// wc_basepair() - create Watson/Crick base pair
#define AT_SEP 8.29
#define CG_SEP 8.27
molecule wc_basepair( residue sres, residue ares )
{
	molecule m, m_sense, m_anti;
	float	sep;
	string arname, srname;
	string xtail, xhead;
	string ytail, yhead;
	matrix	mat;

	m = newmolecule();
	m_sense = newmolecule();
	m_anti = newmolecule();
	addstrand( m, "sense" );
	addstrand( m, "anti" );
	addstrand( m_sense, "sense" );
	addstrand( m_anti, "anti" );

	srname = getresname( sres );
	arname = getresname( ares );
	ytail = "sense::C1'";
	yhead = "anti::C1'";
	if( ( srname == "ADE" ) || ( srname =~ "D*A[35]*" ) ){
		sep = AT_SEP;
		xtail = "sense::C5";
		xhead = "sense::N3";
		addresidue( m_sense, "sense", sres );
		setframe( 2, m_sense, "::C4", "::C5", "::N3", "::C4", "::N1" );
	}else if( ( srname == "CYT" ) || ( srname =~ "D*C[35]*" ) ){
		sep = CG_SEP;
		xtail = "sense::C6";
		xhead = "sense::N1";
		addresidue( m_sense, "sense", sres );
		setframe( 2, m_sense, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else if( ( srname == "GUA" ) || ( srname =~ "D*G[35]*" ) ){
		sep = CG_SEP;
		xtail = "sense::C5";
		xhead = "sense::N3";
		addresidue( m_sense, "sense", sres );
		setframe( 2, m_sense, "::C4", "::C5", "::N3", "::C4", "::N1" );
	}else if( ( srname == "THY" ) || ( srname =~ "DT[35]*" ) ){
		sep = AT_SEP;
		xtail = "sense::C6";
		xhead = "sense::N1";
		addresidue( m_sense, "sense", sres );
		setframe( 2, m_sense, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else if( ( srname == "URA" ) || ( srname =~ "U[35]*" ) ){
		sep = AT_SEP;
		xtail = "sense::C6";
		xhead = "sense::N1";
		addresidue( m_sense, "sense", sres );
		setframe( 2, m_sense, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else{
		fprintf( stderr,"wc_basepair : unknown sres %s\n",srname );
 		exit( 1 );
	}
	if( ( arname == "ADE" ) || ( arname =~ "D*A[35]*" ) ){
		addresidue( m_anti, "anti", ares );
		setframe( 2, m_anti, "::C4", "::C5", "::N3", "::C4", "::N1" );
	}else if( ( arname == "CYT" ) || ( arname =~ "D*C[35]*" ) ){
		addresidue( m_anti, "anti", ares );
		setframe( 2, m_anti, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else if( ( arname == "GUA" ) || ( arname =~ "D*G[35]*" ) ){
		addresidue( m_anti, "anti", ares );
		setframe( 2, m_anti, "::C4", "::C5", "::N3", "::C4", "::N1" );
	}else if( ( arname == "THY" ) || ( arname =~ "DT[35]*" ) ){
		addresidue( m_anti, "anti", ares );
		setframe( 2, m_anti, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else if( ( arname == "URA" ) || ( arname =~ "U[35]*" ) ){
		addresidue( m_anti, "anti", ares );
		setframe( 2, m_anti, "::C6", "::C5", "::N1", "::C6", "::N3" );
	}else{
		fprintf( stderr,"wc_basepair : unknown ares %s\n",arname );
 		exit( 1 );
	}
	
	alignframe( m_sense, NULL );
	alignframe( m_anti, NULL );
	mat = newtransform( 0., 0., 0., 180., 0., 0. );
	transformmol( mat, m_anti, NULL );
	mat = newtransform( 0., sep, 0., 0., 0., 0. );
	transformmol( mat, m_anti, NULL );
	mergestr( m, "sense", "last", m_sense, "sense", "first" );
	mergestr( m, "anti", "last", m_anti, "anti", "first" );

	freemolecule( m_sense );
	freemolecule( m_anti );

	setframe( 2, m, "::C1'", xtail, xhead, ytail, yhead );
	alignframe( m, NULL );
	return( m );
};