# Nuskell Documentation (PREVIEW)¶

## Introduction¶

Nuskell compiles formal chemical reaction networks (CRNs) into domain-level strand displacement (DSD) systems, based on rigorous proofs establishing the correctness of compilation from a higher level to a lower level. To support the wide range of translation schemes that have already been proposed in the literature, as well as potential new ones that are yet to be proposed, Nuskell provides a domain-specific programming language for translation schemes. A notion of correctness is established on a case-by-case basis using the rate-independent stochastic-level theories of pathway decomposition equivalence and/or CRN bisimulation.

Nuskell is a first step to integrate biophysical modelling of nucleic acids with rigorous abstraction hierarchies of modern compilers to design and characterize DSD systems. Our independently developed DSD reaction enumeration library peppercornenumerator is used for translation-independent reaction enumeration with well-defined semantics and based on empirical (sequence-independent) DNA folding parameters. Nuskell also provides an interface to translate these enumerated systems into systems of ordinary differential equations (ODEs), which are printed into stand-alone, executable Python scripts using the crnsimulator library.

Quickstart introduces high-level concepts that are supported by library, for more details on how to write translation schemes visit The Nuskell programming language. For advanced development on the Nuskell library, use the Nuskell API section and the Developer Guidelines. The section Background provides an intuition about the formalisms underlying the Nuskell library. More details can be found in the publications cited below.

Cite:

### Quickstart¶

The following command translates a formal CRN into a DSD system using the built-in translation scheme: qian2011_3D.ts, verifies whether the implementation is correct using CRN bisimulation and prints the domain-level specification in the *.pil file format.

nuskell --ts qian2011_3D.ts --verify bisimulation --pilfile < formal_crn.in


The Nuskell library is meant to help researchers building custom DSD compilers. Nuskell provides (i) a number of different CRN-to-DSD translation schemes, (ii) functions that test different CRN equivalence notions, as well as (iii) DSD reaction enumeration and CRN-to-ODE simulations based on empirical (sequence-independent) DNA folding parameters.

from nuskell import translate, verify

testtube = translate('A+B->C', scheme = 'soloveichik2010.ts')

# Get the enumerated CRN
testtube.enumerate_reactions()

# Interpret the enumerated CRN, i.e. replace history species
interpretation = testtube.interpret_species(['A','B','C'], prune=True)

# Formulate reversible reactions as two irreversible reactions.
fcrn = [[['A','B'],['C']]]
vcrn = []
for r in testtube.reactions:
rxn = [map(str,r.reactants), map(str,r.products)]
vcrn.append(rxn)

v = verify(fcrn, vcrn, fs, method = 'bisimulation')

if v :
print("Input CRN and TestTube-Species are CRN bisimulation equivalent.")
else :
print("Input CRN and TestTube-Species are not CRN bisimulation equivalent.")


## CRN-to-DSD translation¶

### DSD system requirements¶

Automated compilation of DSD system requires DSD systems to follow a particular format. First, all involved species and complexes need to be free of pseudo-knots. Second, we distinguish signal species and fuel species. Signal species are at low concentrations and they present the information (input/output) unit. Fuel species are at high (ideally constant) concentrations and they mediate the information transfer by consuming and/or releasing signal species. After compilation, every species in the formal CRN corresponds to one signal species. Thus, all signal species must have the same domain-level constitution and structure, but they need to be independent of each other. A signal species may be a complex composed of multiple molecules.

History domains are common in many translation schemes. A history domain is considered to be an inert domain of a signal species, but it is unique to the reaction that has produced the signal species. Hence, multiple species that differ only by their history domains map to the same formal species. In the translation scheme language, a history domain is a wildcard: ?. Together with the remainder of the molecule, a species with a wildcard forms a regular-expression, matching every other species in the system that differs only by a single domain instead of ?.

### The Nuskell programming language¶

The Nuskell programming language is used to write translation schemes, i.e. design algorithms which can be interpreted by the Nuskell compiler. Translation schemes translate a CRN into a DSD system. A library of existing schemes can be found in the official Nuskell repository, the links below point to a tutorial to write your own translation scheme:

### Analysis of implementation networks¶

Besides the top-down interface of Nuskell to translate CRNs to DSD systems, there also exists a modular, bottom-up interface where users can analyze, simulate and verify handcrafted or alternatively designed DSD systems.

nuskell --readpil zhang2007_catalyst.pil --verify bisimulation < formal_crn.in


The option --readpil <file> tells Nuskell to load domain-level specifications from a text file, as opposed to automated design via translation schemes. The input format is a variation of the pepper internal language (PIL) kernel notation which allows the specification of constant or initial concentrations in M, mM, uM, nM, pM.

# Use '#' for comments.

# Domains
length d1  = 10
length d2a =  6

# Complexes               # Concentratios
C = d4 d5                 @initial 2 nM
OB = d1 d2a d2b d2c       @constant 100 nM


The concentration specification (e.g. @initial 10 nM) is optional, but relevant for both verification and simulation of DSD systems. Nuskell’s verification has to be provided with the information of which species correspond to signal and fuel species.

