In rformassspectrometry/SpectriPy: Integrating Spectra with Python's matchms

BiocStyle::markdown()

Compiled: r date()

Introduction

The SpectriPy package allows integration of Python MS packages into a Spectra-based MS analysis in R. Python functionality is wrapped into R functions allowing a seamless integration of the functionality of Python's matchms package into R. In addition, functions to convert between R's Spectra objects and Python's matchms spectrum objects are available to the advanced user or developer.

Installation

The package requires a python environment to be available and can be installed with the BiocManager R package using the command BiocManager::install("RforMassSpectrometry/SpectriPy"). All required python packages are installed automatically on demand.

Spectra similarity calculations using `matchms`

The SpectriPy package provides the compareSpectriPy function that allows to perform spectra similarity calculations using the scoring functions from the matchms Python package. Below all currently supported scoring functions are listed along with the parameter class that allows selecting and configuring the algorithm in the compareSpectriPy function. Additional functions will be added in future.

CosineGreedy: CosineGreedyParam.
CosineHungarian: CosineHungarianParam.
ModifiedCosineParam: ModifiedCosineParam.

We next create some simple example spectra and subsequently use the compareSpectriPy function to calculate pairwise similarities between these.

library(Spectra)
library(SpectriPy)

## Create a Spectra object with two MS2 spectra for Caffeine.
caf <- DataFrame(
    msLevel = c(2L, 2L),
    name = "Caffeine",
    precursorMz = c(195.0877, 195.0877)
)
caf$intensity <- list(
    c(340.0, 416, 2580, 412),
    c(388.0, 3270, 85, 54, 10111))
caf$mz <- list(
    c(135.0432, 138.0632, 163.0375, 195.0880),
    c(110.0710, 138.0655, 138.1057, 138.1742, 195.0864))
caf <- Spectra(caf)

## Create a Spectra object with two MS2 spectra for 1-Methylhistidine
mhd <- DataFrame(
    msLevel = c(2L, 2L),
    precursorMz = c(170.0924, 170.0924),
    id = c("HMDB0000001", "HMDB0000001"),
    name = c("1-Methylhistidine", "1-Methylhistidine"))
mhd$mz <- list(
    c(109.2, 124.2, 124.5, 170.16, 170.52),
    c(83.1, 96.12, 97.14, 109.14, 124.08, 125.1, 170.16))
mhd$intensity <- list(
    c(3.407, 47.494, 3.094, 100.0, 13.240),
    c(6.685, 4.381, 3.022, 16.708, 100.0, 4.565, 40.643))
mhd <- Spectra(mhd)

We first calculate pairwise similarities between all spectra defined above and those of caffeine using Spectra's built-in compareSpectra function.

all <- c(caf, mhd)
res_r <- compareSpectra(all, caf)
res_r

Thus, compareSpectra returned the pairwise similarity scores (by default calculated using the normalized dot-product function) between all spectra in all (rows) and all spectra in caf (columns). compareSpectriPy works similar, with the difference that we need to specify and configure the similarity function (from matchms) using a dedicated parameter object. Below we calculate the similarity using the CosineGreedy function changing the tolerance to a value of 0.05 (instead of the default 0.1).

res <- compareSpectriPy(all, caf, param = CosineGreedyParam(tolerance = 0.05))
res

As a result compareSpectriPy returns also a numeric matrix of similarities. Note also that the first compareSpectriPy call takes usually a little longer because the Python setup has to be initialized.

Next we use the ModifiedCosine algorithm that considers also differences between the spectra's precursor m/z in the calculation.

res <- compareSpectriPy(all, caf, param = ModifiedCosineParam())
res

Note that for this calculation all spectra precursor m/z values need to be available, otherwise an error will be thrown. Thus, we should always ensure to remove spectra without precursor m/z values prior to similarity scoring with this similarity method. Below we remove the precursor m/z from one of our input spectra and then show how the Spectra object could be subsetted to valid spectra for this method.

## Remove precursor m/z from the 3rd spectrum
all$precursorMz[3] <- NA

## Filter the input spectra removing those with missing precursor.
all <- all[!is.na(precursorMz(all))]

compareSpectriPy(all, caf, param = ModifiedCosineParam())

Advanced use and internals

For advanced users or developers, the rspec_to_pyspec and pyspec_to_rspec functions are available that enable conversion between MS data representations in R and Python (i.e. between the Spectra object and the Python matchms Spectrum object). To use these functions the reticulate package needs to be installed along with a Python environment and the matchms Python package.

To illustrate their use we initialize below the Python environment that is bundled using the r BiocStyle::Biocpkg("basilisk") package within SpectriPy.

library(SpectriPy)
library(basilisk)
cl <- basiliskStart(SpectriPy:::matchms_env)

We next create a simple Spectra object representing fragment spectra for some small compounds.

library(Spectra)

# create example spectra
spd <- DataFrame(
  msLevel = c(2L, 2L, 2L),
  id = c("HMDB0000001", "HMDB0000001", "HMDB0001847"),
  name = c("1-Methylhistidine", "1-Methylhistidine", "Caffeine"))

## Assign m/z and intensity values.
spd$mz <- list(
  c(109.2, 124.2, 124.5, 170.16, 170.52),
  c(83.1, 96.12, 97.14, 109.14, 124.08, 125.1, 170.16),
  c(56.0494, 69.0447, 83.0603, 109.0395, 110.0712,
    111.0551, 123.0429, 138.0662, 195.0876))
spd$intensity <- list(
  c(3.407, 47.494, 3.094, 100.0, 13.240),
  c(6.685, 4.381, 3.022, 16.708, 100.0, 4.565, 40.643),
  c(0.459, 2.585, 2.446, 0.508, 8.968, 0.524, 0.974, 100.0, 40.994))

sps <- Spectra(spd)

We next convert the Spectra to a matchms Spectrum object.

pysps <- rspec_to_pyspec(sps)
pysps

As a result we got now a Python list with 3 Spectrum objects containing the peak data of sps as well as a reduced set of the available spectra variables in sps. Which spectra variables get copied to Python can be defined with the mapping parameter of rspec_to_pyspec. The default is to convert all variables defined by spectraVariableMapping(), but additional variables along with their respective names in the Python Spectrum object can be defined too. Below we list the pre-selected spectra variables that are converted by default.

spectraVariableMapping()

With our Spectra data converted to Python, we could directly call all routines from the matchms package using the reticulate R package. Below we normalize the intensities of the 3 spectra using the from normalize_intensities functions from the matchms.filtering Python package. We thus need to import first the functionality from that package and can then directly call the function on the objects.

library(reticulate)
filters <- import("matchms.filtering")

res <- vector("list", length(pysps))
for (i in (seq_along(pysps) - 1))
    res[[i + 1]] <- filters$normalize_intensities(pysps[i])
res <- r_to_py(res)
res

We can now convert the list of Python Spectrum objects back to R with pyspec_to_rspec:

sps_r <- pyspec_to_rspec(res)
#' The normalized intensities
intensity(sps_r)
#' The original intensities
intensity(sps)

Intensity values have now been normalized to values between 0 and 1.

Note however that, it would be much better (and likely more efficient) to directly use a Python function on the list of Spectrum objects instead of the mixed R and Python code used in the example above. Below we define a simple python script that iterates over the spectra in python and performs the normalization.

py_script <- paste0("from matchms.filtering import normalize_intensities\n",
                    "for i in range(len(pysps)):\n",
                    "    pysps[i] = normalize_intensities(pysps[i])\n")
py$pysps <- pysps
py_run_string(py_script)

tmp <- pyspec_to_rspec(pysps)
intensity(tmp)