THE INFLUENCE OF AMINO ACID SIDE CHAINS ON WATERTHE INFLUENCE OF AMINO ACID SIDE CHAINS ON WATERBINDING TO THE COPPER(II) IN COPPER(II) COMPLEXES:BINDING TO THE COPPER(II) IN COPPER(II) COMPLEXES:

AN EPR AND A MOLECULAR MECHANICS STUDYAN EPR AND A MOLECULAR MECHANICS STUDY

Krunoslav Mirosavljević1, Jasmina Sabolović2, Vesna Nöthig-Laslo1

1Ruđer Bošković Institute 2Institute for Medical Research and Occupational Health

Zagreb, CroatiaINTRODUCTIONINTRODUCTIONCopper(II) complexes with amino acids and peptides are potentially good enzyme mimetics. Interactions of amino acid side chains in

those complexes may have important role in their temperature dependent behaviour. The EPR studies of the Brownian motion of bis(N,N-dimethyl-L--valinato)copper(II) (Cu(Me2Val)2) and bis(N,N-dimethyl-L--leucinato)copper(II) (Cu(Me2Leu)2) dissolved in different solvents (deuterated methanol, ~280-320 K, and deuterated chloroform, ~ 265-305 K) were combined with molecular

mechanics calculations. The conformational analysis of these compounds was performed in order to find the most stable conformations. The aim of this work was to find whether different EPR behaviour of these two copper(II) complexes could be connected with the conformational (sterical) reasons. Bis(N,N-dimethyl-L--alaninato)copper(II) (Cu(Me2Ala)2) was used as

reference complex. There are no interactions of alanine side chains because –CH3 is small group and the apical water molecule must be present for the stability of the complex. The effective volume of the complex is constant in temperature interval examined.

The EPR spectra of Cu(Me2Leu)2 both in CDCl3 and CD3OD suggest aqua-complex (D2O in coordination sphere of copper) in whole

temperature interval examined. In each solvent the spectra weresimulated with the same parameters (A0 and g0). The conformationalanalysis backs EPR results: the energy difference between the most

stable conformer and first above it increases for aqua-complex. It meansthat the presence of the water molecule additionally stabilizes complex.

Cu(Me2Val)2 in CD3OD shows more complicated dependence of to/T. Since it is not linear (i.e. the volume of the complex changes in

temperature interval examined), there are some conformationalchanges. The energy difference between the most stable conformerand first above it is smaller when water molecule is present which

allows us to suggest some rearrangements of amino acid side chainsand perhaps releasing of apical ligand (water) during the heating.

THEORY AND RESULTSTHEORY AND RESULTS

ΔB(mI) = a + b mI + c mI2

Because of the incomplete averaging of interaction between magnetic moments of an electron and a nuclear spin, line widths in

EPR spectra, B, depend on nuclear magnetic moment, mI . It

provides that line widths obtained in EPR spectra can be expressed:

0

3π

3

4 kT

r

Reorientation correlation time,, depends on viscosity to

temperature ratio,/T, according to Stokes-Einstein relation:

211 ))()(34(10006.0 gAAggub

Parameter b is varied by temperature and it is connected with :

B/mT260 280 300 320 340 360

AII

gII

Figure 1. EPR spectrum of Cu(Me2Leu)2

in CD3OD frozen in liquid nitrogen at 140 K.

The parallel values of g- and A-tensors are shown.

Figure 2. EPR spectra of a) Cu(Me2Leu)2 and b) Cu(Me2Val)2

in CD3OD at 317 K (experimental-red, simulated-black).

B/mT

290 300 310 320 330 340

b)

a)

DISCUSION AND CONCLUSIONDISCUSION AND CONCLUSION

g g g A/mT A/

mT

A/mT

Cu(Me2Ala)2

CD3OD 2.1170.00

1

2.2410.00

1

2.0550.00

1

7.390.01

18.270.01

1.950.01

Cu(Me2Leu)2

CDCl3 2.1160.00

1

2.2430.00

1

2.0530.00

1

9.400.01

17.900.01

5.150.01

CD3OD 2.1330.00

1

2.2440.00

1

2.0780.00

1

7.800.01

17.800.01

2.800.01

Cu(Me2Val)2

CDCl3 2.1170.00

1

2.2510.00

1

2.0500.00

1

9.450.01

17.900.01

5.230.01

CD3OD280-295 K

2.1290.00

1

2.2440.00

1

2.0720.00

1

8.200.01

18.500.01

3.050.01

CD3OD295 -317 K

2.1260.00

1

2.2440.00

1

2.0670.00

1

8.400.01

18.500.01

3.350.01

Table 1. EPR parameters used for calculations of .

106 (/Pa s)/(T/K)

1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20

/ ps

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.010

6 (/Pa s)/(T/K)

1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00

/ ps

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

Figure 3. Dependence of on /T for a) Cu(Me2Val)2

(blue - constant A0 and g0 ; black - different A0 and g0 at

higher temperature) and Cu(Me2Leu)2 (red) in CD3OD, and

b) Cu(Me2Val)2 (blue) and Cu(Me2Leu)2 (red) in CDCl3.

a)

b) Figure 4. The most stable conformations ofaqua-complexes of Cu(Me2Leu)2 and

Cu(Me2Val)2 with both side chains in axial positions.

Cu(Me2Leu)2·4H2O

Ow

Ow

Cu(Me2Val)2 ·4H20