TABLE 2.

Affinity of cyclotides for lipid membranes

The parameters were obtained by fitting the binding response (P/L) versus the peptide concentration injected over the lipid bilayer (see Fig. 2). P/L max is the maximum binding response, and K_d is the peptide concentration needed to achieve half-maximum binding at equilibrium. Although peptide-lipid binding events are more complex than Langmuir binding, one-site specific binding with Hill slope equation was used to fit the parameters and to have a quantitative comparison of the SPR response.

Peptide	POPC/POPE (90:10)		POPC/POPE (80:20)		POPC/POPE/POPG (60:20:20)		POPC/POPE/Chol/SM (17:10:33:40)		POPC/Chol/SM (27:33:40)a
	P/L max	Kd	P/L max	Kd	P/L max	Kd	P/L max	Kd	P/L max	Kd(app)
		μm		μm		μm		μm		μm
kB1	0.18 ± 0.01	73 ± 9	0.24 ± 0.01	24 ± 0.5	0.29 ± 0.01	18 ± 1	0.11 ± 0.003	29 ± 2	0.11	2893
kB2	0.18 ± 0.03	19 ± 4	0.22 ± 0.01	16 ± 1	0.31 ± 0.01	15 ± 1	0.12 ± 0.004	13 ± 1	0.12	1586
kB6	0.23 ± 0.02	25 ± 3	0.31 ± 0.02	22 ± 2	0.35 ± 0.01	10 ± 1	0.13 ± 0.01	16 ± 2	0.13	594
kB7	0.20 ± 0.02	66 ± 15	0.34 ± 0.01	47 ± 2	0.37 ± 0.01	15 ± 1	0.13 ± 0.01	57 ± 6	0.13	1846
kB5	0.12 ± 0.003	6.6 ± 4	0.34b	143 ± 42	0.28b	18 ± 4	0.05 ± 0.003	11 ± 2	0.05	129
kB8	0.17b	19 ± 3	0.34b	190 ± 39	0.28b	10 ± 2	0.14b	112± 4	0.14	983
kB9	0.17b	41± 7			0.10 ± 0.003	13 ± 1	0.08 ± 0.005	32 ± 5	0.08	143
cyO2	0.17 ± 0.003	8.2 ± 0.4	0.34 ± 0.01	8.6 ± 0.6	0.28 ± 0.004	3.8 ± 0.5	0.14 ± 0.002	24 ± 1	0.14	1283
tcA	0.15 ± 0.01	41 ± 6	0.34b	178 ± 12	0.24 ± 0.01	35 ± 4	0.08 ± 0.017	88 ± 48	0.08	700

^a The binding of cyclotides to membranes lacking PE-phospholipids is weak; therefore, the curves do not reach a plateau in the concentration range tested, the P/L_max cannot be defined, and nor can K_d value. Nevertheless to illustrate the weak affinity to membranes lacking PE, binding to POPC/Chol/SM is compared with POPC/POPE/Chol/SM assuming the same P/L_max for both lipid mixtures (which possess identical fluidity properties at 25 °C). As this fit implies extrapolations, the error associated with the K_d is high, and therefore the fitted values are referred to as apparent K_d (K_{d (app)}).

^b The curve did not reach a plateau in the concentration range tested; therefore, the P/L_max was constrained to be equal to the value obtained with cyO2 which is also a bracelet cyclotide.