The solution structure of the isolated antibody heavy chain variable domain 
(VH)-P8 was determined by NMR spectroscopy. The VH had previously been modified 
(camelised) at three positions in its former antibody light chain variable 
domain (VL) interface to reduce hydrophobicity by mimicking camelid heavy chains 
naturally devoid of light chains. The architecture of two pleated beta-sheets 
and the conformation of the H1 and H2 loops in VH-P8 are very similar to those 
in non-camelised, VL-associated VH domains. Major differences concern the H3 
loop, which no longer points towards the now absent VL, and three residues in 
the former VL interface. The side-chains of Val37 and Trp103 are buried and the 
Arg38 side-chain exposed in VH-P8. In non-camelised, VL-associated VH domains 
the side-chains of Val37 and Trp103 are in contact with the VL while the Arg38 
side-chain is buried within the VH. Reorientation of Trp103 is due to the local 
structure in the beta-bulge of strand G. Reorientation of Val37 and Arg38 is 
caused by a disruption of regular beta-structure in strand C opposite the 
beta-bulge in strand C'. These changes, combined with the more hydrophilic 
side-chains of the camelised residues, reduce hydrophobicity and prevent 
non-specific binding of camelised VH domains, which proved critical for their 
use as small recognition units. The VH-P8 structure also indicates structural 
reasons for two other mutations specific for light-chain-lacking camel 
immunoglobins. Absence of the VH-typical Arg94/Asp101 salt bridge at the base of 
the H3 loop in VH-P8 may explain why a positively charged residue at position 94 
is not conserved in camels. Reorientation of Val37 suggests a function of the 
camel-specific phenylalanine residue at this position in the hydrophobic core of 
light-chain-lacking camel heavy chains.