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Mega deals morph into mega problems for Wall Street

* Political uncertainty, antitrust hurdles deterring big deals

* Some inversions still possible but high risk

* ICE rules out structuring any deal for LSE as inversion -sources

* Investment banks looking at new ways to structure their fees

By Carl O'Donnell and Pamela Barbaglia

NEW YORK/LONDON April 7 (Reuters) - If 2015 was a dream year for Wall Street's top dealmakers, 2016 is starting to take a nightmarish turn.

Some of the mega transactions that had champagne corks popping in boardrooms are running into antitrust problems and, in the case of pharmaceutical firm Pfizer Inc's $160 billion takeover of rival Allergan PLC, political opposition to a deal that envisaged the biggest drug company in the United States moving to Ireland to lower its taxes.

The U.S. Treasury unveiled new rules this week that, while they did not name Pfizer and Allergan, had provisions that targeted a specific feature of their agreement and prompted both parties to walk away from what would have been the second-largest deal of all time.

The move by the Obama administration to suddenly change the rules has sent a chilling message to dealmakers and comes on top of a number of legal challenges to big transactions such as Halliburton Co's takeover of rival oil services company Baker Hughes Inc on antitrust grounds.

The political uncertainty and antitrust concerns mean that firms will think twice about future tie-ups that consolidate industries and move tax dollars offshore.

"As uncertainty increases on multiple fronts, companies are markedly more cautious and the number of transformational deals worth $10 billion or more has significantly dropped this quarter compared to last year," said Luigi Rizzo, head of mergers and acquisitions (M&A) for Europe, the Middle East and Africa at Bank of America Merrill Lynch.

The new U.S. rules do not directly affect most inversion deals, in which an American company buys a foreign counterpart and then moves abroad to lower its tax bill, but they have sent a message to company bosses about the risks of attempting to move their tax addresses overseas.

Intercontinental Exchange Inc, the U.S. exchange considering a bid for the London Stock Exchange Group PLC , has ruled out structuring any possible deal for the LSE as an inversion, despite it being possible to do so, according to people familiar with the internal deliberations, who declined to be identified.

Intercontinental Exchange declined to comment.

Tax inversions have been a political hot button issue in Washington for years.

The rules unveiled this week were the Obama administration's third effort to stop U.S. companies renouncing their American citizenship but they are only a temporary stopgap.