A complex with a corresponding name in the formal CRN, is always interpreted as a signal species, independent of whether or not constant or initial concentrations have been specified. Species that are not present in formal CRN default to fuel species if: (i) they have no concentration specified, or (ii) their concentration is higher than 0. Variant (i) allows a compact DSD system specification, which is equivalent to the format of a PIL file when using option --pilfile, and compatible with input for the peppercornenumerator. Variant (ii) enables us to define named intermediate complexes as those which are explicitly initially not present, i.e are followed by the @initial 0 nM tag. Note, do not use @constant 0 nM to specify an intermediate species, as the behavior of Nuskell is currently undefined and might change in future versions.

The following PIL file shows a complete DSD system specification, including initial concentrations for signal species, formal species and all enumerated intermediate species:

#
# Zhang, Turberfield, Yurke, Winfree (2007)
# "Engineering Entropy-Driven Reactions and Networks Catalyzed by DNA"
#
# A DSD implementation of the catalyst reaction (Figure 1A + 1D)
# Note: Domain 2 is actually contains two toeholds (2a, 2b)
#
# CRN:
#   C + S -> C + OB
#   OB -> ROX
#
# verify:
#   echo "C + S -> C + OB; OB -> ROX" | nuskell --readpil zhang2007_catalyst.pil --verify pathway bisimulation
#     => not pathway equivalent
#     => bisimulation equivalent
#   echo "C -> C + OB; OB -> ROX" | nuskell --readpil zhang2007_catalyst.pil --verify pathway bisimulation
#     => not pathway equivalent
#     => bisimulation equivalent
#

# Domains
length d1  = 10
length d2a =  6
length d2b =  6
length d2c = 12
length d3  =  4
length d4  = 16
length d5  =  6
length d6  = 16

# Species
C = d4 d5                 @initial 2 nM     # defaults to fuel
OB = d1 d2a d2b d2c       @initial 0 nM     # defaults to intermediate
ROX = d1 d2a              @initial 0 nM     # defaults to intermediate
S = d1 d2a( d2b( d2c( + d6 d3( d4( + d5* ) ) ) ) )  @initial 100 nM # defaults to fuel
F = d2a d2b d2c d3 d4     @initial 100 nM   # defaults to fuel
OR = d1( d2a( + d2b* ) )  @initial 100 nM   # defaults to fuel
SB = d6 d3 d4             @initial 0 nM     # defaults to intermediate


## Background¶

### DSD enumeration¶

The domain-level representation provides a more coarse-grained perspective on nucleic acid folding than the single-nucleotide-level. At the nucleotide-level every step is a base pair opening or closing reaction and the corresponding rate can be calculated from the free energy change of a reaction. On the domain-level, we consider a more diverse set of reactions in order to compensate for the fine-grained details that can happen on the sequence level. Nuskell uses a domain-level reaction enumeration library [peppercornenumerator], to predict desired and, potentially, undesired reactions emerging from previously compiled signal and fuel species.

The general types of reactions are spontaneous binding and unbinding of domains, 3-way branch migration, 4-way branch migration and remote toehold branch-migration. Peppercorn’s enumeration semantics are justified based on the assumption that the DSD system is operated at sufficiently low concentrations, such that unimolecular reactions always go to completion before the next bimolecular interaction takes place. Under the assumptions of low concentrations, a condensed CRN can be calculated, with reactions that indicate just the eventual results after all unimolecular reactions complete, and with rate constants systematically derived from the detailed reaction network rate constants.

There are many options available to adjust the semantics of reaction enumeration, they are described in detail in [Grun et al. (2014)].

More:

### CRN equivalence¶

The most fundamental requirement towards compilation of large scale DSD systems, is verification. Every formal reaction is translated into multiple implementation reactions. Thus, there are many possibilities to introduce bugs, i.e. unwanted side reactions that alter the implemented algorithm. Nuskell supports currently variants of two case-by-case verification strategies that compare formal CRNs with their implementations. As intended, our approach does not verify the general correctness of a particular scheme, but the correctness of a particular implementation.

Pathway decomposition equivalence. The core idea is to represent each implementation trajectory as a combination of independent pathways of reactions between formal species. Pathway decomposition yields a set of pathways which are indivisible (or prime) and are called the formal basis of a CRN. The formal basis is unique for any valid implementation. Any two CRNs are said to be equivalent if they have the same formal basis. Conveniently, a CRN without intermediate species has itself as the formal basis, but it is worth pointing out that this equivalence relation allows for the comparison of one implementation with another implementation.

CRN bisimulation verification. A CRN bisimulation is an interpretation of the implementation CRN, where every implementation species is mapped to a multiset of formal species. This often yields so-called trivial reactions, where reactants and products do not change according to the interpretation. An interpretation is only a bisimulation, if three conditions are fulfilled: (i) atomic condition – for every formal species there exists an implementation species that interprets to it, (ii) delimiting condition – any reaction in the implementation is either trivial or a valid formal reaction, and (iii) permissive condition – for any initial condition in the implementation CRN, the set of possible next non-trivial reactions is exactly the same as it would be in the formal CRN. CRNs are said to be bisimulation equivalent, if the translation can be interpreted as an implementation of that formal CRN.

More:

### Simulations of DSD systems¶

Translate enumerated CRNs into ODEs

# Developer Guidelines¶

In order to ensure sustainability of the Nuskell compiler package, there are a few rules for developers before submitting a pull request.

The Nuskell repository can be found on GitHub:

https://github.com/DNA-and-Natural-Algorithms-Group/nuskell