Separation of Cyclic Dipeptides (Diketopiperazines) from Their Corresponding Linear Dipeptides by RP-HPLC and Method Validation

Table 2

Linear range, linear equation, correlation coefficient 𝑅², and limit of quantification (LOQ) for DKPs, dipeptides, and amino acids.

Substance	Linear range (mM)	Linear equation	𝑅^2	LOQ; (nmol per 10 μL injection)
Cyclo(Gly-Gly)	0.010–5.00	𝑦 = 1732𝑥 + 107	0.9980	5.00
Gly-Gly	0.010–5.00	𝑦 = 1355𝑥 + 124	0.9989	10.00
Cyclo(L-Ala-D-Ala)	0.025–5.00	𝑦 = 2188𝑥 + 93	0.9991	5.00
Cyclo(L-Ala-L-Ala) and cyclo(D-Ala-D-Ala)	0.025–5.00	𝑦 = 2271𝑥 + 96	0.9991	0.25
L-Ala-L-Ala and D-Ala-D-Ala	0.025–5.00	𝑦 = 1821𝑥 + 111	0.9996	5.00
L-Ala-D-Ala	0.025–5.00	𝑦 = 1642𝑥 + 60	0.9996	5.00
Cyclo(L-Arg-L-Arg)	0.100–50.00	𝑦 = 162𝑥 + 5	1.0000	1.00
L-Arg-L-Arg	0.010–10.00	𝑦 = 1611𝑥 + 86	0.9996	5.00
Cyclo(L-Asp-L-Asp)	0.005–2.50	𝑦 = 6855𝑥 + 307	0.9992	5.00
L-Asp-L-Asp	0.010–5.00	𝑦 = 5192𝑥 + 225	0.9990	5.00
cyclo(L-Asp-L-Phe)	0.005–5.00	𝑦 = 8227𝑥 + 162	0.9998	0.05
L-Asp-L-Phe	0.005–5.00	𝑦 = 8258𝑥 + 132	0.9999	0.05
L-Phe-L-Asp	0.005–2.50	𝑦 = 8389𝑥 + 122	0.9994	0.50
L-Phe (20% MeOH)	0.005–5.00	𝑦 = 5845𝑥 + 131	0.9997	0.10
Cyclo(L-Pro-L-Tyr)	0.005–5.00	𝑦 = 8256𝑥 + 116	0.9999	0.05
L-Pro-L-Tyr	0.005–2.50	𝑦 = 7164𝑥 + 87	0.9997	1.00
L-Tyr-L-Pro	0.010–10.00	𝑦 = 6127𝑥 + 172	0.9997	5.00
L-Tyr	0.010–2.50	𝑦 = 4929𝑥 + 43	1.0000	10.00
Cyclo(Gly-L-Phe)	0.005–5.00	𝑦 = 7662𝑥 + 165	0.9997	0.05
Gly-L-Phe	0.005–2.50	𝑦 = 8360𝑥 + 106	0.9995	0.10
L-Phe-Gly	0.005–2.50	𝑦 = 7392𝑥 + 109	0.9993	0.10
L-Phe (30% MeOH)	0.005–2.50	𝑦 = 6121𝑥 + 60	0.9997	0.10