% Returns the fraction of dsDNA in a solution containing equal concentrations
% of two complementary DNA oligos as a functino of total DNA concentration,  
% temperature, entropy change, and enthalpy change. 
% Gas constant is an optional parameter.
%
% USAGE: f = DnaFraction(Ct, T, DeltaS, DeltaH, <GasConstant>)
% RETURN VALUE: fraction of dsDNA
%
% Default units are molar, Kelvin, cal/mole, and cal/(mole-Kelvin). For
% other unit systems, supply the appropriate value of R in the optional
% fifth parameter. The default value is 1.987 cal/(degree K * mole).

function f = DnaFraction(Ct, T, DeltaS, DeltaH, GasConstant)
% Gas constant
if(nargin < 5)              % determine if caller supplied R value or not
    R = 1.987;              % set to default: cal/(degree K * mole)
else
    R = GasConstant;        % otherwise use caller's value
end

% Compute Ct * Keq
CtKeq = Ct * exp(DeltaS / R - DeltaH ./ (R * T));

%now compute f
f = (1 + CtKeq - sqrt(1 + 2 * CtKeq)) ./ CtKeq;

% Written 4/9/2008 by SCW