Concentration of Amino acid calculation?
To clarify:
$\mathrm{p}K_\mathrm{a1}: \ce{NH3^{+}-R-COOH &lt;=&gt; NH3^{+}-R-COO- + H+}$
$\mathrm{p}K_\mathrm{a2}: \ce{NH3^{+}-R-COO- &lt;=&gt; NH2-R-COO- + H+}$
For aminoacids, the the pH calculation is the same as for acidic salts of diprotic weak acids $\ce{MHA}$.
Unfortunately for you, $\mathrm{pH}=\dfrac{\mathrm{p}K_\mathrm{a1}+\mathrm{p}K_\mathrm{a2}}{2}$ as the very good approximation, so it is almost independent on the concentration, so you cannot determine the concentration from $\mathrm{pH}$.
More exactly. you could, but by a much more complex formula as you listed above, as it would be very inacurate because of high $\dfrac{\mathrm{d}c}{\mathrm{d(pH)}}$
By other words, aminoacid solutions act as low capacity pH buffers ( because of quite a big difference of their $\mathrm{p}K_\mathrm{a1}$ and $\mathrm{p}K_\mathrm{a2}$.