconfirmed. The next logical steps — receptor docking, in vitro stability assay, and head-to-head PK comparison against native Selank — are clearly defined.
research data
known activity
// not yet provided by clinical agent
biohacker use
// not yet provided by clinical agent
mechanism class
// not yet provided by clinical agent
AI research brief
Selank C-terminal amidation predicts a clean, high-confidence structure (pLDDT 0.90) with pharmacophore intact. The half-life extension hypothesis is rational but unanchored — no PK data exist for native Selank, and no receptor complex was modeled. PROMISING: structurally sound, biologically unverified.
Fold #8 — C-terminal Amidation of Selank (TKPRPGP-NH2)
Verdict: PROMISING | pLDDT: 0.90 | Modification: Pro-7 -COOH → -CONH2
Mechanism of action
Selank is a synthetic heptapeptide (Thr-Lys-Pro-Arg-Pro-Gly-Pro) built on the scaffold of tuftsin, an endogenous tetrapeptide (Thr-Lys-Pro-Arg) with immunomodulatory and neuromodulatory properties. The N-terminal Thr-Lys-Pro-Arg sequence constitutes the primary pharmacophore, putatively engaging a tuftsin receptor — the molecular identity of which remains debated, with Neuropilin-1 (NRP1) as a leading candidate. Downstream effects attributed to this receptor engagement include:
- GABAergic modulation: Indirect or allosteric, not via direct GABA-A receptor binding (PMID:26924987, PMID:28293190). The mechanism appears context- and cell-type-dependent.
- BDNF regulation: Selank prevents ethanol-induced dysregulation of BDNF in hippocampus and prefrontal cortex (PMID:31625062) and is classified among neuroactive peptides that upregulate neuroplasticity pathways (PMID:41490200).
- Anxiolysis and stress protection: Demonstrated across multiple rodent models. A single IP dose of 0.3 mg/kg reduces morphine withdrawal index by ~40%, approaching the effect of diazepam (PMID:36322304).
- Immune and hepatic modulation: Anti-inflammatory and hepatoprotective effects documented in rodent models (PMID:31243679, PMID:32621722).
- Acute CNS effect in humans: fMRI confirms measurable functional connectivity changes within 5–20 minutes of intranasal administration in healthy volunteers (PMID:32342318).
The C-terminal Pro-Gly-Pro glyprolin extension was designed to improve metabolic stability relative to native tuftsin; this fold asks whether further stabilization via terminal amidation extends that benefit.
Performance applications
The intended application domain remains unchanged from native Selank: cognitive enhancement, anxiolysis, stress resilience, and potentially neuroprotection in the context of acute stress, learning consolidation, and neuroplasticity support. The amidated analogue is not hypothesized to produce qualitatively different effects — rather, the same pharmacological actions delivered with greater temporal persistence, potentially enabling:
- Lower effective doses due to prolonged receptor exposure
- Less frequent dosing in multi-day protocols (consistent with the 7–20 day regimens used in efficacy studies)
- Improved CNS bioavailability through marginally increased lipophilicity from charge neutralization
No new mechanism or indication is claimed for the modified peptide.
Modification rationale
C-terminal amidation (replacement of the free -COOH with -CONH2) at Pro-7 was selected for three converging reasons:
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Carboxypeptidase blockade: Terminal proline residues are substrates for prolyl carboxypeptidase and ACE-like enzymes. Amidation removes the carboxylate required for carboxypeptidase recognition, eliminating this degradation route by analogy to established pharmaceutical practice.
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Charge neutralization: Removing the C-terminal negative charge at physiological pH is expected to reduce electrostatic repulsion from membrane surfaces, modestly increasing passive permeability — consistent with BBB penetration principles for therapeutic peptides.
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Orthogonal strategy to Fold #1: Fold #1 demonstrated that N-terminal acetylation of Semax — another Pro-rich cognitive peptide of the same class — produced a REFINED result (pLDDT 0.80) with preserved backbone geometry. Amidating the C-terminus of Selank is the logical complementary maneuver: together, these two approaches bracket the full exopeptidase vulnerability window (N-terminal aminopeptidases + C-terminal carboxypeptidases) for proline-rich cognitive peptides. The lab is building a consistent modification grammar for this peptide class.
The modification is chemically minimal: it changes a single terminal functional group without introducing new stereocenters, altering the backbone, or affecting the charge state of any other residue.
Predicted properties (where signal is moderate)
| Property | Native Selank (estimated) | TKPRPGP-NH2 (predicted) | Confidence |
|---|---|---|---|
| Backbone conformation | Extended/PPII-like | Preserved (pLDDT 0.90) | High |
| N-terminal pharmacophore geometry | Intact | Intact | High |
| Aggregation propensity | Low | 0.0 (heuristic) | Low-moderate |
| Stability score | ~0.65–0.70 | 0.696 (heuristic) | Low |
| BBB penetration score | ~0.09–0.11 | 0.102 (heuristic) | Low |
| Half-life estimate | Short (15–45 min) | Short (15–45 min, with theoretical gain) | Very low |
| Receptor binding affinity | Unknown | Not modeled (no complex run) | N/A |
Where the signal is strong: The structural prediction is high-confidence. pLDDT 0.90 for a 7-mer is excellent and confirms that amidation does not deform the backbone or disrupt the Pro-rich geometry of the pharmacophore. This is the expected result for a terminal functional group change on a conformationally constrained short peptide, and it validates the structural safety of the modification.
