Mass calculation

Comparison of results with weighted mean and without

As it is described in Data processing article, the data within each replicate of the experiment is aggregated using weighted mean. Below we present the effect it has on the results.

The whole workflow is described in the mentioned article. Here the focus is on the first aspect of the processing.

For the analysis, we use the example file from HaDeX.

library(HaDeX)
dat <- read_hdx(system.file(package = "HaDeX", "HaDeX/data/KD_180110_CD160_HVEM.csv"))

As described, the first step is to transform the Center value (geometric centroid of the isotopic envelope for given peptide in a given state in given time point) and then to aggregate the values measured for different charge values. This is all done within each replicate.

The aggregation in our workflow is a weighted mean, with Inten (intensity) values as weights, as shown below.

avg_exp_mass = weighted.mean(exp_mass, Inten, na.rm = TRUE)

Later, the results from replicates are aggregated using the mean, and their uncertainty is calculated as the standard deviation of the mean.

How the weighted mean change the result in comparison with the simple mean? Let’s see.

Below are calculated values of mass in two approaches and the difference between them for the example peptide.

##          Sequence Start End      State Exposure no_weight weight     diff
## 1  ARSQKSGIRLQGHF    88 101      CD160    0.000      1585   1585 -0.00675
## 2  ARSQKSGIRLQGHF    88 101      CD160    0.001      1585   1585  0.00171
## 3  ARSQKSGIRLQGHF    88 101      CD160    0.167      1591   1591  0.00266
## 4  ARSQKSGIRLQGHF    88 101      CD160    1.000      1592   1592  0.00190
## 5  ARSQKSGIRLQGHF    88 101      CD160    5.000      1592   1592  0.00173
## 6  ARSQKSGIRLQGHF    88 101      CD160   25.000      1592   1592  0.00168
## 7  ARSQKSGIRLQGHF    88 101      CD160  120.000      1592   1592 -0.00247
## 8  ARSQKSGIRLQGHF    88 101      CD160 1440.000      1593   1593  0.00232
## 9  ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.000      1585   1585 -0.00675
## 10 ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.001      1585   1585  0.00171
## 11 ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.167      1591   1591  0.00327
## 12 ARSQKSGIRLQGHF    88 101 CD160_HVEM    1.000      1592   1592  0.00363
## 13 ARSQKSGIRLQGHF    88 101 CD160_HVEM    5.000      1592   1592  0.00349
## 14 ARSQKSGIRLQGHF    88 101 CD160_HVEM   25.000      1593   1593  0.00915
## 15 ARSQKSGIRLQGHF    88 101 CD160_HVEM  120.000      1593   1593  0.00777
## 16 ARSQKSGIRLQGHF    88 101 CD160_HVEM 1440.000      1593   1593  0.00557

Below are calculated values of the uncertainty of mass in two approaches and the difference between them for the example peptide.

##          Sequence Start End      State Exposure no_weight weight      diff
## 1  ARSQKSGIRLQGHF    88 101      CD160    0.000    0.0000 0.0000  0.00e+00
## 2  ARSQKSGIRLQGHF    88 101      CD160    0.001    0.0000 0.0000  0.00e+00
## 3  ARSQKSGIRLQGHF    88 101      CD160    0.167    0.0185 0.0193  7.89e-04
## 4  ARSQKSGIRLQGHF    88 101      CD160    1.000    0.0133 0.0141  7.22e-04
## 5  ARSQKSGIRLQGHF    88 101      CD160    5.000    0.0985 0.0998  1.36e-03
## 6  ARSQKSGIRLQGHF    88 101      CD160   25.000    0.0157 0.0178  2.12e-03
## 7  ARSQKSGIRLQGHF    88 101      CD160  120.000    0.0144 0.0152  8.69e-04
## 8  ARSQKSGIRLQGHF    88 101      CD160 1440.000    0.0318 0.0329  1.07e-03
## 9  ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.000    0.0000 0.0000  0.00e+00
## 10 ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.001    0.0000 0.0000  0.00e+00
## 11 ARSQKSGIRLQGHF    88 101 CD160_HVEM    0.167    0.0378 0.0381  3.71e-04
## 12 ARSQKSGIRLQGHF    88 101 CD160_HVEM    1.000    0.0602 0.0602  1.02e-05
## 13 ARSQKSGIRLQGHF    88 101 CD160_HVEM    5.000    0.0119 0.0130  1.09e-03
## 14 ARSQKSGIRLQGHF    88 101 CD160_HVEM   25.000    0.0181 0.0159 -2.21e-03
## 15 ARSQKSGIRLQGHF    88 101 CD160_HVEM  120.000    0.0128 0.0144  1.59e-03
## 16 ARSQKSGIRLQGHF    88 101 CD160_HVEM 1440.000    0.0193 0.0205  1.18e-03

And for the whole set of peptides, the average mass difference is:

## [1] 0.00499

and uncertainty difference is:

## [1] -0.00925

Below are the histograms of these differences.