[leanswer]

https://pubchem.ncbi.nlm.nih.gov/compound/588 --> Creatinine
https://pubchem.ncbi.nlm.nih.gov/compound/7243 --> o-Phenylenediamine

I am determining the template-monomer interactions between creatinine and o-phenylenediamine (OPD) and for this I need to know if the amines in OPD will be charged or not at neutral pH. The pKa1 and pKa2 of OPD are around 0.98 and 4.5 respectively, and the one of creatinine was around 11 so at the pH that I am working (7.3) I expect creatinine (basic) to have a =NH2 positively charged. Judging by the pKas, OPD has an acidic nature, but as amines are not usually negatively charged, I don't know if they are positively charged below pH=1 and Thus neutral at pH=7.3, or neutral below pH=1 and negatively charged at pH=7.3

TL;DR How are the amines of OPD gonna be (charge-wise) at pH=7.3?

Thanks!

[Dan]

Judging by the pKas, OPD has an acidic nature, but as amines are not usually negatively charged, I don't know if they are positively charged below pH=1 and Thus neutral at pH=7.3, or neutral below pH=1 and negatively charged at pH=7.3

The "pK_a" of 11 for creatinine is the pK_a of creatinine's conjugate acid, or creatinine's pK_aH.

This is a common and annoying ambiguity in amine pKa tables. You will often see the pKa of ammonia listed as ~9 for example - what is meant is that the pKa of ammonium ion is ~9 (or the pKaH of ammonia is ~9). The pKa of ammonia is actually ~35.

Similarly here, the pKa values given for o-phenylenediamine refer to the mono- and diprotonated forms (not to the deprotonation of neutral o-phenylenediamine). It means:

OPD·2H⁺  OPD·H⁺ + H⁺   pKa = 0.98
OPH·H⁺  OPD + H⁺  pKa = 4.5

[pgk]

Furthermore,
The bacicity is given by the pKb:
pKb = pKw – pKa
where, pKa refers to the conjugate acid
and pKw = 14
However, pKa is used for both acids and bases in tables, mainly for homogeneity reasons that help understanding how acidic or basic a compound is.
Thus, o-phenylenediamine will be non-protonated in blood, as being rather a weak base.
On the other hand and though containing amine groups, creatinine is not an amine but a guanidine, which is both a borderline acid and a borderline base and can form both salts with strong acids, e.g. HCl, as well as alkali salts under drastic condition, e.g. with reaction with alkoxides.
Thus, creatinine will partially be protonated in blood at a low amount. The concentrations ratio of protonated/non-protonated forms, can be calculated by the Henderson-Hasselbach equation.
The blood volume of an average sized adult, is ≈ 5 L.