Table 1.

Expected diffraction ratios for potential applications of resolved anomalous phasing

Molecule	NP	NA	$\frac{\langle \mid \mathrm{\Delta }F\mid \rangle }{\langle F_{\mathrm{P}}\rangle }$	$\frac{\langle F_{\mathrm{A}}\rangle }{\langle F_{\mathrm{P}}\rangle }$
Sulphur-rich proteins:
Crambin	400	6S	1.4%	29%
Snake neurotoxin38	500	8S	1.5%	30%
Metalloproteins:
Hi PIP39	700	4Fe/8S	5.3%	38%
Haemerythrin19	1,000n	2nFe	3.0%*	17%*
Oligonucleotides:
d(CPGPCPGPCPG)40	300	10P	1.6%	39%
Oligopeptides:
Antamanide41	100	10O	0.5%†	37%
Heavy-atom complexes:
Myoglobin42	1,300	1Hg	4.5%	31%
Lysozyme43	1,100	1U	8.5%	39%

N_P is the approximate number of non-hydrogen atoms in the total molecule and N_A is the number of anomalous scatterers. (Omission of hydrogen atoms has a negligible effect on these calculations: 〈F_P〉 with hydrogens is ~1.01〈F_P〉 without hydrogens.) The diffraction ratios are estimates for zero scattering angle. In the case of only one kind of anomalous scatterer these ratios are $\langle \mid \mathrm{\Delta }F\mid \rangle /\langle F_{\mathrm{P}}\rangle \simeq 2^{1/2}(N_{\mathrm{A}}^{1/2}\mathrm{\Delta }{f}_{\mathrm{A}}^{″})/(N_{\mathrm{P}}^{1/2}{Z}_{\text{eff}})$ and $\langle F_{\mathrm{A}}\rangle /\langle F_{\mathrm{P}}\rangle \simeq (N_{\mathrm{A}}^{1/2}{Z}_{\mathrm{A}}^{\prime })/(N_{\mathrm{P}}^{1/2}{Z}_{\text{eff}})$ where Z_eff is the effective atomic number (~6.7 for non-hydrogen protein atoms) and $Z_{\mathrm{A}}^{\prime }$ is $Z_{\mathrm{A}}+\mathrm{\Delta }{f}_{\mathrm{A}}^{\prime }$. F_P and F_A are structure-factor magnitudes of real contributions from the complete molecule and from anomalous scatterers only, respectively. Of course, the diffraction ratios will vary with scattering angle in the same way as the ratio of thermal and scattering factors for the anomalous scatterers to those of the other atoms. Except where noted all estimates are for CuKα radiation.

^*At low resolution (for example, 5.5 Å) these values are enhanced by a factor of 2^1/2 because the dimeric iron centre is then unresolved.

^†Oxygen anomalous scattering is computed for CrKα radiation.