Where the signal is moderate: The heuristic property estimates (stability, BBB penetration, half-life) are sequence-derived algorithmic outputs, not experimental or simulation data. The half-life estimate does not distinguish native from amidated Selank — the gain is theoretically expected from carboxypeptidase blockade but was not quantified in this run.
Where there is no signal: Receptor binding impact. No complex was modeled; no affinity delta was computed. Whether -CONH2 at Pro-7 improves, preserves, or subtly disrupts binding to the tuftsin receptor / NRP1 is entirely unknown from this fold.
What would strengthen this signal
Computational next steps:
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Receptor complex prediction: Model TKPRPGP-NH2 docked to Neuropilin-1 (PDB: 2QQN or equivalent) using Boltz-2 or Chai-1 with the full receptor chain. Compare predicted binding conformation and interface contacts against native Selank. This is the single highest-priority follow-on prediction.
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Native vs. amidated head-to-head run: Run both sequences through the same structural pipeline in the same session to directly compare pLDDT, pTM, and any heuristic metrics. Isolate the delta attributable solely to amidation.
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MD-based stability simulation: Short molecular dynamics runs (10–50 ns) of both peptides in explicit solvent would provide a more grounded estimate of conformational stability and solvent exposure at the C-terminus.
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Dual-terminus analogue: Motivated by Fold #1 (N-terminal acetylation of Semax → REFINED), a doubly-modified variant Ac-TKPRPGP-NH2 combining N-terminal acetylation with C-terminal amidation could be predicted and compared — this would maximally protect both termini simultaneously.
Experimental validation needed:
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In vitro plasma stability assay: Incubate native Selank and TKPRPGP-NH2 in human plasma at 37°C; measure remaining intact peptide by LC-MS/MS at multiple timepoints. This directly tests the carboxypeptidase blockade hypothesis and is the minimum required experiment to convert PROMISING → REFINED or DISCARDED.
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Carboxypeptidase-specific degradation assay: Expose both peptides to purified carboxypeptidase B or ACE and measure cleavage kinetics. This isolates the mechanism proposed in the hypothesis.
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Receptor binding competition assay: If a tuftsin receptor binding assay can be established (e.g., displacement of labeled tuftsin from immune cells or recombinant NRP1), measure IC50 for native vs. amidated Selank to confirm pharmacophore preservation.
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Rodent anxiolytic behavioral assay: Elevated plus maze or forced swim test comparing equipotent doses of native and amidated Selank, ideally with pharmacokinetic sampling to correlate exposure with effect.
folding metrics
// no per-residue pLDDT trace — Boltz-2 returned summary metrics only
aggregation propensity (window)
3 windowsconfidence metrics
domain annotations
// not yet annotated by clinical / structural agents
structural caption
The predicted structure of C-terminally amidated Selank (TKPRPGP-NH2) shows a confidently modeled short peptide backbone (pLDDT ≈ 0.90) with the canonical extended/polyproline-II-like geometry expected from a Pro-rich heptapeptide. The N-terminal Thr-Lys-Pro-Arg tuftsin pharmacophore is preserved in conformation, and the C-terminal Pro-Gly-Pro segment retains the turn-like geometry seen in native Selank, with the only chemical change being the -COOH → -CONH2 substitution at Pro-7. No target receptor was included in this run, so no interface can be evaluated.
peptide profile
These are sequence-based heuristic estimates, not wet-lab measurements. Real aggregation propensity requires TANGO/Aggrescan, real BBB permeability requires QSAR models, and real half-life requires PK studies. Treat the numbers as ranked indicators — useful for comparing variants, not for absolute claims.
known binders
// no ChEMBL binders found for this target
agent findings
caveats
- ─in silico prediction only — requires wet lab validation
- ─single-run prediction (not ensembled)
- ─predicted properties may not reflect real-world biological behavior
- ─this is research, not medical advice
- ─no receptor complex was modeled in this fold — binding affinity impact of C-terminal amidation is entirely unquantified
- ─heuristic peptide property estimates (stability 0.696, BBB 0.102, half-life 15–45 min) are sequence-derived algorithmic outputs, not simulation or experimental values
- ─the half-life extension benefit from carboxypeptidase blockade is theoretically motivated but not supported by published pharmacokinetic data for native Selank — the dominant degradation pathway has not been experimentally characterized
- ─the tuftsin receptor / Neuropilin-1 target identity is putative and unconfirmed by direct binding assay in the available Selank literature
- ─pTM 0.168 reflects monomer-only prediction — no docking or interface score should be inferred
data
works cited
- [1]
(2022). Selank, a Peptide Analog of Tuftsin, Attenuates Aversive Signs of Morphine Withdrawal in Rats
- [2]
(2026). Therapeutic Peptides in Orthopaedics: Applications, Challenges, and Future Directions
- [3]
(2021). The Influence of Selank on the Level of Cytokines Under the Conditions of 'Social' Stress
- [4]
(2019). Effect of Selank on Morphological Parameters of Rat Liver in Chronic Foot-Shock Stress
- [5]
(2019). Selank, Peptide Analogue of Tuftsin, Protects Against Ethanol-Induced Memory Impairment by Regulating of BDNF Content in the Hippocampus and Prefrontal Cortex in Rats
- [6]
(2016). Selank Administration Affects the Expression of Some Genes Involved in GABAergic Neurotransmission
- [7]
(2017). GABA, Selank, and Olanzapine Affect the Expression of Genes Involved in GABAergic Neurotransmission in IMR-32 Cells
- [8]
(2020). Functional Connectomic Approach to Studying Selank and Semax Effects