Antonino Ingargiola Merge branch 'master' of https://github.com/tritemio/fretbursts_paper  almost 8 years ago

Commit id: 2ace2a53b11ab31c0e65bee8eaf60185819b659d

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comprising a fixed number $n$ of photons,  and to compare the empirical variance of acceptor counts of all sub-bursts in a burst,  with the theoretical shot-noise-limited variance.  An empirical variance of sub-bursts larger than the shot-noise limited shot-noise-limited  value indicates the presence of dynamics.Since the estimation of the sub-bursts variance is affected  by uncertainty, BVA analysis provides and indication of an higher or lower probability  of observing dynamics.  In a FRET (sub-)population originating from a single static FRET efficiency,  the sub-bursts acceptor counts $n_a$ can be modeled as a binomial-distributed random variable  

\operatorname{Std}(E_{\textrm{sub}}) = \left( \frac{E_p\,(1 - E_p)}{n} \right)^{1/2}  \end{equation}  BVA analysis consists of in  four steps: 1) dividing bursts into consecutive sub-bursts containing a constant number of consecutive photons~\textit{n}, 2) computing the PR   of each sub-burst, 3) calculating the empirical standard deviation ($s_E$) of sub-bursts  PR in each burst, and 4) comparing $s_E$ to the expected standard deviation  

non-interconverting molecules) characterized by distinct FRET efficiencies,   $s_E$ of each burst is only affected by shot-noise and will follow the expected   standard deviation curve based on eq.~\ref{eq:binom_std}.   Conversely, if the observed distribution originates from biomolecules belonging to a single specie, species,  which interconverts between different FRET sub-populations (over times comparable to the diffusion   time), as in figure~\ref{fig:bva_dynamic}, $s_E$ of each burst will be larger than the expected   shot-noise-limited standard deviation, and will be located above the shot-noise standard          

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\label{fig:bva_dynamic} \textbf{BVA distribution for a hairpin sample undergoing dynamics.}  The left panel shows the E-S histogram for a single stranded DNA sample ($A_{31}$-TA, see in~\cite{Tsukanov_2013}), designed to form a transient hairpin in 400mM NaCl. The right panel shows the corresponding BVA plot. Since the transition between hairpin and open structure causes a significant change in FRET efficiency, $s_E$ lies largely above the static standard deviation curve (\textit{red curve}). Data courtesy of XXX.         

Unfortunately, the method is not of straightforward application for   freely-diffusing data as it requires a special selection   criterion for filtering bursts with quasi-Poisson rates.  Santoso~\cite{santoso_probing_2009} Santoso et al.~\cite{santoso_probing_2009}  and Kalinin~\cite{Kalinin2010} Kalinin et al.~\cite{Kalinin2010}  extended the PDA approach to estimate conversion rates between different  states by comparing FRET histograms as a function of the time-bin size.  In addition, Gopich and Szabo~\cite{Gopich2009, gopich_theory_2011} developed 

In case of measurement including lifetime, the multiparameter fluorescence  detection (MFD) method allows to identify dynamics from the deviation   from the linear relation between lifetime and E~\cite{sisamakis_accurate_2010}.  Hoffman~\cite{hoffmann_quantifying_2011} Hoffman et al.~\cite{hoffmann_quantifying_2011}  proposed a method called RASP (recurrence analysis of single particles) to extend   the timescale of detectable kinetics.  Hoffman computes et al. compute  the probability that two nearby bursts are due to the same molecule and therefore allows setting a time-threshold  for considering consecutive bursts as the same single-molecule event. 

Finally, two related methods for discriminating between static heterogeneity  and sub-millisecond dynamics are Burst Variance Analysis  (BVA) proposed by Torella~\cite{Torella_2011} Torella et al.~\cite{Torella_2011}  and two-channel kernel density estimator (2CDE) proposed by   Tomov~\cite{Tomov_2012}. Tomov et al.~\cite{Tomov_2012}.  The BVA method is described in the next section. The 2CDE method, which has been implemented in FRETBursts, computes local  photon rates from timestamps within bursts using  Kernel Density Estimation (KDE)  (FRETBursts includes general-purpose functions  to compute KDE of photon timestamps in the \verb|phrates| module,   (\href{http://fretbursts.readthedocs.io/en/latest/phrates.html}{link})).  From time variations of local rates rates, it  is possible to detect infer  the occurrence presence  of some  dynamics. In particular particular,  the 2CDE method builds, for each burst, a quantity $(E)_D$ (or $(1-E)_A$) $(1-E)_A$),  which is equal to the burst average $E$ when no dynamics is present, butit  is biased toward an higher (or lower) value in presence of dynamics. From these  quantities quantities,  a burst ``estimator'' (called FRET-2CDE) is derived. For a user user,  the 2CDE method consists  in amounts  to  plotting the 2-D histogram of $E$ versus FRET-2CDE   in FRET-2CDE, and  assessing the vertical position of the various populations: populations centered around FRET-2CDE=10 have undergo  no dynamics dynamics,  while population biased towards higher FRET-2CDE values have undergo  dynamics. The BVA and 2CDE methods are implemented   in two notebooks included with FRETBursts 

and run the anaysis therein.  The other methods mentioned in this section are not currently   implemented in FRETBursts.  However, users can easily  implement their any  additional favorite method  taking advantage of FRETBursts functions for methods  in FRETBursts, using its built-in  burst analysis and timestamps/bursts manipulation.  To facilitate this task, in manipulation functions.  In  the next section, we show how to perform low-level analysis of timestamps and bursts data   by implementing the BVA method from scratch.  An additional example showing how to split bursts in constant time-bins